Name
Western Governors University
D311 Microbiology Lab Report: Identification Tests & Results
Prof. Name
Date
The primary aim of Task 2 in this microbiology laboratory exercise is to precisely identify an unknown bacterial species using a restricted set of diagnostic tools and laboratory tests. This simulation reflects real-world laboratory limitations, where resources and time are finite. The exercise promotes critical thinking and analytical skills by requiring students to interpret laboratory data, make evidence-based decisions about further testing, and ultimately confirm the bacterial species. This task is designed to enhance problem-solving capabilities and decision-making skills essential for understanding microbial identification within complex biological systems.
You are tasked with identifying an unknown bacterial species by strategically applying a series of laboratory tests. Initially, select the organism group based on the first letter of your last name to narrow the scope. Given typical constraints in laboratory environments, it is impractical to perform every available test on the sample. Instead, you will use a decision tree to guide your choices, progressively eliminating options until a final identification is made.
Your lab report should emphasize the Gram stain result and two additional tests selected through the decision tree based on prior results. Each test selection must be clearly justified in your report. Upon completion of identification, analyze the Kirby-Bauer antibiotic sensitivity test results to recommend an effective antibiotic treatment.
Before starting, review Section 4, Lesson 2, which covers the Gram staining technique and the decision tree that supports your diagnostic process.
Save your final report as a Word document with the title format: D311 Task 2 Lab Report [Last Name]. Ensure the unknown organism number is clearly indicated for prompt evaluation.
Start by performing a Gram stain to classify the bacterium as either Gram-positive or Gram-negative. This classification is based on the color observed due to differences in cell wall structure—purple for Gram-positive and pink/red for Gram-negative bacteria. Also, observe the shape of the cells to determine if the bacterium is rod-shaped (bacillus) or spherical (coccus).
| Question | Answer |
|---|---|
| Is the organism Gram-positive or Gram-negative? | Identify based on stain color and cell wall characteristics: purple indicates Gram-positive, pink/red indicates Gram-negative. |
| What is the morphology of the organism? | Specify the cell shape observed under the microscope: bacilli (rods) or cocci (spheres). |
Following the Gram stain results, use the decision tree to select the next appropriate test. Decisions should be made purposefully, relying on prior test outcomes. The decision tree includes tests such as endospore staining, catalase activity, glucose fermentation, citrate utilization, blood agar hemolysis, capsule staining, motility, lactose fermentation, Voges-Proskauer, coagulase, and Kirby-Bauer antibiotic susceptibility testing.
This test detects the presence of resistant spores formed by certain bacteria. Spores appear as distinct colored structures that stand out from the vegetative cells.
| Question | Response |
|---|---|
| What colors are observed in the stain? | Describe the staining colors, such as green spores against pink vegetative cells, and their significance. |
| Does the organism form spores? | Confirm whether spores are present based on staining results. |
| What is the next test chosen? | Justify the next test choice using spore presence and decision tree guidance. |
Capsule staining reveals whether the bacterium produces a polysaccharide capsule, typically visualized as a clear halo surrounding the cells.
| Question | Response |
|---|---|
| Is there visible space around cells? | Note if a clear halo indicative of a capsule is visible around bacterial cells. |
| Does the organism form a capsule? | Confirm capsule presence based on staining evidence. |
| What is the next test chosen? | Explain the selection of the subsequent test according to capsule results and decision tree. |
This test assesses bacterial growth on blood agar and characterizes hemolysis as alpha (partial), beta (complete), or gamma (none).
| Question | Response |
|---|---|
| Is there bacterial growth? What is the color? | Describe colony growth and coloration on blood agar plates. |
| What type of hemolysis is observed? | Identify hemolysis type with descriptive observations (alpha, beta, or gamma). |
| What is the next test chosen? | Provide reasoning for the next test based on hemolytic activity. |
This test determines if the bacterium can use citrate as its sole carbon source, indicated by a color change in the medium.
| Question | Response |
|---|---|
| What is the color of the medium? | Record the color after incubation to assess citrate metabolism. |
| Are results positive or negative? | Interpret if the organism can metabolize citrate. |
| What is the next test chosen? | Justify test selection based on citrate utilization results. |
This test identifies the presence of acetoin, a metabolic intermediate, through a color change in the reagent tube.
| Question | Response |
|---|---|
| What color is the reagent tube? | Note the color after reagent addition. |
| Are results positive or negative? | Interpret the presence or absence of acetoin. |
| What is the next test chosen? | Explain test choice grounded on metabolic activity results. |
This test evaluates whether the bacterium ferments glucose, which may result in acid production and sometimes gas.
| Question | Response |
|---|---|
| What color is the tube? Is gas produced? | Describe color changes and presence of gas bubbles in the test tube. |
| Are results positive or negative? | Conclude on glucose fermentation capability. |
| What is the next test chosen? | Support test selection based on fermentation results. |
Like the glucose test, this assay tests lactose fermentation and potential gas production.
| Question | Response |
|---|---|
| What color is the tube? Is gas produced? | Record tube color and note any gas production. |
| Are results positive or negative? | Determine lactose metabolism status. |
| What is the next test chosen? | Justify the subsequent test based on lactose fermentation outcomes. |
Catalase activity is indicated by bubbling when hydrogen peroxide is added, reflecting the breakdown of peroxide into water and oxygen.
| Question | Response |
|---|---|
| Were bubbles observed? | Yes or no. |
| Are results positive or negative? | Explain the significance of catalase presence or absence. |
| What is the next test chosen? | Provide rationale for the next step informed by catalase results. |
This assay detects coagulase enzyme production, which causes plasma clotting.
| Question | Response |
|---|---|
| Was the tube liquid or clumped? | Describe the physical state of the plasma after incubation. |
| Are results positive or negative? | Interpret coagulase presence based on clotting. |
| What is the next test chosen? | Justify the following test decision based on coagulase results. |
Motility testing determines if bacterial growth spreads diffusely (motile) or remains localized (non-motile).
| Question | Response |
|---|---|
| Was the growth diffuse or defined? | Describe the growth pattern in the medium. |
| Are results positive or negative? | Conclude whether the organism is motile. |
| What is the next test chosen? | Explain the choice for the next test based on motility. |
This test evaluates the susceptibility of the organism to various antibiotics. The results classify the organism as resistant, intermediate, or susceptible, guiding effective treatment choices.
| Question | Response |
|---|---|
| What is the resistance profile to each antibiotic? | List each antibiotic tested and the organism’s susceptibility or resistance. |
| Which antibiotics are unsuitable for treatment? | Identify antibiotics to avoid based on resistance patterns. |
| Which antibiotic is optimal for treatment? | Recommend the best antibiotic according to susceptibility data. |
Accurate identification of unknown bacteria in clinical microbiology necessitates a systematic and evidence-based approach. Starting with the Gram stain to broadly classify bacteria, the use of a decision tree directs efficient test selection, conserving laboratory resources while enhancing diagnostic precision. Subsequent confirmatory tests narrow down the possibilities to achieve definitive identification. Finally, the Kirby-Bauer antibiotic sensitivity test informs the selection of effective antimicrobial therapy. This stepwise methodology integrates practical laboratory techniques with analytical reasoning, ensuring precise microbial identification and optimized clinical management.
Cappuccino, J. G., & Welsh, C. T. (2017). Microbiology: A Laboratory Manual (11th ed.). Pearson.
Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock Biology of Microorganisms (15th ed.). Pearson.
Western Governors University. (n.d.). D311 Task 2: Identifying Unknown Microorganisms in Lab 9. Microbiology with Lab: A Fundamental Approach.
