Question 1

Which stain would you use to initially color all bacterial cells purple in the Gram stain procedure, and why do Gram-positive bacteria retain this color after decolorization?

Accepted Answer

Crystal violet; Gram-positive bacteria have a thick peptidoglycan cell wall that retains the dye.

Question 2

A microbiologist performs a catalase test by adding hydrogen peroxide to a bacterial colony. Immediate bubbling is observed. What is the correct interpretation and the chemical reaction involved?

Accepted Answer

The bacteria are catalase positive; $$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 3

Which result would you expect if you inoculate a Gram-negative, lactose-fermenting bacterium on MacConkey agar?

Accepted Answer

Growth with pink/red colonies due to acid production from lactose fermentation.

Question 4

A student observes green-stained structures inside pink/red cells after performing an endospore stain. What is the correct interpretation?

Accepted Answer

The green structures are endospores, and the pink/red are vegetative cells.

Question 5

Which selective medium would you use to isolate Staphylococcus aureus from a mixed sample, and what result would indicate its presence?

Accepted Answer

Mannitol Salt Agar; yellow media due to mannitol fermentation.

Question 6

A bacterium produces a dark purple color within 10 seconds when tested with oxidase reagent. What does this indicate about the bacterium?

Accepted Answer

It possesses cytochrome c oxidase and is likely Gram-negative.

Question 7

Which of the following best describes the principle behind the Kirby-Bauer disk diffusion test?

Accepted Answer

Antibiotic disks are placed on an agar plate, and the size of the clear zone around each disk is measured to determine bacterial sensitivity.

Question 8

A tube of Simmons citrate agar turns blue after incubation with a bacterial sample. What does this result indicate?

Accepted Answer

The organism can use citrate as its sole carbon source, producing alkaline byproducts.

Question 9

In an ELISA assay, what does a color change after substrate addition indicate?

Accepted Answer

The presence of a specific antigen or antibody in the sample.

Question 10

A bacterium is inoculated into phenylalanine agar, incubated, and then ferric chloride is added. The medium turns green. What enzyme is present and what reaction has occurred?

Accepted Answer

Phenylalanine deaminase; phenylalanine is deaminated to phenylpyruvic acid.

Question 11

Which of the following best explains why Gram-negative bacteria appear pink/red after Gram staining?

Accepted Answer

Their thin peptidoglycan layer cannot retain crystal violet after decolorization, so they absorb safranin.

Question 12

A microbiologist observes a clear zone around bacterial colonies on blood agar. What type of hemolysis is this and what does it indicate?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells.

Question 13

Which test would you use to differentiate Staphylococcus (catalase positive) from Streptococcus (catalase negative), and what result would you expect for Staphylococcus?

Accepted Answer

Catalase test; immediate bubbling after adding hydrogen peroxide.

Question 14

A bacterium is inoculated into urea broth and after incubation, the medium turns bright pink. What does this indicate about the bacterium?

Accepted Answer

It produces urease, which hydrolyzes urea to ammonia and carbon dioxide.

Question 15

Which of the following best describes the process and result of a positive bile esculin test?

Accepted Answer

Esculin is hydrolyzed in the presence of bile, producing a black complex in the medium.

Question 16

During a pGLO transformation experiment, what result would indicate that bacteria have successfully taken up the plasmid and expressed the GFP gene?

Accepted Answer

Colonies grow on ampicillin-containing media and glow green under UV light.

Question 17

A microbiologist observes purple coloration in a decarboxylase test tube after incubation. What does this indicate about the metabolic activity of the bacterium?

Accepted Answer

The bacterium can decarboxylate the amino acid, producing alkaline amines.

Question 18

Which medium would you use to selectively grow Gram-positive bacteria while inhibiting Gram-negative bacteria, and what are the key inhibitory agents?

Accepted Answer

Columbia CNA Agar; colistin and nalidixic acid.

Question 19

A patient sample is tested using an ELISA for a specific viral antigen. The well shows no color change after substrate addition. What is the correct interpretation?

Accepted Answer

The sample does not contain the viral antigen.

Microbiology Lab Techniques: Staining, Biochemical Tests, and Selective Media

Study Guide - Practice Questions

Study Guide - Flashcards