Question 1

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

Accepted Answer

It suppresses the growth of unwanted bacteria while encouraging the growth of desired organisms.

Question 2

A clinical microbiologist is attempting to isolate Staphylococcus aureus from a wound swab. Which medium would be most appropriate and why?

Accepted Answer

Mannitol Salt Agar, because it is selective for salt-tolerant bacteria and differential for mannitol fermentation.

Question 3

On a blood agar plate, a bacterial colony is surrounded by a clear, colorless zone. What type of hemolysis is this, and what does it indicate?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells.

Question 4

Which of the following best explains why Eosin-Methylene Blue (EMB) agar is used in water quality testing?

Accepted Answer

It selects for Gram-negative enterics and differentiates E. coli by a metallic green sheen, indicating fecal contamination.

Question 5

A stool sample is plated on Hektoen Enteric (HE) agar. Colonies appear green with black centers. What is the most likely organism, and what is the significance of the black center?

Accepted Answer

Salmonella; the black center indicates hydrogen sulfide ($$H_2S) $$production.

Question 6

Which component in MacConkey (MAC) agar is responsible for inhibiting the growth of Gram-positive bacteria?

Accepted Answer

Crystal violet and bile salts

Question 7

A urine sample is plated on MacConkey agar. After incubation, colonies appear bright fuchsia. What does this indicate about the organism?

Accepted Answer

It is a Gram-negative bacterium that ferments lactose, producing acid.

Question 8

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

Accepted Answer

It is a pH indicator that turns yellow when mannitol is fermented, indicating acid production.

Question 9

A microbiologist observes yellow colonies surrounded by a yellow zone on Mannitol Salt Agar after incubation. Which organism is most likely present, and what does this result indicate?

Accepted Answer

Staphylococcus aureus; it ferments mannitol, producing acid.

Question 10

Which of the following equations best represents the chemical reaction responsible for the black centers seen in Salmonella colonies on Hektoen Enteric agar?

Accepted Answer

$$\mathrm{H_2S} + \mathrm{Fe$$^{2+}$$} \rightarrow \mathrm{FeS} + 2\mathrm{H^+}$$

Question 11

A patient’s throat swab is plated on blood agar. After incubation, the colonies are surrounded by a greenish halo. What is the most likely type of hemolysis, and what is the underlying process?

Accepted Answer

Alpha hemolysis; partial reduction of hemoglobin to methemoglobin.

Question 12

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

Accepted Answer

It contains high NaCl to select for salt-tolerant bacteria and mannitol with phenol red to differentiate mannitol fermenters.

Question 13

A laboratory technician is testing a sample suspected of containing Shigella. Which medium would be most appropriate for isolating this organism, and what colony color would you expect?

Accepted Answer

Hektoen Enteric Agar; green colonies without black centers.

Question 14

Which of the following statements about Rose agar is correct?

Accepted Answer

It is selective for Gram-positive bacteria and contains blood to assess hemolytic activity.

Question 15

A sample is plated on EMB agar and produces colonies with a metallic green sheen. What does this result indicate?

Accepted Answer

The organism is a Gram-negative bacterium that ferments lactose with abundant acid production.

Question 16

Which pH indicator is used in MacConkey agar, and what color does it turn in the presence of acid from lactose fermentation?

Accepted Answer

Neutral red; bright fuchsia

Question 17

A microbiologist wants to differentiate between Enterobacter and Salmonella in a stool sample. Which medium and colony color would help in this differentiation?

Accepted Answer

Hektoen Enteric Agar; Enterobacter forms yellow colonies, Salmonella forms green with black centers.

Question 18

Which of the following best describes gamma hemolysis on blood agar?

Accepted Answer

No lysis of red blood cells, resulting in no change in the medium.

Question 19

A sample is plated on MacConkey agar and produces colorless colonies. What does this indicate about the organism?

Accepted Answer

It is a Gram-negative bacterium that does not ferment lactose.

Question 20

Which of the following best explains why blood agar is not considered a selective medium?

Accepted Answer

It allows the growth of a wide variety of organisms and is used to assess hemolytic activity.

Selective and Differential Media in Microbiology

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