Question 1

Which of the following best describes the purpose of selective media?

Accepted Answer

To suppress unwanted microbes and encourage the growth of desired microbes

Question 2

A microbiologist wants to isolate Staphylococcus aureus from a mixed sample. Which medium should they use and why?

Accepted Answer

Mannitol Salt Agar, because it selects for halophiles and differentiates mannitol fermenters

Question 3

Which component in Mannitol Salt Agar acts as the selective agent, and what is its concentration?

Accepted Answer

NaCl at 7.5%

Question 4

On a blood agar plate, a colony is surrounded by a clear zone. What type of hemolysis is this, and what enzyme is responsible?

Accepted Answer

Beta hemolysis; hemolysin

Question 5

A sample is streaked on MacConkey agar. After incubation, pink colonies are observed. What does this indicate about the organism?

Accepted Answer

It is Gram-negative and ferments lactose

Question 6

Which of the following best explains why Hektoen Enteric (HE) agar is useful for isolating Salmonella and Shigella?

Accepted Answer

It differentiates based on lactose, sucrose, and salicin fermentation and sulfur reduction

Question 7

A microbiologist observes black colonies on Hektoen Enteric agar. What does this result indicate?

Accepted Answer

The organism is a Gram-negative non-fermenter that reduces sulfur

Question 8

Which of the following equations represents the fermentation of glucose to lactic acid?

Accepted Answer

$$\mathrm{C_6H_{12}O_6$} \rightarrow 2\ \mathrm{CH_3CH(OH)COOH}$$

Question 9

Which of the following is NOT a function of enrichment media?

Accepted Answer

Differentiating colonies based on metabolic properties

Question 10

A clinical sample is streaked on Columbia CNA agar with 5% sheep blood. Only Gram-positive cocci grow, and one colony shows a greenish zone around it. What is the most likely explanation?

Accepted Answer

The colony is Gram-positive and alpha-hemolytic

Question 11

Which indicator is used in Mannitol Salt Agar to detect fermentation, and what color change indicates a positive result?

Accepted Answer

Phenol red; red to yellow

Question 12

Why are bile salts and crystal violet added to MacConkey agar?

Accepted Answer

To inhibit Gram-positive bacteria and select for Gram-negative bacteria

Question 13

A microbiologist streaks a sample on Hektoen Enteric agar and observes blue-green colonies with no black centers. What is the most likely identity of the organism?

Accepted Answer

Gram-negative, non-fermenter, non-sulfur reducer (possible Shigella)

Question 14

Which of the following best describes the role of substrates in differential media?

Accepted Answer

They provide nutrients that allow differentiation based on metabolic activity

Question 15

A patient sample is streaked on MacConkey agar. After incubation, there is no growth. What does this result suggest?

Accepted Answer

The organism is Gram-positive

Question 16

Which of the following is a correct pairing of media type and its primary purpose?

Accepted Answer

Differential media – differentiation of colonies based on metabolic properties

Question 17

Which of the following best explains why MacConkey agar is both selective and differential?

Accepted Answer

It contains bile salts and crystal violet to select for Gram-negative bacteria and lactose plus neutral red to differentiate lactose fermenters

Question 18

Which of the following equations represents the fermentation of glucose to ethanol and carbon dioxide?

Accepted Answer

$$\mathrm{C_6H_{12}O_6$} \rightarrow 2\ \mathrm{C_2H_5OH} + 2\ \mathrm{CO_2$}$$

Question 19

A clinical microbiologist is testing a stool sample for fecal contamination using a medium that differentiates lactose fermenters. Which medium should be used and what result would indicate fecal coliforms?

Accepted Answer

MacConkey agar; pink colonies

Selective and Differential Media in Microbiology: Principles and Applications

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