Question 1

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

Accepted Answer

It inhibits the growth of unwanted bacteria while allowing only the desired bacteria to grow.

Question 2

A microbiologist streaks a sample on MacConkey agar and observes pink colonies. What does this indicate about the bacteria?

Accepted Answer

The bacteria are Gram negative and ferment lactose, producing acid.

Question 3

Which of the following is a differential medium and what is its primary purpose?

Accepted Answer

MacConkey Agar; to distinguish between lactose fermenters and non-fermenters.

Question 4

During the oxidase test, a microbiologist observes a colorless result after 30 seconds. What does this indicate?

Accepted Answer

The organism is negative for cytochrome oxidase.

Question 5

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

Accepted Answer

It inhibits Gram positive bacteria and differentiates lactose fermenters by color change.

Question 6

A Kligler Iron Agar (KIA) slant shows a yellow butt and a red slant after incubation. What does this result indicate about the organism's metabolism?

Accepted Answer

The organism ferments glucose only.

Question 7

Which of the following statements about antibiotic susceptibility testing using the Kirby-Bauer method is correct?

Accepted Answer

The diameter of the zone of inhibition is compared to a standard table to determine susceptibility.

Question 8

Which of the following is a common mechanism by which bacteria acquire antibiotic resistance?

Accepted Answer

Horizontal gene transfer, such as conjugation, transformation, or transduction.

Question 9

A patient is infected with a Gram negative rod that is oxidase positive and grows as colorless colonies on MacConkey agar. Which organism is most likely responsible?

Accepted Answer

Pseudomonas aeruginosa

Question 10

Why is it important to use aseptic technique when inoculating culture media in the laboratory?

Accepted Answer

To ensure that only the desired organism is transferred and to prevent contamination.

Question 11

Which of the following best describes a bacteriostatic antibiotic?

Accepted Answer

It slows down the growth and replication of bacteria.

Question 12

A microbiologist adds ferric chloride to a phenylalanine deaminase agar (PDA) slant and observes a green color. What does this indicate?

Accepted Answer

The organism is capable of deaminating phenylalanine.

Question 13

Which of the following is a reason for the widespread development of antibiotic resistance in bacteria?

Accepted Answer

Unnecessary prescription and misuse of antibiotics.

Question 14

Which equation would you use to calculate the diameter of the zone of inhibition in a Kirby-Bauer test if you measure the radius as $$r$$?

Accepted Answer

$$d = 2r$$

Question 15

A Gram negative rod is tested on KIA and produces a black precipitate in the butt of the tube. What does this result indicate?

Accepted Answer

The organism produces hydrogen sulfide ($$H_2S).$$

Question 16

Which of the following best explains why fastidious organisms require enriched media?

Accepted Answer

They require additional growth factors not present in standard media.

Question 17

A patient’s bacterial isolate is resistant to penicillin but sensitive to ciprofloxacin. What is the best explanation for this observation?

Accepted Answer

The bacterium produces beta-lactamase, which inactivates penicillin.

Question 18

Which of the following is NOT a member of the Enterobacteriaceae family?

Accepted Answer

Pseudomonas aeruginosa

Question 19

Why is it important to label all plates and tubes with organism name, group number, and test type in a microbiology lab?

Accepted Answer

To ensure accurate identification and prevent mix-ups during analysis.

Question 20

Which of the following best describes the main difference between selective and differential media?

Accepted Answer

Selective media inhibits unwanted bacteria, while differential media distinguishes between organisms based on metabolic differences.

Identification and Laboratory Study of Enterobacteriaceae and Gram-Negative Rods

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