Which of the following statements best describes the result of a Kirby-Bauer diffusion test for a bacterium that is resistant to an antibiotic?

• A. The bacterium will exhibit a large zone of inhibition.
• B. The bacterium will be completely killed in the test.
• C. The bacterium will exhibit a small or no zone of inhibition.
• D. The bacterium will change color in the presence of the antibiotic.

Answer: (C)

Verified step by step guidance

1. Understand the purpose of the Kirby-Bauer diffusion test: it is used to determine the susceptibility of bacteria to antibiotics by measuring the zone of inhibition around an antibiotic disk on an agar plate.
2. Recall that the zone of inhibition is the clear area around the antibiotic disk where bacterial growth is prevented due to the antibiotic's effect.
3. Recognize that if a bacterium is resistant to an antibiotic, it means the antibiotic does not effectively inhibit or kill the bacteria.
4. Therefore, in the case of resistance, the bacterium will show a small or no zone of inhibition because the bacteria continue to grow close to the antibiotic disk.
5. Conclude that the correct description of the result for a resistant bacterium in the Kirby-Bauer test is a small or no zone of inhibition, indicating the antibiotic is ineffective against that bacterium.