Question 1

Which of the following best describes the role of mannitol salt agar in microbiology labs?

Accepted Answer

It is a selective medium that inhibits Gram-negative bacteria and differentiates bacteria based on mannitol fermentation.

Question 2

A microbiologist streaks Staphylococcus aureus and Staphylococcus epidermidis on mannitol salt agar. After incubation, S. aureus colonies are yellow, while S. epidermidis colonies remain pink. What does this result indicate?

Accepted Answer

S. aureus ferments mannitol, producing acid that turns the indicator yellow, while S. epidermidis does not ferment mannitol.

Question 3

Which ingredient in MacConkey agar acts as the selective agent, and what is its function?

Accepted Answer

Bile salts and crystal violet; they inhibit the growth of Gram-positive bacteria.

Question 4

A student inoculates MacConkey agar with Escherichia coli and Salmonella enterica. After incubation, E. coli colonies are pink, while S. enterica colonies are colorless. What is the most accurate interpretation?

Accepted Answer

E. coli ferments lactose, producing acid that turns the colonies pink; S. enterica does not ferment lactose.

Question 5

Which formula correctly represents the hydrolysis of starch by amylase?

Accepted Answer

$$\mathrm{(C_6H_{10}O_5$)_n + nH_2O \rightarrow nC_6H_{12}O_6$}$$

Question 6

In a sugar fermentation test, a yellow color and gas bubble in the Durham tube indicate which of the following?

Accepted Answer

Acid and gas production due to fermentation of the sugar substrate.

Question 7

A blood agar plate shows clear zones around colonies. What type of hemolysis is this, and what does it indicate?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells.

Question 8

Which of the following is the correct interpretation of a positive urease test?

Accepted Answer

The medium turns pink due to ammonia production, increasing the pH.

Question 9

A microbiologist tests a bacterial isolate on citrate agar. After incubation, the medium turns blue. What does this result indicate?

Accepted Answer

The bacterium can utilize citrate as its sole carbon source, producing alkaline products.

Question 10

Which of the following best describes the function of the oxidase test in bacterial identification?

Accepted Answer

It detects the presence of cytochrome c oxidase, distinguishing between oxidase-positive and oxidase-negative bacteria.

Question 11

A motility test medium contains tetrazolium as an indicator. What color change would indicate bacterial motility?

Accepted Answer

Red color diffusing away from the stab line, indicating motile bacteria.

Question 12

Which of the following equations represents the reaction catalyzed by catalase?

Accepted Answer

$$\mathrm{2H_2O_2 \rightarrow 2H_2O + O_2$}$$

Question 13

A patient’s wound culture grows Streptococcus pyogenes. Which test result would help confirm this identification?

Accepted Answer

Beta hemolysis on blood agar and sensitivity to bacitracin.

Question 14

Which statement best describes the difference between selective and differential media?

Accepted Answer

Selective media inhibit the growth of certain microbes, while differential media distinguish microbes based on biochemical reactions.

Question 15

Which of the following is a correct interpretation of a positive coagulase test?

Accepted Answer

Clot formation in plasma, indicating the presence of Staphylococcus aureus.

Question 16

A microbiologist observes green colonies with black centers on Hektoen enteric agar. What does this result suggest?

Accepted Answer

The organism is likely Salmonella, which produces H2S.

Question 17

Which of the following best describes the action of penicillin?

Accepted Answer

It inhibits bacterial cell wall synthesis by blocking transpeptidase enzymes.

Question 18

Which statement best explains why Gram-negative bacteria are generally more resistant to antibiotics than Gram-positive bacteria?

Accepted Answer

Gram-negative bacteria have an outer membrane that limits antibiotic penetration.

Question 19

Which of the following is a correct match between a fungal structure and its function?

Accepted Answer

Hyphae: filamentous structures that absorb nutrients.

Microbiology Lab Exam 2 Study Notes: Differential and Selective Media, Microbial Identification, and Eukaryotic Microbes

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