In the Kirby-Bauer disk diffusion test used to guide antimicrobial therapy, what does the clear zone around an antibiotic disk (zone of inhibition) indicate?

• A. The antibiotic diffuses poorly through agar, so the drug is ineffective regardless of zone size.
• B. The organism is producing a bacteriocin that kills nearby cells, creating a clear zone unrelated to the antibiotic.
• C. The organism is inhibited by that antibiotic at the tested concentration, and the zone diameter can be interpreted as susceptible/intermediate/resistant using standardized criteria.
• D. The organism metabolizes and grows best in the presence of that antibiotic, indicating resistance.

Answer: (C)

Verified step by step guidance

1. Understand the purpose of the Kirby-Bauer disk diffusion test: it is used to determine the susceptibility of bacteria to specific antibiotics by observing bacterial growth around antibiotic-impregnated disks placed on an agar plate.
2. Recognize that the antibiotic diffuses from the disk into the agar, creating a concentration gradient where the highest concentration is near the disk and decreases with distance.
3. Identify that the clear zone around the antibiotic disk, called the zone of inhibition, represents an area where bacterial growth has been prevented due to the antibiotic's effect.
4. Interpret the size of the zone of inhibition by comparing its diameter to standardized charts that classify the bacteria as susceptible, intermediate, or resistant to the antibiotic at the tested concentration.
5. Conclude that a clear zone indicates the organism is inhibited by the antibiotic, and the zone size helps guide antimicrobial therapy decisions.