Question 1

Which of the following best describes the main purpose of the Kirby-Bauer disk diffusion test?

Accepted Answer

To assess the effectiveness of multiple antibiotics against a specific bacterial organism.

Question 2

Why is it important to standardize the turbidity of bacterial cultures using the 0.5 McFarland standard before performing the Kirby-Bauer test?

Accepted Answer

To guarantee that the same number of bacteria are plated for each test, ensuring reliable and comparable results.

Question 3

A student measures the zone of inhibition around an antibiotic disk and finds it to be 18 mm. According to the interpretive chart, a zone of $$\geq 20 mm is $$considered susceptible. How should this result be interpreted?

Accepted Answer

The organism is resistant to the antibiotic.

Question 4

Which of the following is a critical reason for pouring Mueller-Hinton agar plates to a specific depth in the Kirby-Bauer test?

Accepted Answer

To standardize the rate of antibiotic diffusion through the agar.

Question 5

What is the significance of the size of the zone of inhibition in the Kirby-Bauer test?

Accepted Answer

It determines whether the bacteria are susceptible, intermediate, or resistant to the antibiotic.

Question 6

A laboratory technician accidentally uses a bacterial suspension with a turbidity much higher than the 0.5 McFarland standard. What is the most likely effect on the Kirby-Bauer test results?

Accepted Answer

Zones of inhibition will appear smaller than expected, possibly leading to false resistance.

Question 7

Which of the following best explains why the Kirby-Bauer test cannot distinguish between bactericidal and bacteriostatic antibiotics?

Accepted Answer

Both types of antibiotics can inhibit visible bacterial growth, resulting in a clear zone of inhibition.

Question 8

A student forgets to let the inoculated Mueller-Hinton plate dry before applying antibiotic disks. What is the most likely consequence?

Accepted Answer

The zones of inhibition may be irregular or difficult to interpret.

Question 9

Why is it necessary to use sterile technique when preparing the bacterial lawn for the Kirby-Bauer test?

Accepted Answer

To avoid contamination that could interfere with the interpretation of results.

Question 10

Which of the following steps is performed immediately after adjusting the bacterial suspension to match the 0.5 McFarland standard in the Kirby-Bauer test?

Accepted Answer

Use a sterile cotton applicator to prepare a bacterial lawn on the Mueller-Hinton plate.

Question 11

A clinical isolate of Escherichia coli is tested using the Kirby-Bauer method. The zone of inhibition for ampicillin is 10 mm, and the interpretive chart states that resistance is defined as $$\leq 13 mm. $$What is the correct interpretation?

Accepted Answer

The E. coli isolate is resistant to ampicillin.

Question 12

Which of the following best describes the role of interpretive charts in the Kirby-Bauer test?

Accepted Answer

They allow comparison of measured zones of inhibition to standardized criteria for susceptibility, intermediate, or resistance.

Question 13

If a zone of inhibition is present but smaller than the susceptible cutoff on the interpretive chart, what does this indicate?

Accepted Answer

The bacteria are resistant to the antibiotic.

Question 14

Which of the following is a limitation of the Kirby-Bauer disk diffusion test?

Accepted Answer

It cannot distinguish between bactericidal and bacteriostatic effects.

Question 15

A plate is inoculated with Staphylococcus epidermidis and incubated at $$37^\circC $$for 18 hours. After incubation, no zones of inhibition are observed around any antibiotic disks. What is the most likely explanation?

Accepted Answer

The organism is resistant to all antibiotics tested.

Question 16

Which equation best represents the relationship between the number of bacteria and the turbidity of a suspension as used in the McFarland standard?

Accepted Answer

$$1.5 	imes 10^8$ cells/mL = 0.5 McFarland standard

Question 17

Which of the following best explains why even resistant microbes may show a zone of inhibition in the Kirby-Bauer test?

Accepted Answer

High concentrations of antibiotic near the disk can inhibit growth, but this does not indicate true susceptibility.

Question 18

A student uses a ruler to measure the diameter of a zone of inhibition and records 22 mm. If the interpretive chart lists susceptible as $$\geq 18 mm$$, what is the correct interpretation?

Accepted Answer

The organism is susceptible to the antibiotic.

Question 19

Which of the following is the correct sequence of steps for performing the Kirby-Bauer test?

Accepted Answer

Standardize culture turbidity, prepare bacterial lawn, apply antibiotic disks, incubate plate.

Antimicrobial Susceptibility Testing: The Kirby-Bauer Disk Diffusion Method

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