Question 1

Which of the following best describes a selective medium?

Accepted Answer

A medium that contains agents to inhibit the growth of certain microorganisms while allowing others to grow.

Question 2

Why is mannitol salt agar (MSA) considered both selective and differential?

Accepted Answer

It selects for halotolerant bacteria and differentiates based on mannitol fermentation.

Question 3

A culture is streaked onto MacConkey agar. After incubation, no growth is observed. What is the most likely explanation?

Accepted Answer

The organism is Gram-positive and is inhibited by the bile salts and crystal violet.

Question 4

Which of the following components makes MacConkey agar selective?

Accepted Answer

Bile salts and crystal violet

Question 5

On Columbia CNA with 5% Sheep Blood Agar, a bacterial colony is surrounded by a clear, colorless zone. What type of hemolysis is this, and what does it indicate?

Accepted Answer

Beta (β) hemolysis; complete lysis of red blood cells

Question 6

Which genus is most commonly selected for by Mannitol Salt Agar (MSA), and what is the typical salt concentration in this medium?

Accepted Answer

Staphylococcus; 7.5% NaCl

Question 7

A sample is streaked onto Hektoen Enteric (HE) agar. After incubation, colonies appear black. What does this indicate about the organism?

Accepted Answer

It reduces sulfur to hydrogen sulfide ($$\mathrm{H_2S}$$), which reacts with ferric ammonium citrate to form a black precipitate.

Question 8

Which of the following best describes the function of phenol red in Mannitol Salt Agar?

Accepted Answer

It is a pH indicator that detects acid production from mannitol fermentation.

Question 9

Which two genera are primarily identified using Hektoen Enteric (HE) agar?

Accepted Answer

Salmonella and Shigella

Question 10

A Gram-negative rod is streaked onto MacConkey agar and forms pink colonies. What does this indicate about the organism?

Accepted Answer

It ferments lactose, producing acid that causes the pH indicator to turn pink.

Question 11

Which of the following best describes an enrichment culture?

Accepted Answer

A medium designed to favor the growth of a particular microorganism by providing specific nutrients or conditions.

Question 12

Which of the following is NOT a component that can make a medium differential?

Accepted Answer

Antibiotic

Question 13

A culture is streaked onto Columbia CNA with 5% Sheep Blood Agar and shows no growth. What is the most likely explanation?

Accepted Answer

The organism is Gram-negative and is inhibited by colistin and nalidixic acid.

Question 14

Which of the following best explains why blood agar is used in clinical microbiology laboratories? (Select the most comprehensive answer.)

Accepted Answer

To differentiate bacteria based on their ability to hemolyze red blood cells and to support the growth of fastidious organisms.

Question 15

Which of the following sugars is NOT present in Hektoen Enteric (HE) agar?

Accepted Answer

Mannitol

Question 16

What is the function of ferric ammonium citrate in Hektoen Enteric (HE) agar?

Accepted Answer

It reacts with $$\mathrm{H_2S} to $$form a black precipitate, indicating sulfur reduction.

Question 17

A patient sample is streaked onto MSA. After incubation, yellow colonies are observed. What does this result indicate?

Accepted Answer

The organism is a halophile and ferments mannitol, producing acid that turns the medium yellow.

Question 18

Which of the following best describes the role of bile salts in MacConkey agar?

Accepted Answer

They inhibit the growth of Gram-positive bacteria, making the medium selective for Gram-negative bacteria.

Question 19

Which of the following scenarios best demonstrates the use of an indicator in differential media?

Accepted Answer

Including phenol red in MSA to detect acid production from mannitol fermentation.

Question 20

Which of the following statements about enrichment culture is correct?

Accepted Answer

Enrichment culture is more similar to selective media because it favors the growth of specific organisms.

Question 21

A microbiologist wants to isolate Salmonella from a stool sample. Which medium should they use and what result would indicate possible Salmonella?

Accepted Answer

Hektoen Enteric Agar; blue-green colonies with black centers

Microbiology Study Guide: Selective, Differential, and Enrichment Media

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