D311 Microbiology Lab Report: Identification Tests & Results

Grace J.
November 12, 2025November 12, 2025
BSN, D311, Western Governors University

Student Name

Western Governors University

D311 Microbiology with Lab: A Fundamental Approach

Prof. Name

Date

Microbiology Lab First Identification Test: Gram Stain

What did the Gram stain reveal about the bacterial specimen?

Microscopic analysis of the Gram-stained bacterial sample revealed that the cells appeared purple in color and were arranged in spherical clusters. These structural features characterize the bacteria as cocci. The retention of the purple color after the staining process confirms that the microorganism is Gram-positive.

This result occurs due to significant structural variations in bacterial cell walls. Gram-positive bacteria possess a thick peptidoglycan layer, which comprises approximately 90% of the total cell wall. This dense peptidoglycan structure effectively traps the crystal violet–iodine complex during the decolorization step of Gram staining, resulting in the bacteria appearing blue or violet under microscopic observation. In contrast, Gram-negative bacteria, with their thinner peptidoglycan layer and outer membrane, lose the crystal violet stain and appear pink or red after counterstaining with safranin (WGU-CGP-OEX).

Second Identification Test: Catalase Test

Why is the catalase test performed, and what does it indicate about the bacteria?

Following the Gram-stain results, the catalase test was performed to further differentiate the Gram-positive cocci. This biochemical test determines whether the organism produces the enzyme catalase, which decomposes hydrogen peroxide (H₂O₂) into water and oxygen. Hydrogen peroxide is a toxic byproduct of aerobic metabolism that can damage essential cellular components, including DNA, proteins, and lipids.

To perform the test, a small amount of bacterial culture is mixed with hydrogen peroxide on a microscope slide or in a test tube. The rapid appearance of oxygen bubbles indicates a positive result, confirming catalase production. Conversely, no bubble formation indicates a negative result, suggesting that the organism does not produce catalase (WGU-CGP-OEX).

What were the catalase test results for the unknown sample, and what do they imply?

In Unknown Lab Manual No. 6, the catalase test was negative, as evidenced by the absence of bubble formation after hydrogen peroxide application. This finding implies that the bacterium lacks the catalase enzyme and cannot neutralize reactive oxygen species effectively. As a result, the organism is more vulnerable to oxidative stress and tends to thrive in anaerobic or microaerophilic environments. This characteristic narrows down the identification to catalase-negative Gram-positive cocci, commonly found in the genera Enterococcus or Streptococcus (WGU-CGP-OEX).

Third Identification Test: Blood Agar

What does the blood agar test assess in bacterial identification?

The blood agar test is designed to evaluate an organism’s ability to lyse red blood cells (RBCs) through the production of hemolysins, which are extracellular enzymes. The blood agar medium typically contains 5% sheep blood and nutrient agar, providing an enriched environment suitable for differentiating species based on hemolytic activity. After incubation, bacterial colonies are observed for color and transparency changes surrounding their growth (WGU-CGP-OEX).

How are the hemolysis types distinguished?

Hemolysis Type	Description	Visual Indicator on Agar Plate
Alpha-hemolysis	Partial degradation of hemoglobin with production of biliverdin	Greenish halo around colonies
Beta-hemolysis	Complete lysis of red blood cells	Clear, transparent zones around colonies
Gamma-hemolysis	No red blood cell lysis	No color change or clearing in the medium

What were the blood agar results for the unknown specimen?

The Unknown Lab No. 6 bacterial culture demonstrated gamma-hemolysis, characterized by no visible change in the agar medium surrounding the colonies. This result indicates that the organism does not produce hemolysins and therefore cannot degrade red blood cells. Gamma-hemolytic bacteria are typically nonpathogenic or opportunistic pathogens, consistent with members of the Enterococcus genus (WGU-CGP-OEX).

Organism Identification

Based on the tests performed, what is the identity of the bacterial organism?

Combining the results of the Gram stain (Gram-positive cocci), catalase test (negative), and blood agar test (gamma-hemolysis), and applying the Decision Tree method for bacterial classification, the unknown organism from Lab Manual No. 6 was identified as Enterococcus faecalis.

Enterococcus faecalis is a facultative anaerobic bacterium commonly found in the human gastrointestinal tract. While typically nonpathogenic in healthy individuals, it can act as an opportunistic pathogen, contributing to infections such as urinary tract infections (UTIs), bacteremia, and endocarditis in immunocompromised patients.

Kirby-Bauer Diffusion Test

What is the purpose of the Kirby-Bauer test?

The Kirby-Bauer disk diffusion test is a standardized method used to determine the antibiotic susceptibility of bacterial isolates. By assessing the bacteria’s resistance or sensitivity to various antibiotics, clinicians can make informed decisions about effective treatment options. The size of the growth inhibition zone around each antibiotic disk is compared to standardized interpretation charts to categorize bacterial responses as susceptible, intermediate, or resistant (WGU-CGP-OEX).

How is the Kirby-Bauer test performed and interpreted?

During the procedure, antibiotic-impregnated disks are placed on the surface of an agar plate uniformly inoculated with the bacterial culture. As the antibiotic diffuses through the agar, it forms a concentration gradient. Areas where the antibiotic concentration inhibits bacterial growth appear as clear zones—known as zones of inhibition.

The diameter of these zones is measured in millimeters and interpreted according to the Clinical and Laboratory Standards Institute (CLSI) guidelines. The interpretation helps determine the organism’s resistance profile, guiding therapeutic decisions.

Antibiotic	Zone of Inhibition Result	Interpretation
Amoxicillin	Very small or absent zone	Resistant
Doxycycline	Intermediate-sized zone	Intermediate sensitivity
Doxycycline*	Large inhibition zone	Susceptible

Note: Doxycycline was tested twice, showing different inhibition sizes, possibly due to concentration variation, diffusion rate, or procedural inconsistencies.

What antibiotic is recommended for treating Enterococcus faecalis?

Based on the Kirby-Bauer diffusion test results, doxycycline exhibited the largest inhibition zone, suggesting it is the most effective antibiotic against the Enterococcus faecalis isolate from Unknown Lab No. 6. Therefore, doxycycline is the recommended treatment option, although clinical correlation and patient-specific factors should always be considered.