In the Kirby-Bauer disk diffusion test, what does a clear zone around an antimicrobial disk on an agar plate indicate?

• A. The agar plate was contaminated during inoculation.
• B. The microorganism is resistant to the antimicrobial agent.
• C. The antimicrobial agent has evaporated from the disk.
• D. The microorganism is sensitive to the antimicrobial agent.

Answer: (D)

Verified step by step guidance

1. Understand the purpose of the Kirby-Bauer disk diffusion test: it is used to determine the sensitivity or resistance of a microorganism to specific antimicrobial agents by observing the effect of the agent on bacterial growth.
2. Recognize that the antimicrobial disk contains a specific concentration of an antibiotic or antimicrobial agent that diffuses into the agar medium surrounding the disk.
3. Identify that a clear zone, called the zone of inhibition, forms around the disk where bacterial growth is prevented due to the effectiveness of the antimicrobial agent.
4. Interpret the presence of a clear zone as an indication that the microorganism is sensitive (susceptible) to the antimicrobial agent, meaning the agent inhibits or kills the bacteria in that area.
5. Contrast this with no clear zone or growth up to the disk edge, which would indicate resistance, and understand that contamination or evaporation would not produce a clear, defined zone of inhibition.