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How to Write a Microbiology Lab Report

Last Updated: October 13, 2023 Fact Checked

This article was co-authored by Bess Ruff, MA . Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 77,932 times.

Whether you’re studying microbiology in high school or as a college student, you’ll need to write a number of lab reports. The lab-report genre does have several sections you’ll need to meet in your report, which include: a Purpose Statement, Methods, Results, a Discussion or Conclusion, and References. Depending on your instructor's preferences, your report may also include an introduction. Scientific writing should always focus on concision and clarity. Write your lab report without any flowery or figurative language, and focus on clearly describing the experiment you’ve performed. [1] X Research source

Using Clear Scientific Writing

Step 1 Write the lab report in the passive voice.

  • So, instead of writing, “I used plastic pipettes to fill the beakers with 25 mL of water,” write “The beakers were filled with 25 mL of water using plastic pipettes.”
  • Use as few pronouns as possible when writing your lab report. Pronouns to avoid using include “I,” “we,” and “they.”

Step 2 Compose the majority of the lab report using the past tense.

  • For instance, instead of saying, "The results prove the hypothesis is correct," say, "The results of the experiment proved the hypothesis was correct."
  • The Introduction is one of the few parts of the lab report which can be written in the present tense.

Step 3 Review the lab-report rubric before you start writing.

  • Add/subtract/merge certain structural elements of a report.
  • Grade one part of a report more heavily than another.
  • Require reports to be typed, using a specific font and size.
  • Require reports to be handwritten in a research notebook.

Composing the Introduction and Purpose Statement

Step 1 Write an Introduction only if your instructor requests one.

  • For example, your Introduction could begin, “In this laboratory experiment, the ability of a lab microscope to differentiate between different species of single-cell organisms was tested.”
  • Methods and Results should all be written in the past tense, since you’ll be summarizing actions that you’ve already performed as part of the lab.

Step 2 Include your purpose and hypothesis in the Purpose section.

  • In the Purpose section you should also include background information about the experiment, including the reason that you’re performing the experiment. This information can be found in the lab manual or related microbiology textbook.
  • For example, begin your Purpose statement by writing something like, “In this lab experiment, 3 different types of bacteria were separated using a nutrient agar plate.”

Step 3 State your hypothesis at the end of the Purpose section.

  • For example, write: “The initial hypothesis suggested that the bacteria in group 1 would outnumber bacteria in groups 2 and 3 by a rate of 5:1.”
  • Finally, the Purpose section should state, but not extrapolate on, all techniques or tests used in the experiment. Keep things cursory here, though, since you’ll give detailed information about the techniques and methods used in the Methods section.

Writing the Methods and Results Sections

Step 1 State the materials you used in the experiment in the Methods section.

  • The type of agar (if agar was used).
  • The type of microorganism used (if the organism types were known beforehand).
  • The size of all test tubes, beakers, calipers, and any other type of science equipment.
  • For example, the materials description could include a sentence like: “Five 50-mL beakers were used to contain the water and single-celled organisms. The water was applied to microscope slides using 1-mL plastic pipettes.”

Step 2 Describe the steps performed during the experiment in the Methods section.

  • If your instructor deviated from the original experiment, make adjustments as needed.
  • For example, write something like, “After a plastic pipette was used to place the single-celled organisms on the center of microscope slides, a slide cover was placed over each water sample. Organisms were then identified through the microscope using 50x and 100x magnification.”

Step 3 Record your results using specific data measurements in the Results section.

  • However, do not interpret the scientific data in the Results section. Only interpret data in the Discussion section.
  • For example, write something like, “When the microscope was set to 100x magnification, single-celled organisms that were at least 0.25 mm smaller or larger than the surrounding organisms could be identified.”

Step 4 Focus the Results on trends and phenomena you were asked to test.

  • For example, if a bacteria that you were asked to observe had consistent physical traits, describe these in the Results section.
  • Write something like, “The reactions of single-celled organisms to different water temperatures and chemical additives were noted. It was noted that, as less-diluted chemicals were added, the organisms acted in increasingly unpredictable ways.”

Step 5 Include figures and...

  • Figures and tables should also be mentioned and explained in the main text of your Results section.

Putting Together the Discussion and References Sections

Step 1 Interpret and contextualize your data findings in the Discussion section.

  • The Discussion section is usually the most important section of the lab report. It shows that you’ve understood the experiment you just performed and are able to engage with the scientific implications.
  • For example, write, “The amoebas were observed displaying consistent behavior throughout the observation period. The data suggests that the organisms were unable to detect the variety of chemicals that were added to different water samples, which the amoebas were then suspended in.”

Step 2 Explain whether the results supported or disproved your hypothesis in the Discussion section.

  • You could state something as simple as, “The results disproved the initial hypothesis, which failed to account for the similar sizes and colors of many of the single-celled organisms that were identified.”
  • If your results do not support your hypothesis, ask questions like, was there any error during the experiment? Did you miss a step in the experiment? Did you use proper techniques? Were your results accurate?

Step 3 Reference all sourced...

  • If you should include a References section instead of a Bibliography, you’ll only need to include citation information for sources that were cited in the lab report.
  • Ask your instructor which citation style you should use when compiling your Bibliography. For example, most microbiology TAs will ask you to use Chicago style.
  • Most lab reports have short Bibliographies, since very few lab reports cite more than 1 or 2 sources (if any).

Community Q&A

Community Answer

  • Remember to always ask your instructor about correct formatting before writing a lab report if you’re confused or unsure about any aspect of the report. The instructor may have specific stylistic or content-focused requirements for the report that they can clarify to you. Thanks Helpful 0 Not Helpful 0
  • Never pay a website to write a lab report for you. Not only will you be paying someone to do work that you could do for free, but your instructor will almost certainly see through the lab report and realize that you didn’t write it yourself. Thanks Helpful 0 Not Helpful 0

how to write unknown lab report microbiology

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  • ↑ http://microbiology.science.oregonstate.edu/files/micro/Writing%20Manual%20for%20Science%20Majors_0.pdf
  • ↑ https://www.hcs-k12.org/userfiles/354/Classes/18203/conclusionwriting.pdf
  • ↑ https://wic.oregonstate.edu/microbiology-writing-guide-lab-report-format
  • ↑ http://www.utm.edu/staff/cerkal/report.html

About This Article

Bess Ruff, MA

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Unknown Sample Lab Report: What Is It?

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In your BIO 150 Lab class you might write the sections of the report  individually  as practice for the final report. Whether the assignment is to write an individual section of the report or the whole report, the procedure  is the same , and your research sources should  address the bigger picture.

Read all assignments completely, and don't forget to read the rubric (grading criteria) to know what is expected of you. 

How to Write the Report

A lab report is a formal scientific document, and as such it differs from other forms of writing (such as literature). The aim of the report is to provide information about the experiement or study that was conducted by the researcher(s), and about the findings and implications.  There are several rules that apply to the report:

  • The language should be formal, direct, and plain.
  • The passive voice should be used. For example, instead of writing "We used a blood agar plate," you would write "A blood agar plate was used."
  • Most of the report (introduction, summary, materials & methods, and results) is written in the past tense ( was used  as opposed to  is used ).
  • The conclusion of the report is usually written in the present tense. 
  • Microbial nomenclature (the names of the bacteria) must be correct and free of spelling mistakes. Names should be in italics, with the genus capitalized and the species in lowercase. Example:  Proteus mirabilis .  
  • After the first mention of the bacterium, subsequent mentions may be shortened (still maintaining italics). Example:  P. mirabilis .

All reports must be typed. Read your instructor's instructions to make sure you follow any other requirement that s/he may have. It is important to be very precise and follow instructions to the letter. 

For more information, please visit these links:

How to Write Scientific Reports, from the University of North Carolina

How to Use the Passive Voice, from the University of North Carolina

References Section

For this assignment you will cite your sources using the Journal of Microbiology style. Please note this style is particular to this one journal ; to see some examples of formatted citations check the class handout linked in this section. Please note this document is limited in the type of citations it gives examples of. This is the only available document with information about this citation style.

This style abbreviates the titles of journals.  To find the abbreviation, go to  the  National Library of Medicine Catalog  and in the search box type the title of the journal whose abbreviation you need, then click "Search."

Your results will include the journal abbreviation:

Red arrow pointing to example of search result from NLM

Report Sections

An lab report usually includes the following sections:

  • Introduction
  • Materials and Methods
  • Discussion 

Note : You may not be asked to do all of these. 

Finding Research for Your Report

In the  Introduction  section of your report you should discuss "the bigger picture." For example, why is there value in identifying the bacterium you found? 

Lab samples

For example, if your unknown sample was  Staphylococcus aureus , you might look for articles that describe the impact of  S. aureus  on populations and relate that to the need to identify this bacterium in order to prevent the damage it may cause. Look for sources that tie into  the "bigger picture" issues that you have identified, and that backup the arguments you made. 

You may be required to have one or more  primary  sources. A primary source, in the sciences, is  empirical research . This means a study or experiment where the experimenter is also the author of the article.

How do you recognize whether the article you are reading is a primary source? As you begin to read the article, you should see that the writers describe a study they conducted or an experiment they carried out. In fact, the article will sound a bit similar to your report in that it will talk about objectives, materials and methods, and will have a conclusion and a discussion. If the article you are reading is about an experiment or study someone other than the writer conducted, then you do not have a primary source. 

Find sources here

Provides access to whole articles, in addition to citations and abstracts.

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How To Write A Lab Report | Step-by-Step Guide & Examples

Published on May 20, 2021 by Pritha Bhandari . Revised on July 23, 2023.

A lab report conveys the aim, methods, results, and conclusions of a scientific experiment. The main purpose of a lab report is to demonstrate your understanding of the scientific method by performing and evaluating a hands-on lab experiment. This type of assignment is usually shorter than a research paper .

Lab reports are commonly used in science, technology, engineering, and mathematics (STEM) fields. This article focuses on how to structure and write a lab report.

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Table of contents

Structuring a lab report, introduction, other interesting articles, frequently asked questions about lab reports.

The sections of a lab report can vary between scientific fields and course requirements, but they usually contain the purpose, methods, and findings of a lab experiment .

Each section of a lab report has its own purpose.

  • Title: expresses the topic of your study
  • Abstract : summarizes your research aims, methods, results, and conclusions
  • Introduction: establishes the context needed to understand the topic
  • Method: describes the materials and procedures used in the experiment
  • Results: reports all descriptive and inferential statistical analyses
  • Discussion: interprets and evaluates results and identifies limitations
  • Conclusion: sums up the main findings of your experiment
  • References: list of all sources cited using a specific style (e.g. APA )
  • Appendices : contains lengthy materials, procedures, tables or figures

Although most lab reports contain these sections, some sections can be omitted or combined with others. For example, some lab reports contain a brief section on research aims instead of an introduction, and a separate conclusion is not always required.

If you’re not sure, it’s best to check your lab report requirements with your instructor.

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Your title provides the first impression of your lab report – effective titles communicate the topic and/or the findings of your study in specific terms.

Create a title that directly conveys the main focus or purpose of your study. It doesn’t need to be creative or thought-provoking, but it should be informative.

  • The effects of varying nitrogen levels on tomato plant height.
  • Testing the universality of the McGurk effect.
  • Comparing the viscosity of common liquids found in kitchens.

An abstract condenses a lab report into a brief overview of about 150–300 words. It should provide readers with a compact version of the research aims, the methods and materials used, the main results, and the final conclusion.

Think of it as a way of giving readers a preview of your full lab report. Write the abstract last, in the past tense, after you’ve drafted all the other sections of your report, so you’ll be able to succinctly summarize each section.

To write a lab report abstract, use these guiding questions:

  • What is the wider context of your study?
  • What research question were you trying to answer?
  • How did you perform the experiment?
  • What did your results show?
  • How did you interpret your results?
  • What is the importance of your findings?

Nitrogen is a necessary nutrient for high quality plants. Tomatoes, one of the most consumed fruits worldwide, rely on nitrogen for healthy leaves and stems to grow fruit. This experiment tested whether nitrogen levels affected tomato plant height in a controlled setting. It was expected that higher levels of nitrogen fertilizer would yield taller tomato plants.

Levels of nitrogen fertilizer were varied between three groups of tomato plants. The control group did not receive any nitrogen fertilizer, while one experimental group received low levels of nitrogen fertilizer, and a second experimental group received high levels of nitrogen fertilizer. All plants were grown from seeds, and heights were measured 50 days into the experiment.

The effects of nitrogen levels on plant height were tested between groups using an ANOVA. The plants with the highest level of nitrogen fertilizer were the tallest, while the plants with low levels of nitrogen exceeded the control group plants in height. In line with expectations and previous findings, the effects of nitrogen levels on plant height were statistically significant. This study strengthens the importance of nitrogen for tomato plants.

Your lab report introduction should set the scene for your experiment. One way to write your introduction is with a funnel (an inverted triangle) structure:

  • Start with the broad, general research topic
  • Narrow your topic down your specific study focus
  • End with a clear research question

Begin by providing background information on your research topic and explaining why it’s important in a broad real-world or theoretical context. Describe relevant previous research on your topic and note how your study may confirm it or expand it, or fill a gap in the research field.

This lab experiment builds on previous research from Haque, Paul, and Sarker (2011), who demonstrated that tomato plant yield increased at higher levels of nitrogen. However, the present research focuses on plant height as a growth indicator and uses a lab-controlled setting instead.

Next, go into detail on the theoretical basis for your study and describe any directly relevant laws or equations that you’ll be using. State your main research aims and expectations by outlining your hypotheses .

Based on the importance of nitrogen for tomato plants, the primary hypothesis was that the plants with the high levels of nitrogen would grow the tallest. The secondary hypothesis was that plants with low levels of nitrogen would grow taller than plants with no nitrogen.

Your introduction doesn’t need to be long, but you may need to organize it into a few paragraphs or with subheadings such as “Research Context” or “Research Aims.”

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A lab report Method section details the steps you took to gather and analyze data. Give enough detail so that others can follow or evaluate your procedures. Write this section in the past tense. If you need to include any long lists of procedural steps or materials, place them in the Appendices section but refer to them in the text here.

You should describe your experimental design, your subjects, materials, and specific procedures used for data collection and analysis.

Experimental design

Briefly note whether your experiment is a within-subjects  or between-subjects design, and describe how your sample units were assigned to conditions if relevant.

A between-subjects design with three groups of tomato plants was used. The control group did not receive any nitrogen fertilizer. The first experimental group received a low level of nitrogen fertilizer, while the second experimental group received a high level of nitrogen fertilizer.

Describe human subjects in terms of demographic characteristics, and animal or plant subjects in terms of genetic background. Note the total number of subjects as well as the number of subjects per condition or per group. You should also state how you recruited subjects for your study.

List the equipment or materials you used to gather data and state the model names for any specialized equipment.

List of materials

35 Tomato seeds

15 plant pots (15 cm tall)

Light lamps (50,000 lux)

Nitrogen fertilizer

Measuring tape

Describe your experimental settings and conditions in detail. You can provide labelled diagrams or images of the exact set-up necessary for experimental equipment. State how extraneous variables were controlled through restriction or by fixing them at a certain level (e.g., keeping the lab at room temperature).

Light levels were fixed throughout the experiment, and the plants were exposed to 12 hours of light a day. Temperature was restricted to between 23 and 25℃. The pH and carbon levels of the soil were also held constant throughout the experiment as these variables could influence plant height. The plants were grown in rooms free of insects or other pests, and they were spaced out adequately.

Your experimental procedure should describe the exact steps you took to gather data in chronological order. You’ll need to provide enough information so that someone else can replicate your procedure, but you should also be concise. Place detailed information in the appendices where appropriate.

In a lab experiment, you’ll often closely follow a lab manual to gather data. Some instructors will allow you to simply reference the manual and state whether you changed any steps based on practical considerations. Other instructors may want you to rewrite the lab manual procedures as complete sentences in coherent paragraphs, while noting any changes to the steps that you applied in practice.

If you’re performing extensive data analysis, be sure to state your planned analysis methods as well. This includes the types of tests you’ll perform and any programs or software you’ll use for calculations (if relevant).

First, tomato seeds were sown in wooden flats containing soil about 2 cm below the surface. Each seed was kept 3-5 cm apart. The flats were covered to keep the soil moist until germination. The seedlings were removed and transplanted to pots 8 days later, with a maximum of 2 plants to a pot. Each pot was watered once a day to keep the soil moist.

The nitrogen fertilizer treatment was applied to the plant pots 12 days after transplantation. The control group received no treatment, while the first experimental group received a low concentration, and the second experimental group received a high concentration. There were 5 pots in each group, and each plant pot was labelled to indicate the group the plants belonged to.

50 days after the start of the experiment, plant height was measured for all plants. A measuring tape was used to record the length of the plant from ground level to the top of the tallest leaf.

In your results section, you should report the results of any statistical analysis procedures that you undertook. You should clearly state how the results of statistical tests support or refute your initial hypotheses.

The main results to report include:

  • any descriptive statistics
  • statistical test results
  • the significance of the test results
  • estimates of standard error or confidence intervals

The mean heights of the plants in the control group, low nitrogen group, and high nitrogen groups were 20.3, 25.1, and 29.6 cm respectively. A one-way ANOVA was applied to calculate the effect of nitrogen fertilizer level on plant height. The results demonstrated statistically significant ( p = .03) height differences between groups.

Next, post-hoc tests were performed to assess the primary and secondary hypotheses. In support of the primary hypothesis, the high nitrogen group plants were significantly taller than the low nitrogen group and the control group plants. Similarly, the results supported the secondary hypothesis: the low nitrogen plants were taller than the control group plants.

These results can be reported in the text or in tables and figures. Use text for highlighting a few key results, but present large sets of numbers in tables, or show relationships between variables with graphs.

You should also include sample calculations in the Results section for complex experiments. For each sample calculation, provide a brief description of what it does and use clear symbols. Present your raw data in the Appendices section and refer to it to highlight any outliers or trends.

The Discussion section will help demonstrate your understanding of the experimental process and your critical thinking skills.

In this section, you can:

  • Interpret your results
  • Compare your findings with your expectations
  • Identify any sources of experimental error
  • Explain any unexpected results
  • Suggest possible improvements for further studies

Interpreting your results involves clarifying how your results help you answer your main research question. Report whether your results support your hypotheses.

  • Did you measure what you sought out to measure?
  • Were your analysis procedures appropriate for this type of data?

Compare your findings with other research and explain any key differences in findings.

  • Are your results in line with those from previous studies or your classmates’ results? Why or why not?

An effective Discussion section will also highlight the strengths and limitations of a study.

  • Did you have high internal validity or reliability?
  • How did you establish these aspects of your study?

When describing limitations, use specific examples. For example, if random error contributed substantially to the measurements in your study, state the particular sources of error (e.g., imprecise apparatus) and explain ways to improve them.

The results support the hypothesis that nitrogen levels affect plant height, with increasing levels producing taller plants. These statistically significant results are taken together with previous research to support the importance of nitrogen as a nutrient for tomato plant growth.

However, unlike previous studies, this study focused on plant height as an indicator of plant growth in the present experiment. Importantly, plant height may not always reflect plant health or fruit yield, so measuring other indicators would have strengthened the study findings.

Another limitation of the study is the plant height measurement technique, as the measuring tape was not suitable for plants with extreme curvature. Future studies may focus on measuring plant height in different ways.

The main strengths of this study were the controls for extraneous variables, such as pH and carbon levels of the soil. All other factors that could affect plant height were tightly controlled to isolate the effects of nitrogen levels, resulting in high internal validity for this study.

Your conclusion should be the final section of your lab report. Here, you’ll summarize the findings of your experiment, with a brief overview of the strengths and limitations, and implications of your study for further research.

Some lab reports may omit a Conclusion section because it overlaps with the Discussion section, but you should check with your instructor before doing so.

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A lab report conveys the aim, methods, results, and conclusions of a scientific experiment . Lab reports are commonly assigned in science, technology, engineering, and mathematics (STEM) fields.

The purpose of a lab report is to demonstrate your understanding of the scientific method with a hands-on lab experiment. Course instructors will often provide you with an experimental design and procedure. Your task is to write up how you actually performed the experiment and evaluate the outcome.

In contrast, a research paper requires you to independently develop an original argument. It involves more in-depth research and interpretation of sources and data.

A lab report is usually shorter than a research paper.

The sections of a lab report can vary between scientific fields and course requirements, but it usually contains the following:

  • Abstract: summarizes your research aims, methods, results, and conclusions
  • References: list of all sources cited using a specific style (e.g. APA)
  • Appendices: contains lengthy materials, procedures, tables or figures

The results chapter or section simply and objectively reports what you found, without speculating on why you found these results. The discussion interprets the meaning of the results, puts them in context, and explains why they matter.

In qualitative research , results and discussion are sometimes combined. But in quantitative research , it’s considered important to separate the objective results from your interpretation of them.

Cite this Scribbr article

If you want to cite this source, you can copy and paste the citation or click the “Cite this Scribbr article” button to automatically add the citation to our free Citation Generator.

Bhandari, P. (2023, July 23). How To Write A Lab Report | Step-by-Step Guide & Examples. Scribbr. Retrieved March 4, 2024, from https://www.scribbr.com/academic-writing/lab-report/

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Microbiology Writing Guide: Lab Report Format

Organization and format, basic outline.

Scientific writing can be in the form of a laboratory report, a thesis, a journal article, or some other written communication used to disseminate the results of scientific research. The exact format required depends upon the type of written communication and often will vary from source to source.

Preparation of a Laboratory Report

A lab report differs from a paper in that it has defined sections. The sections required vary from laboratory to laboratory but the standard outline for most lab reports in the biological science include: title, your name, purpose of the experiment, methods, results, discussion and conclusion, references. Some lab reports may include a section of questions that must be answered concerning the experiment. Most laboratory courses will require that data be immediately written into a lab notebook in pen. Some labs will require you to attach these data pages to your report. Normally a lab report should be typed, spell checked and proofread before being submitted.

When writing a thesis, article for publication, or a report to turn into your supervisor, your first draft will be reviewed by your mentor and/or co-workers and then undergo revision. No matter how good a writer is, most reports require some revision. It is best to write your first draft and then let it sit for a few days before you read it the next time. Many times you are too “close” to the material after the first writing to see obvious errors. (This has definitely been true of this document!)

Sections of a Laboratory Report

Title : The title should be concise and specific and tell the reader what you did

Purpose : Most lab reports do not include a formal introduction and instead substitute a purpose. The purpose of the experiment should be stated in one or two sentences. You should know the purpose of the experiment before you start.

Methods: Most lab reports do not include all the details a journal article requires. Normally the procedure can be listed and referenced to the appropriate laboratory manual pages. If modifications have been made to the methods in the lab manual, these need to be clearly described.

Results : All data and observations should be included in the lab book; however, what you think should have happened or the methods section are not included. Types of results may include:

  • Measurements. Report measurements using standard metric units. Any time a number is presented, it must have units. Abbreviations of units are used without a following period. Use the prefixes m, m, n, and p for 10 -3 , 10 -6 , 10 -9 , and 10 -12 , respectively. Numbers should be written as numerals when they are greater than ten or when they are associated with measurements; for example, 8 mm or 20 g. In a list of objects including both numbers over and under ten, all numbers may be expressed as numerals. Example: 17 bacteria, 2 yeast, and 1 protozoan. If a number starts a sentence spell out the number, do not use a numeral. Example: ten mannitol salt agar plates were streaked…
  • Calculations. The equation should be indicated. In a lab report, even if you use a calculator, you must set up the problem.
  • Tables. Number each table and provide a title and legend that contains all the information needed to interpret the data. The reader should be able to understand the content without the text. The title should be located at the top of the table. Columns and rows should be labeled clearly. All notes should be placed below tables. Any abbreviations, units, calculations, or statistics used should be described in headers or footnotes (see Table 1 for an example). Symbols such as #, *, ! ; and superscripts such as 1 and 2 can be used to identify these footnotes. Use bold type to make these obvious.
  • Figures. Figures include graphs, photographs, drawings, diagrams, maps, and all other illustrations. All figures should be numbered and have a title and legend that contains all the information needed to interpret the data. The reader should be able to understand the content without the text. Figures should be labeled at the bottom. For a graph, units are specified on the abscissa and ordinate. If the photograph is of an object under the microscope, the total magnification should be indicated. Photographs of gel electrophoresis data should include a number on each lane, and the legend (or the figure itself) should indicate the contents of each lane.
  • Plate counts. Include results for all dilutions, even if they are too numerous to count (TNTC) or 0. You should indicate the type of medium plated and temperature of incubation. See Table 1.

Table 1. Results of viable cell count of diluted Escherichia coli grown at 37 o C in nutrient broth (1 ml plated).

*In this example, only 249, 235 and 35 are significant counts. These data are averaged:

249/10 -3 + 235/10 -3 + 35/10 -4 or 2.5 X 10 5 + 2.4 X 10 5 + 3.5 X 10 5 /3

= 2.8 X 10 5 CFU/ml

The text should refer to each table and figure and they should appear after, but close to, text that refers to them, (i.e., at the end of a paragraph or section). Alternatively, tables and figures may be placed at the end of the paper. Tables and figures are numbered independently of each other, and they are assigned numbers in the order they are mentioned in the text. The in-text reference to a table or figure should not repeat the caption (e.g. ‘table 1 shows “Title on table” ’). Instead, it should draw attention to key features (e.g. “Table 1 shows that the number of bacteria in the culture increased markedly between hours 1 and 4.”).

Discussion/Conclusion: The discussion section interprets the meaning of the results and draws conclusions from the data that have been presented. The authors should show how their observations relate to each other to form a cohesive story. If data can be interpreted in more than one way, all possibilities should be mentioned and the authors should indicate which alternative they think is correct and why. Results should be discussed even if they are unexpected or negative. For example, the presence of unexpected bands on agarose gels should be explained. This section should also address any discrepancies between these results and other papers. Material obtained from another source should be referenced.

The meaning of your results should be summarized in two to three sentences at the end of the section. This includes the potential implications of the research, and possibilities for future research that would contribute more to the field. In lab reports, experiments do not always work. This section allows the researcher to explain what might have gone wrong with an experiment.

References: The reference section gives complete details about sources that were cited, in any section of the text. A "Bibliography," on the other hand, refers to a list of materials used to obtain background knowledge on a subject. There are several standard styles for listing references. Depending on what type of scientific writing you are doing, you may be directed to follow a particular format. If so, follow the format that has been specified exactly. When references are cited, either the reference number or the author’s last name and the publication year are used. Example: “Some strains of E. coli can grow in orange juice (1)…” or “Some strains of E. coli can grow in orange juice (Brown, 1999)….” In this class, we will use the reference style of the American Society for Microbiology Journals. When references are cited within the paper , only the number is used. References are numbered in the order in which they appear in the article (citation-sequence reference system). No reference should be included that is not cited in the paper. Remember that ALL information within the report that is not your original work or idea should be referenced. Statements by other authors are usually paraphrased or summarized – direct quotations are rare in scientific writing.

<< Scientific Style || Citing Sources >>

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Microbiology Unknown Lab Report

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The identifying the unknown bacteria was started over again. The unknown tube A with mixed culture containing 2 types of bacteria was given out by the instructor again. The streak plate was made from original broth of mixed culture on the Nutrient Agar plate, using a method described in the lab manual. In order to be able to have a pure culture of unknown bacteria, this procedure was made. The plate was left in the incubator for 24 hours for growing. After observation of the plate, it was clear that there was a lot of growth. Some of that growth was taken for the making of another streak plate on Nutrient Agar, in order to isolate a pure colony. After the plate was taken from the incubator on the next day, the examination of growth was observed.

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Biology LibreTexts

1.42: Unknown Bacteria Identification Project

  • Last updated
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  • Page ID 79464

  • Rosanna Hartline
  • West Hills College Lemoore

Learning Objectives

  • Apply microbiological tools to isolate and identify bacterial species of unknown identities.
  • Carefully document results of microbiological tests.
  • Effectively collaborate with a classmate.
  • Successfully identify the unknown bacterial species.

Introduction

In this project you will experience the type of process that microbiologists have traditionally used to identify a bacterial species. This will involve:

  • Isolating bacteria (one species per culture - must begin with an isolated colony to insure that there is only one species) - if bacteria are not isolated, you cannot rely on the results of any of the other tests you do.
  • Conducting biochemical tests to narrow down the possible species of the unknown bacteria
  • Collaborating with a partner and contributing equally to the work
  • Writing a scientifically-written report detailing the project, its experiments, and its results

In scientific research and throughout the professional world, collaboration occurs often and is an essential skill. Employers want you to be able to collaborate effectively with others. You are building your job and career skills here!

In scientific research and throughout the professional world, being able to clearly communicate through writing is an essential skill. Empolyers want you to be able to communicate effectively with others. You are building your job and career skills here!

Activity Log

For this project you will work in a pair. You are BOTH responsible for contributing equally to identifying the unknown bacteria.

Keep the following activity log to show your contributions to identifying your unknown bacteria. This insures that this group project is fair and all members of the group are contributing equally to the project. You will submit your activity log in your report. If your contributions to the project don't demonstrate equal contributions, deductions will be made to the report.

Instructions for Identifying Unknown Bacteria

  • Obtain a culture of unknown bacteria from your instructor. Your unknown culture will have a letter or a number. What letter or number is your unknown bacterial culture?
  • Your culture of unknown bacteria will have two different species in it. Follow the steps below carefully to identify these bacterial species.

Step 1: Isolate the Two Bacterial Species & Do Initial Gram Stain

  • Conduct eight (8) streak plates on TSA in order to obtain isolated colonies (each partner will conduct four). By doing eight streak plates, this increases the chances that you and your partner will obtain isolated colonies of both your bacterial species.
  • Make sure your names, unknown number/letter, and the date are on the Petri plates, invert the Petri plates, tape together, and put into the bin to be incubated.
  • Whether the mixture of bacterial species contains a Gram positive and a Gram negative species, both Gram positive, or both Gram negative.
  • The cell shape of each bacterial species (i.e. coccus, bacillus, vibrio, spirillum, or spirochete).
  • The cell arrangement of each bacteria species (i.e. single, pairs, chains (strepto-), or bunches (staphylo-).
  • Record results in the table below. Refer back to this table in Step 2 to make sure that the bacteria you isolate match the bacteria you saw in this Gram stain.

Taking photos of results at every stage of the project will make a stand-out report! Include these photos in your report to provide visuals and evidence of your results. Make sure each photo is accompanied by a caption telling what the photo shows. If the photo is of a microscopic sample, make sure to indicate the magnification used when the photo was taken.

Step 2: Characterize Bacterial Colonies, Create Stock Cultures of Isolated Bacteria, & Do Gram Stain on Isolates

petri plate differentiating isolated colonies versus non-isolated colonies

Figure 1: A quadrant streak plate after growth. Use this as a guide to help identify what are and are not isolated colonies. Isolated colonies originate from a single cell, and therefore should contain only one bacterial species. Therefore, growing a culture from an isolated colony is an effective way to separate different species.

illustrations of different colony forms, margins, and elevations

Figure 2: A guide for classifying colony characteristics. Colony form is the overall shape of the colony (circular, irregular, filamentous, or rhizoid) when viewed from above. Colony elevation examines the height and shape of the growth above the surface of the petri plate agar (raised, convex, flat, umbonate, or crateriform) and is best determined by viewing the colony from its side. Colony margin examines the shape of the edge of the colony when magnified (entire, undulate, filiform, curled, or lobate).

  • Carefully examine streak plates. Identify two bacterial colonies that have differences in appearance.
  • For the two different bacterial colonies you find, describe differences in the colony forms in the tables below. Do the best you can keeping your Petri plate closed to prevent contamination. It is essential that these bacterial colonies do not become contaminated with other bacteria or microbes floating in the air.
  • Label two TSA slants. Give each Unknown bacterial species a name (you can use names "A" and "B" or "1" and "2" or "Yessica" and "Yoli" - choose any names you and your partner would like - you will just need to keep straight which colony is which).
  • Transfer one third (1/3) of one of the isolated colonies to one of the TSA slants.
  • Transfer one third (1/3) of the other isolated colony to one of the TSA slants.
  • Use one third (1/3) of each of the colonies to make separate bacterial smears on a slide and conduct a Gram stain .
  • If one or both bacterial species are not isolated (they are mixed with another species), re-examine the petri plates to see if there is another colony that appears different and create a new TSA slant, and create a new bacterial smear on a slide. Do the Gram stain on this if there is still time. If there is not time, save your bacterial smear for next class to see if the bacteria are now isolated.
  • If you do not have better isolated colonies or continue to find that you have a mixture, conduct more streak plates to isolate bacterial colonies again.
  • When you have successfully isolated your bacteria, record results from the Gram stain in the tables below.

Step 3: Follow the Flow Chart to Identify Bacterial Species

  • For example, if your Bacteria "A" is Gram-positive, use your TSA stock slant to inoculate a starch plate to do a starch hydrolysis test.
  • For example, if your Bacteria "B" is Gram-negative, use your TSA stock slant to to inoculate a SIM deep culture to test for H 2 S production.
  • Use the results (positive or negative) from each test to determine the next test you should do on the flow chart.
  • starch hydrolysis
  • oxidase test
  • coagulase test and/or mannitol salts agar
  • fermentation test

unknown identification flow chart shows the steps to take after each test result

Figure 3: Unknown bacterial species identification flow chart. Once your bacterial species are isolated and you have good Gram stain results, begin to follow the flow chart for both of your unknown bacterial species. The tests that are a part of this project are in boxes. Use the results from each test to determine what the next test is or what your unknown bacterial species is.

Attributions

  • Laboratory Exercises in Microbiology: Discovering the Unseen World Through Hands-On Investigation by Joan Petersen and Susan McLaughlin is licensed under CC BY-NC-SA 4.0
  • Legionella Plate 01.png by CDC/James Gathany is in the public domain .

BIO205 - Microbiology

  • C.R.E.A.T.E.S.

Books on Scientific Writing

Unknown report instructions and rubric, citing with apa, citing with cse, ebooks on lab reports.

Cover Art

  • Unknown Report Instructions and Rubric Your unknown report must be written in APA format and with Times New Roman 12-point font. It must be double spaced and have 1 inch margins all around. It must be 4-5 pages in length (excluding tables, title page, this grading sheet, data table, and a reference page). It must include a title page that states the title of your report, the name and # of your unknown, the date, and your name. Include page numbers and a header with your title on each page. You must have at least five references, three of which cannot be Bergey’s, the lab manual, or the lecture book. Your references must be listed on a reference page in the correct APA format. You must refer to all of your references in the body of the report (“in-text” citations) using correct APA formatting.

IMPORTANT: INCLUDE THIS GRADING SHEET AND YOUR DATA TABLE TO THE BACK OF YOUR REPORT

APA formatting:

  • https://owl.english.purdue.edu/owl/resource/560/01  
  • APA Formatting in Word 2013   

Your report is worth 70 points and will be graded using the following rubric:

Part III    BIOCHEMICAL REACTIONS                    /20

  • Analysis and explanation of results                                /10
  • Comparison of unknown results to                                   /5 known/control (including possible explanations for discrepancies)
  • Comparison of unknown results to                                   /5 Bergey’s Manual (including possible explanations for discrepancies)

Total (%)                                                                                                                          /100

TOTAL (prorated)                                                                           /70

  • APA - In Text Citation
  • APA - Print & Media Sources
  • APA - Online & Database Sources
  • Tips for Using APA Citation Format & Style Guide
  • The Writer's Handbook: CSE Name-Year Documentation In this CSE citation system, references in your text give the last name of the author or authors and the year of publication within parentheses. These parenthetical refer to sources listed at the end of the document.
  • CSE - Phoenix College Council of Science Editors (CSE) style format.

how to write unknown lab report microbiology

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HOW TO WRITE AN UNKNOWN LAB REPORT IN MICROBIOLOGY

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<strong>HOW</strong> <strong>TO</strong> <strong>WRITE</strong> <strong>AN</strong> <strong>UNKNOWN</strong> <strong>LAB</strong> <strong>REPORT</strong> <strong>IN</strong> <strong>MICROBIOLOGY</strong><br />

GENERAL<br />

Unknown reports in microbiology are written in scientific format. Scientific writing is written<br />

differently from other types of writing. The results of the exercise or experiment are what are being<br />

showcased, not the writing. The purpose of scientific writing is not to entertain, but to inform. The<br />

writing should be simple and easy to understand. There is a specific style that must be followed<br />

when writing scientific reports.<br />

Scientific writing is typically written in the passive voice. The pronouns "I", "We" and "They" are<br />

not typically used.. For example, instead of writing "I used a TSA agar plate to isolate my<br />

unknown," it is customary to write, "A trypticase soy agar (TSA) plate was used to isolate the<br />

unknown."<br />

It is also customary to write in the past tense for most of the report. This includes the introduction,<br />

the summary, the description of the materials and methods and the results. The present tense is<br />

reserved for the conclusions about the results. See the examples given below.<br />

Some other general rules that should be followed are:<br />

Microbial nomenclature: The name of the bacterium should written and spelled correctly. The name<br />

should be italicized or underlined. Italicized is preferred. For example, Staphylococcus aureus.<br />

The genus is capitalized but the species is not. After the full genus name is given in the paper, it can<br />

be written as S. aureus, but still italicized. This is as long as there in no other genera in the paper<br />

that starts with the same letter.<br />

PARTS <strong>TO</strong> THE <strong>UNKNOWN</strong> <strong>LAB</strong> <strong>REPORT</strong><br />

(Note: Other than the title page, the pages of the report must be numbered)<br />

TITLE PAGE<br />

There should always be a title page and should include the following information:<br />

EXAMPLE OF TITLE PAGE<br />

Title should be centered and at the top or in the middle of the page<br />

<strong>UNKNOWN</strong> <strong>LAB</strong> <strong>REPORT</strong> # 1<br />

This information should be centered and at the bottom part of the title page:<br />

<strong>UNKNOWN</strong> LETTER (OR NUMBER)<br />

YOUR NAME<br />

DATE (the due date)<br />

<strong>LAB</strong> <strong>IN</strong>STRUC<strong>TO</strong>R'S NAME<br />

COURSE NAME<br />

SEMESTER / YEAR<br />

SECTION NUMBER

<strong>IN</strong>TRODUCTION<br />

This section introduces the reader to the study and why the study was done. This should only be a<br />

few sentences long.<br />

Example: "There are many reasons for knowing the identity of microorganisms. The<br />

reasons range from the knowing the causative agent of a disease in a patient, so as to know how it<br />

can be treated, to knowing the correct microorganism to be used for making certain foods or<br />

antibiotics. This study was done by applying all of the methods that have been learned so far in the<br />

microbiology laboratory class for the identification of an unknown bacterium."<br />

MATERIALS <strong>AN</strong>D METHODS<br />

This is where the details of the study are listed. Where did the specimen come from, and what<br />

methods were used to identify it? Be specific, but do not re write the lab manual. One way is to<br />

mention the names of the materials used and reference the lab manual for the procedure or method<br />

and then continue to elaborate when necessary. See example 1.<br />

Example 1: "An unknown labeled as letter G was given out by the lab instructor. The<br />

methods that have been learned thus far for identifying bacteria have been applied to this unknown.<br />

Procedures were followed as stated in the course laboratory manual by De Mers (1), unless<br />

otherwise noted.<br />

The first procedure that needed to be done was to streak the unknown out on a Trypticase Soy Agar<br />

plate, using the T streak method described in the lab manual.. This needed to be done in order to test<br />

the purity of the unknown. After the plates were incubated and grown, the morphology was<br />

observed and recorded and a Gram stain was performed. Quality control bacteria were Gram stained<br />

along with the unknown to make sure that the Gram stain reaction was done correctly . After<br />

determining the Gram reaction, specific biochemical tests were performed. The biochemical tests<br />

were chosen from the unknown identification tables that were in the lab manual. Since unknown G<br />

was determined to be a Gram negative rod, an oxidase test was performed and the organism was<br />

inoculated into a BCP lactose tube. Note all of these tests were performed by the methods listed in<br />

the lab manual by De Mers (1). Table 1 lists the test, purpose, reagents and results. All of the<br />

following tests were performed on this unknown:<br />

1. Oxidase test<br />

2. BCP Lactose<br />

3. Indole<br />

4. H2S<br />

5. Citrate<br />

6. Motility<br />

7. Methyl Red<br />

8. Urea"<br />

Another way is to write out the methods in detail in either a paragraph form or listed. This way is<br />

not necessary for this type of paper, since this is lab report for the identification of an unknown<br />

bacterium and the methods are explained in detail in the lab manual. If there is a procedure that the<br />

instructor added or made changes to, or the student used another procedure not in the course lab<br />

manual, then it should be written out and referenced. See some of the examples of papers<br />

identifying an unknown from the web sited below.

RESULTS<br />

This is where the results are summarized. The method results should be in a table format (see<br />

examples below). This is also where the flow chart showing how you arrived at the answer is stated.<br />

A short paragraph explaining how the results are presented can be included.<br />

Example: Unknown G had the following morphology on a TSA plate: medium sized opaque<br />

cream colored colony. After determining that it was a Gram negative rod, an oxidase test was<br />

performed and it was inoculated into a BCP lactose tube and onto a TSA slant. Table I lists all of the<br />

biochemical tests, their purpose and results. The results are also shown in a flow chart form.<br />

Example: Table 1: Biochemical Test Results<br />

TEST PURPOSE REAGENTS OBSERVATIONS RESULTS<br />

Gram stain To determine the Gram<br />

reaction of the<br />

bacterium<br />

Oxidase test To determine the<br />

presence of<br />

cytochrome c<br />

BCP Lactose To determine the<br />

ability of a bacterium<br />

to ferment a specific<br />

carbohydrate<br />

Indole Test To determine the<br />

ability of an organism<br />

to split indole from<br />

tryptophane<br />

ETC.<br />

Crystal violet,<br />

Iodine, Alcohol,<br />

Safranin<br />

Oxidase paper Purple / black<br />

color change<br />

Pink rods Gram<br />

negative<br />

None Color change<br />

from purple to<br />

yellow<br />

Kovac's added to<br />

1 ml of tryptone<br />

broth<br />

Red Ring at top<br />

of broth<br />

rods<br />

Positive<br />

oxidase<br />

test<br />

lactose<br />

fermenter<br />

indole test<br />

Another type of table: Table 1: Physiological and Biochemical Results<br />

TEST REAGENTS OR TEMP OBSERVATIONS RESULTS <strong>IN</strong>TERPRETATIONS<br />

MEDIA<br />

Citrate Citrate slant 35<br />

(green)<br />

0 C Color changed from Positive Organism is able to<br />

green to blue<br />

utilize citrate as a<br />

carbon source<br />

Methyl MRVP 35<br />

Red<br />

0 C After adding Negative Organism is not able<br />

methyl red to one<br />

to produce large<br />

ml, color changed<br />

amounts of acid from<br />

from light yellow to<br />

a darker yellow<br />

glucose fermentation

FLOWCHART<br />

<strong>UNKNOWN</strong> G<br />

Gram stain<br />

Gram negative Rod<br />

Oxidase test (positive)<br />

Positive Negative<br />

Citrobacter freundii Proteus vulgaris<br />

Enterobacter aerogenes Proteus mirabilis<br />

Escherichia coli Serratia marcescens<br />

Klebsiella oxytoca Morganella morganii<br />

Klebsiella pneumoniae<br />

Pseudomonas aeruginosa<br />

Pseudomonas aureofaciens<br />

Indole test ( Positive)<br />

Escherichia coli Citrobacter freundii<br />

Klebsiella oxytoca Enterobacter aerogenes<br />

Citrate Test (negative)<br />

Klebsiella oxytoca Escherichia coli<br />

Motility Test (positive)<br />

Escherichia coli<br />

Methyl Red Test (positive)<br />

Unknown B - Escherichia coli

DISCUSSION / CONCLUSION<br />

This section interprets the meaning of the results. The following questions should be answered here:<br />

How did the test result lead to identification? Was it the correct identification? If not, why not. What<br />

problems were encountered? This is also where the background information on the organism<br />

(environment/pathogenicity) that was identified is mentioned.<br />

Example of a discussion: After several differential tests, it was concluded that unknown G was<br />

Escherichia coli. After performing the Gram stain to determine that the unknown was a Gram negative rod,<br />

the organism was grown on a TSA slant for use in inoculating the rest of the biochemical tests. All of the<br />

biochemical tests worked well except for the indole test. It gave a false negative result at first. This was<br />

determined since it was inconsistent with the rest of the result. The TA suggested that the test be repeated and<br />

it was repeated. The repeated test gave a positive result, consistent with the other data. Therefore it was<br />

concluded that the unknown was Escherichia coli. THIS C<strong>AN</strong> BE E<strong>LAB</strong>ORATED MORE FOR EACH<br />

TEST.<br />

E. coli is in the Enterobacteriaceae family. It is typically found in the human intestines, as well as<br />

other animals. It can cause disease in the right host. THE REST OF THIS <strong>IN</strong>FORMATION SHOULD BE<br />

RESEARCHED FROM THE TEXTBOOK, <strong>IN</strong>TERNET OR OTHER <strong>MICROBIOLOGY</strong> RESOURCES.<br />

REFERENCES:<br />

Note: the minimum number of references is three, the lab manual, textbook and atlas. More can be<br />

used. Correct reference format must be used. References should be numbered and the number<br />

added to the report when necessary. See example 1 under materials and methods. Spelling of the<br />

authors of the references must be correct.<br />

Example:<br />

1. De Mers, Marlene. Fundamentals of Microbiology Laboratory Manual, 4 th edition. San<br />

Diego: Montezuma publishing, 2004<br />

2. Tortora, Funk, Case, etc........<br />

3. Leboffe, Michael, etc.........<br />

4. http://oregonstate.edu/dept/microbiology/writingmanual/WIC_WritingManual.pdf<br />

SOME ADDITIONAL PO<strong>IN</strong>TS:<br />

1. Attach both the working worksheet and the final worksheet<br />

2. Attach the unknown identification table that was used for identifying the unknown with the<br />

marks used to help narrow down results.<br />

3. Number the pages<br />

4. Use a spell checker<br />

5. Check out the following book from the library: "Successful Lab Reports" a manual for science<br />

students by Christopher S. Lobban and Maria Schefter.<br />

6. Use the comments from the instructor for the first report to write the second report even better.<br />

7. Check out the following PDF file on the web:<br />

http://oregonstate.edu/dept/microbiology/writingmanual/WIC_WritingManual.pdf

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<strong>HOW</strong> <strong>TO</strong> <strong>WRITE</strong> <strong>AN</strong> <strong>UNKNOWN</strong> <strong>LAB</strong> <strong>REPORT</strong> <strong>IN</strong> <strong>MICROBIOLOGY</strong> GENERAL Unknown reports in microbiology are written in scientific format. Scientific writing is written differently from other types of writing. The results of the exercise or experiment are what are being showcased, not the writing. The purpose of scientific writing is not to entertain, but to inform. The writing should be simple and easy to understand. There is a specific style that must be followed when writing scientific reports. Scientific writing is typically written in the passive voice. The pronouns "I", "We" and "They" are not typically used.. For example, instead of writing "I used a TSA agar plate to isolate my unknown," it is customary to write, "A trypticase soy agar (TSA) plate was used to isolate the unknown." It is also customary to write in the past tense for most of the report. This includes the introduction, the summary, the description of the materials and methods and the results. The present tense is reserved for the conclusions about the results. See the examples given below. Some other general rules that should be followed are: Microbial nomenclature: The name of the bacterium should written and spelled correctly. The name should be italicized or underlined. Italicized is preferred. For example, Staphylococcus aureus. The genus is capitalized but the species is not. After the full genus name is given in the paper, it can be written as S. aureus, but still italicized. This is as long as there in no other genera in the paper that starts with the same letter. PARTS <strong>TO</strong> THE <strong>UNKNOWN</strong> <strong>LAB</strong> <strong>REPORT</strong> (Note: Other than the title page, the pages of the report must be numbered) TITLE PAGE There should always be a title page and should include the following information: EXAMPLE OF TITLE PAGE Title should be centered and at the top or in the middle of the page <strong>UNKNOWN</strong> <strong>LAB</strong> <strong>REPORT</strong> # 1 This information should be centered and at the bottom part of the title page: <strong>UNKNOWN</strong> LETTER (OR NUMBER) YOUR NAME DATE (the due date) <strong>LAB</strong> <strong>IN</strong>STRUC<strong>TO</strong>R'S NAME COURSE NAME SEMESTER / YEAR SECTION NUMBER

  • Page 2 and 3: INTRODUCTION This section introduce
  • Page 4 and 5: FLOWCHART UNKNOWN G Gram stain Gram

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  • Lab Protocols

Identifying an Unknown Staphs, Strep, or Enteric

Lab procedure, coagulase test, bacitracin/sxt sensitivity.

  • Salt Tolerance

Simmons Citrate Agar Slant

Eosin methylene blue (emb) agar, macconkey (mac) agar.

This lab should give you the background information and techniques you will need to successfully perform biochemical tests in order to identify unknown bacterial samples. The micro lab website, your textbook, the web and assorted books available in lab will be the reference materials necessary for you to successfully complete the next several weeks of lab work.

  • Each pair will receive one unknown organism to identify. You will conduct tests appropriate for your organism to determine genus and species identification.
  • Each pair may have to present information on the specific organism they identified including: test results, where that organism is part of the normal flora, when and where that organism becomes a pathogen, possible diseases the organism causes.
  • What does a positive test result look like?
  • What is the biochemical basis of the test?

View Flowchart »

Staphylococcus species are normal flora widespread over the body surface. They are also important pathogens. Some of the most common diseases caused by Staphylococcus species include: impetigo, toxic shock syndrome, bacteremia, endocarditis, folliculitis furuncle (boils), and osteomyelitis (bone abscesses). Many species of Staphylococcus have the ability to form biofilms which can then colonize structures such as medical catheters, stents, heart valves, prostheses, shunts, and valves.

The clinically significant species are generally separated into coagulase-positive staphs (S. aureus) and coagulase-negative (CoNS) staphs (S. epidermidis, S. haemolyticus, and S. saprophyticus).

Many members of the Streptococcus genus are normal flora to the mouth, nose, and throat. The genus Streptococcus is a complex group causing a wide range of diseases such as: rheumatic fever, impetigo, pharyngitis, laryngitis, toxic shock syndrome, scarlet fever, and endocarditis. Streptococci are often classified based on hemolysis which can be seen by their reaction on blood agar. Alpha hemolytic species produce alpha-hemolysin which reduces hemoglobin (red) to methemoglobin (green) causing a brownish or greenish zone around the colony. Beta hemolytic species produce a hemolysin that forms a clear zone around the colony, indicating complete lysis of red blood cells. Gamma hemolytic species are non-hemolytic, having no apparent effect on red blood cells.

The Gram negative enterics are important both as natural flora in the intestinal tract and as pathogens of disease in the gastrointestinal tract and other sites. Four main families with numerous genera and species comprise the Gram negative enteric: Enterobacteriacea, Pseuodmonadaceae, Vibrionaceae, and Camplyobacteraceae. You will only be working with organisms from the first two families.

We have included the basic procedure for doing many common biochemical tests below. You will find more specific procedures for specific biochemical test on the following pages. More complete information on selective & differential media can be obtained by consulting the Difco manuals in lab. You will need to look up the individual test for a more detailed description, including the biochemical basis of each test.

Biochemical Tests for Staphylococcus Organisms

Table 1: Brief Description of Biochemical Tests for Staphylococcus Organisms.

Table 2: Probable Results for Staphylococcus Organisms

Hemolysis - Blood Agar

Intended use.

Blood agar is used to support the growth of fastidious organisms and to determine the type of hemolysis (destruction of red blood cell walls) an organism produces.

Blood agar is a rich medium that has been supplemented with fresh 5-10% blood. The hemolytic response can be dependent upon the type of blood. Sheep blood is commonly used, but some organisms require rabbit or bovine blood.

Test Procedure

  • Optional: Do your last streak with a needle and poke into the agar. This usually gives clear, reliable zones of beta hemolysis and is especially important to see the effects of streptolysin O which is oxygen labile. See page 84 of the Difco/BBL Manual.
  • The plate will be a brownish red color after 48hours.

You can differentiate four types of hemolysis by the appearance of the agar.

  • Beta hemolysis is indicated by a clear colorless zone surrounding the colonies. There has been total lysis of the red blood cells.
  • Alpha hemolysis is indicated by a small zone of greenish to brownish discoloration of the media. This is caused by the reduction of hemoglobin to methemoglobin and its subsequent diffusion into the surrounding medium.
  • Alpha prime hemolysis is indicated by a zone of complete hemolysis, surrounded by a zone of partial hemolysis, a pink halo. This pattern can be easier to see if you scrape off the colony.
  • Gamma hemolysis is indicated by no change in the media.

Limitations

  • The patterns of hemolysis can vary with the incubation atmosphere and the type of blood in the media.
  • Some Staph organisms will only show hemolysis after they have been refrigerated following incubation.

Differentiates Staphylococcus aureus from other Staphylococcus species.

The coagulase test detects the presence of free and bound staphylcoagulase. This enzyme is excreted extracellularly by human strains of Staph. aureus . The mechanism of action is unknown.

  • Thaw a tube of 0.5 mL rabbit plasma.
  • Inoculate a loop-full of organism into the tube. Chose a well isolated colony.
  • Ideally you should incubate the tube at 35°C for 4 hours checking every 30 minutes for clot formation. We incubate them overnight and put them in the refrigerator until the next lab period with comparable results.
  • Check for clot formation.
  • Dispose of the tube in the biohazard container.

The formation of a clot in the bottom of the tube is considered a positive result. The clot will not move as you tilt the tube. Unclotted plasma will flow in the tube.

  • Methicillin resistant Staph. aureus have reduced clumping factor.
  • Do not shake or agitate the tube as this could break up the clot.
  • Some staphylococci strains produce fibrolysin after prolonged incubation at 35°C that can break up the clot resulting in false negative. Incubate the tube overnight at room temperature if you do not get a clot in 4 hours.
  • Some other rarely encountered staph species are also coagulase positive by the tube method.

Biochemical Tests for Streptococcus Organisms

Table 3: Brief Description of Biochemical Tests for Streptococcus Organisms.

Table 4: Probable Results for Streptococcus Organisms

Bacitracin differential disks are used to presumptively identify Group A, beta-hemolytic streptococci from other beta-hemolytic streptococci. The combination of SXT sensitivity increases the accuracy of the results.

Bacitracin is an antibiotic isolated from Bacillus subtilis. It inhibits cell wall synthesis mainly through inhibiting the biosynthesis of peptidoglycan. SXT inhibits folate metabolism which interferes with bacterial DNA synthesis. Group A, beta-hemolytic streptococci are more sensitive to bacitracin than other beta-hemolytic streptococci.

The standard protocol has been modified for our lab.

  • Adjust the turbidity to 0.5 McFarland standard.
  • Use the procedure outlined in antimicrobial susceptibility testing to swab the entire plate to obtain confluent growth.
  • Save the other section for the optochin disk.
  • Invert the plates and incubate them for 18-24 hours at 35°C in 5-10% CO2.
  • Incubate another 24 hours if the results are negative.
  • Any zone of inhibition around the disk is considered sensitive (S).
  • No zone of inhibition with growth up to the disk is considered resistance (R).

This table is from MacFaddin, Biochemical Tests for Identification of Medical Bacteria.

  • Only beta-hemolytic streptococci should be tested.
  • While this test is accurate it is not highly specific. Other biochemical or serological tests are required for accurate identification.
  • The growth should be confluent. Too light of a growth could cause some non-group A streptococci to appear susceptible to bacitracin.

Salt Tolerance Broth

Salt tolerance broth is intended to differentiate non-beta-hemolytic strains of streptococci.

Principle of Use

Brain Heart Infusion (BHI) broth is supplemented with 6.5% sodium chloride and bromcresol purple as a pH indicator. The indicator is included to make reading the test results easier. The broth also includes dextrose. The fermentation of dextrose (glucose) results in the production of acid. This changes the pH of the media causing the media to turn from purple to yellow.

  • It is important to lightly inoculate the tube otherwise you may get a false positive.
  • Loosen the cap and incubate aerobically for 24 hours at 37°C.
  • Continue incubation up to 72 hours if you get a negative result at 24 hours.

A positive reaction is indicated by obvious turbidity in the media with or without a color change. A negative result is indicated by no growth after 72 hours. Enterococcus spp. typically changes the media color within 24 hours.

  • Many staphylococci can grow in media containing 10% salt. Mannitol salt agar has 7.5% salt.
  • Salt tolerance media was intended to differentiate catalase negative gram-positive cocci. Be sure to perform a catalase test before you proceed with the salt tolerance broth test.
  • Other species of catalase negative gram-positive organisms can grow in this media.

Biochemical Tests for Enteric Organisms

Table 5: Brief Description of Biochemical Tests for Enteric Organisms.

Simmons Citrate Agar Image

Used for the differentiation and identification of Enterobacteriaceae on the basis of citrate utilization, citrate being the sole carbon source.

Colonies capable of utilizing citrate as a carbon source produce a local increase in pH, changing the color of the medium from green to blue. Only citrate positive organisms will grow on this medium.

  • Inoculate the organism directly onto the surface of a Citrate slant.
  • Incubate aerobically at 35-37°C.
  • Examine for growth and color change after 18-24 hours of incubation.

Interpretations

Good growth with the medium color turning blue indicative of Enterobacter aerogenes and Salmonella choleraesuis .

Eosin Methylene Blue Agar Image

A differential plating medium for the detection & isolation of the gram-negative enteric bacteria.

  • To aid in the differentiation of lactose fermenting bacteria from non-lactose fermenting bacteria.
  • To aid in the differentiation of Enterobacter aerogenes and Escherichia coli .
  • Inoculate the organism directly onto the surface of an EMB agar plate and streak for isolation.
  • Incubate inoculated plate aerobically at 37°C.
  • Examine for growth after 18-24 hours of incubation.
  • Coliforms that utilize the lactose and/or sucrose are blue/black with a greenish metallic sheen. Indicative of Escherichia coli .
  • Coliforms such as Klebsiella pneumoniae have mucoid colonies that may be purple and/or exhibit a green metallic sheen.
  • Good to excellent, colorless colonies indicative of Proteus vulgaris, Salmonella choleraesuiss, and Shigella spp.

MacConkey Agar Image

  • A differential plating medium recommended for use in the isolation and differentiation of lactose-fermenting organisms from lactose non-fermenting gram negative enteric bacteria.
  • Selectivity of the medium is due to the presence of crystal violet and bile salts which markedly to completely inhibit the growth of gram positive organisms.
  • Organisms capable of fermenting lactose produce a localized pH drop which, followed by the absorption of neutral red, imparts a red/pink/purple color to the colony. A zone of precipitated bile may also be present due to this localized drop in pH.
  • Organisms that do not ferment lactose remain colorless and translucent. They are easily detected by transmitted light and appear as colorless colonies against a red background.
  • Inoculate the organism directly onto the surface of a MacConkey agar plate and streak for isolation.
  • Incubate inoculated plate aerobically at 35-37°C.

Interpretation

  • Good to excellent growth, red/pink/purple colonies with bile precipitate indicative of Escherichia coli .
  • Good to excellent growth, red/pink/purple colonies without bile precipitate indicative of Enterobacter aerogenes .
  • Good to excellent, colorless colonies without bile precipitate indicative of Proteus vulgaris, Salmonella typhimurium, and Shigella spp.

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  • J Microbiol Biol Educ
  • v.22(1); 2021

Implementing a Virtual Midterm to Identify Unknown Bacteria in a Microbiology Lab Course †

Associated data.

Identification of unknown bacteria is an integral module in most introductory Microbiology laboratory courses. This laboratory activity typically involves identifying bacteria based on Gram staining for morphology and gram reaction followed by studying their biochemical characteristics. When instruction moved to online mode earlier this year due to the pandemic, we faced the challenge of moving this hands-on, skill-intensive laboratory activity to remote mode. Here we describe the modular approach we designed to implement the midterm practical of bacterial identification remotely while trying to keep the online format as close to the in-person format as possible in a multi-section synchronous laboratory course. This virtual module was implemented successfully in the summer and fall 2020 semesters.

INTRODUCTION

Queensborough Community College (QCC) is an open-enrollment urban community college located in Queens, New York. The Microbiology laboratory class format in QCC’s Department of Biological Sciences and Geology is similar to contemporary course syllabi at many 2- and 4-year schools ( 1 , 2 ). As in most Microbiology lab courses, practical exams are administered to assess students’ abilities to perform the techniques and skills they learned in the laboratory and apply their theoretical knowledge to practical laboratory situations. This became challenging with the transition to virtual instruction as the pandemic hit the world.

Identification of unknown bacteria is an integral module in most introductory Microbiology laboratory courses that typically involves identifying bacteria based on the morphology, Gram reaction, and biochemical characteristics of bacteria. Most of the literature regarding virtual microbiology instruction is focused on the use of online simulations, used either to enhance learning in an in-person lab or to replace in-person lab exercises ( 3 – 5 ). A web-based virtual bacterial identification process ( 6 ) also has been described. Here we describe how we transitioned the in-person Microbiology lab practical of identification of unknown bacteria to a fully virtual format using the resources that we already had in place for our in-person lab course.

As in most Microbiology lab courses, the first 4 weeks, i.e., Labs 1 to 4, cover an introduction to microscopy, microorganisms, growth media, aseptic technique, and Gram staining ( 1 ). The midterm practical was designed as an open book activity spanning Labs 5 through 8 (see Appendix 4 for a lab schedule outline). Grading was scaffolded, and a final lab report was required where the students described the entire identification process ( Appendix 3 ). Though virtual, our classes met synchronously at scheduled times, so the midterm practical exam was conducted synchronously. However, with little modification, this practical could be conducted asynchronously; if using a platform such as Blackboard, specific elements of the practical exam could still be presented as time-restricted activities (e.g., Gram stain analysis) on Blackboard asynchronously.

Virtual practical exam

To ensure the investigative format of the practical, we wanted to retain the same general format for the virtual lab practical (see Appendix 1 for a table summarizing the in-person practical format) and tried to maintain the “feel” of the physical lab practical exam as much as possible.

Students were provided with two images of their unknown Gram-stained bacteria. The images were displayed in the “Course Documents” folder of the course Blackboard site for 15 minutes. Students were asked to note the morphology and Gram reaction of their “Unknown A and B” and submit their answers in the “Assignments” section of Blackboard within 15 minutes. The time limit was enforced to assess students’ visual memory skills and conceptual knowledge about bacterial morphology and variations in Gram reaction. The Gram stain results were graded and returned to the students by Lab 6. During Lab 5, students also learned about the biochemical tests (lab schedule, Appendix 4 ) that would later be used to identify their unknowns.

No changes to this process were necessary for the virtual format. Students constructed a dichotomous key and provided a list of selected media for the tests they chose to “perform” based on their dichotomous key. The dichotomous key/media selection was graded and returned to students by Lab 7.

If students were doing a physical lab practical, obtaining accurate results would require that they know how to inoculate the media for their selected biochemical tests, and what procedures needed to be done post-inoculation prior to analyzing their results. Instead of performing inoculations in this lab, students were asked to submit an assignment describing the pre- and post-inoculation procedures that they would normally do for the in-person practical ( Appendix 2 ). This was graded and returned to the students by Lab 8.

Results of the biochemical tests were provided to students as images in Blackboard, which they analyzed. They then filled out the same form used for in-person labs ( Appendix 2 ).

Students wrote a lab report describing the entire process of bacterial identification ( Appendix 2 ).

We implemented the above virtual midterm practical exam during the summer and fall 2020 semesters. Although it worked well for the most part, there are challenges inherent to online learning that are not an issue with in-person classes. In the virtual lab practical, students missed the valuable experience of handling and culturing bacteria aseptically and performing Gram staining; videos of procedures (which we used extensively) do not adequately make up for this loss. In addition, we could not assess students’ technical prowess with the microscope or their ability to perform aseptic technique, grow bacteria aseptically, correctly prepare bacterial smears, and perform a Gram stain.

We came up with some solutions to conquer the ongoing issue of technical difficulties. When presenting pictures to students through Blackboard, it was prudent to have a backup plan (e.g., PDFs that can be emailed to students), ready just in case a student’s Internet connection fails or Blackboard is disrupted. We also found it was especially important to provide the students with clear instructions early on in the process about how the virtual practical would be conducted. It helped relieve some of their anxiety about online learning and made everything run more smoothly for both instructors and students.

Implementing our virtual practical exam required a significant time investment by faculty and the course coordinators. To facilitate the process, we organized Gram stain and metabolic test results into separate PowerPoint files for each bacterium. This helped the faculty to provide students with individualized results. We created a Blackboard course site for students enrolled in this multi-section course where all the resources, such as images and videos covered in the lab course, were available to students asynchronously ( Appendix 3 ). Students’ feedback told us that this go-to resource site was extremely useful. We also have a faculty-only Blackboard site where faculty teaching the course exchange ideas and resources useful for transition to virtual instruction.

Although implementation of our virtual practical exam was done out of necessity, this approach could make online instruction more accessible (e.g., to deployed members of the military; to those with physical disabilities that may prevent them from participating in a physical lab). In these special circumstances, these individuals could take part (either synchronously or asynchronously) in the theoretical aspects of a practical exam. Though no virtual experience can completely replace an in-person hands-on laboratory experience, this midterm module attempted to provide students with an experience that closely resembled our in-person practical exam.

SUPPLEMENTARY MATERIALS

Appendix 1: schedule and format for in-person practical exam, appendix 2: practical forms submitted by students, appendix 3: grading for practical exam, appendix 4: microbiology laboratory outline, appendix 5: resources used for the virtual midterm experience, acknowledgments.

The authors have no conflicts of interest to declare.

† Supplemental materials available at http://asmscience.org/jmbe

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