Question 1

Which of the following best describes the primary function of selective media in microbiology?

Accepted Answer

To inhibit the growth of some organisms while encouraging the growth of others.

Question 2

A microbiologist wants to isolate Gram-negative bacteria from a fecal sample. Which medium should they use and why?

Accepted Answer

MacConkey Agar, because it contains bile salts and crystal violet that inhibit Gram-positive bacteria.

Question 3

On a Mannitol Salt Agar plate, a yellow zone appears around a colony. What does this indicate about the organism?

Accepted Answer

It is a halophilic Gram-positive bacterium that ferments mannitol.

Question 4

Which component in MacConkey Agar allows for differentiation between lactose fermenters and non-fermenters?

Accepted Answer

Lactose and the pH indicator neutral red

Question 5

A plate shows pink colonies on MacConkey Agar. What can you conclude about the organism?

Accepted Answer

It is a Gram-negative bacterium that ferments lactose.

Question 6

Which of the following equations best represents the reduction of sulfur to hydrogen sulfide gas, as detected on Hektoen Enteric Agar?

Accepted Answer

$$\mathrm{S$$^{2-}$$ + 2H^+ \rightarrow H_2S}$$

Question 7

A microbiologist observes black-centered colonies on Hektoen Enteric Agar. What does this indicate?

Accepted Answer

The organism reduces sulfur to hydrogen sulfide, forming FeS.

Question 8

Which of the following best explains why Columbia CNA Agar is selective for Gram-positive bacteria?

Accepted Answer

It contains antibiotics colistin and nalidixic acid that inhibit Gram-negative bacteria.

Question 9

A clinical sample is streaked on MacConkey Agar and shows no growth. What is the most likely explanation?

Accepted Answer

The organism is a Gram-positive bacterium inhibited by the medium.

Question 10

Which of the following is a differential component in Mannitol Salt Agar?

Accepted Answer

Phenol red

Question 11

A student streaks Staphylococcus aureus and Escherichia coli on Mannitol Salt Agar. After incubation, only S. aureus grows and turns the medium yellow. What can be concluded?

Accepted Answer

S. aureus is a halophile and ferments mannitol; E. coli is inhibited by high salt.

Question 12

Which of the following best describes the role of indicators in selective and differential media?

Accepted Answer

They make a desired or expected reaction visible, such as a color change.

Question 13

On Hektoen Enteric Agar, a colony appears yellow-pink with no black center. What does this indicate?

Accepted Answer

The organism is a Gram-negative fermenter that does not reduce sulfur.

Question 14

Which of the following combinations would you expect to see for Escherichia coli on MacConkey Agar?

Accepted Answer

Growth with pink colonies

Question 15

A microbiologist streaks a sample on Hektoen Enteric Agar and observes green colonies with black centers. Which organism is most likely present?

Accepted Answer

Salmonella species

Question 16

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

Accepted Answer

It contains bile salts and crystal violet to inhibit Gram-positives, and lactose plus neutral red to differentiate fermenters.

Question 17

Which of the following is the correct interpretation for a colony that grows on Columbia CNA Agar and causes a clear zone around the colony?

Accepted Answer

The organism is Gram-positive and beta-hemolytic.

Question 18

Which of the following best describes the purpose of sodium thiosulfate and ferric ammonium citrate in Hektoen Enteric Agar?

Accepted Answer

To detect sulfur reduction by forming a black precipitate.

Question 19

A clinical microbiologist is testing a wound swab on Mannitol Salt Agar. After incubation, there is growth but the medium remains red. What is the most likely explanation?

Accepted Answer

The organism is a halophile that does not ferment mannitol.

Question 20

Which of the following best explains why environmental microbiologists use selective media to detect coliforms in water samples?

Accepted Answer

Selective media inhibit non-coliforms and allow for differentiation based on lactose fermentation.

Selective and Differential Media in Microbiology: Principles and Applications

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