Which of the following statements about the Kirby-Bauer disk diffusion assay is correct?

• A. It cannot distinguish between bacteriostatic and bactericidal agents.
• B. It determines the minimum inhibitory concentration (MIC) of antibiotics.
• C. It is used to identify bacterial species based on colony morphology.
• D. It directly measures whether an agent is bactericidal.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the purpose of the Kirby-Bauer disk diffusion assay. This test is designed to evaluate the effectiveness of antibiotics against bacteria by measuring the zone of inhibition around antibiotic-impregnated disks placed on an agar plate inoculated with the test bacteria.
2. Step 2: Recognize that the Kirby-Bauer assay measures the susceptibility of bacteria to antibiotics by observing the size of the clear zone where bacterial growth is inhibited, but it does not provide a direct measurement of whether the antibiotic kills the bacteria (bactericidal) or merely inhibits their growth (bacteriostatic).
3. Step 3: Know that the minimum inhibitory concentration (MIC) is determined by other methods, such as broth dilution or E-test strips, not by the Kirby-Bauer disk diffusion assay.
4. Step 4: Understand that the Kirby-Bauer assay is not used to identify bacterial species based on colony morphology; identification requires other microbiological techniques such as biochemical tests or molecular methods.
5. Step 5: Conclude that the correct statement is that the Kirby-Bauer disk diffusion assay cannot distinguish between bacteriostatic and bactericidal agents because it only measures the inhibition zone size, not the killing effect.