In the Kirby-Bauer disc diffusion susceptibility test, which variable is critical when testing Pseudomonas spp. against aminoglycosides?

In the Kirby-Bauer disc diffusion susceptibility test, which variable is critical when testing Pseudomonas spp. against aminoglycosides?

• A. Incubation temperature
• B. Duration of incubation
• C. Cation content of the media
• D. Depth of agar

Answer: (C)

The key idea is that the environment around the bacteria shapes how the antibiotic behaves in a diffusion test. For aminoglycosides tested against Pseudomonas, the level of cations in the agar—especially calcium and magnesium—significantly affects both how the drug diffuses and how readily the bacteria take it up. These divalent ions stabilize the outer membrane and can reduce drug entry, and they can also interact with the drug itself, lowering the amount that reaches its target. Because of this, the media’s cation content becomes the most critical variable to control for accurate and reproducible zone sizes. While incubation temperature, duration, and agar depth also influence results, they do not have the same specific impact on aminoglycoside activity in Pseudomonas as the media’s cation content.

The key idea is that the environment around the bacteria shapes how the antibiotic behaves in a diffusion test. For aminoglycosides tested against Pseudomonas, the level of cations in the agar—especially calcium and magnesium—significantly affects both how the drug diffuses and how readily the bacteria take it up. These divalent ions stabilize the outer membrane and can reduce drug entry, and they can also interact with the drug itself, lowering the amount that reaches its target. Because of this, the media’s cation content becomes the most critical variable to control for accurate and reproducible zone sizes. While incubation temperature, duration, and agar depth also influence results, they do not have the same specific impact on aminoglycoside activity in Pseudomonas as the media’s cation content.