In Kirby-Bauer testing, which factor directly affects diffusion of antibiotics through the agar?

In Kirby-Bauer testing, which factor directly affects diffusion of antibiotics through the agar?

• A. Incubation time
• B. Depth of agar
• C. pH of the medium
• D. Inoculum density

Answer: (B)

The main idea is how the gel’s depth controls how far antibiotics must diffuse in the Kirby-Bauer assay. In this test, antibiotic from the disk spreads into the surrounding agar to create a zone where bacteria don’t grow. The distance the antibiotic molecules must travel is set by how thick the agar layer is. A shallower layer shortens the diffusion path and lets the antibiotic spread farther, producing a larger inhibition zone; a deeper layer lengthens the path and reduces diffusion, giving a smaller zone. That direct relationship is why agar depth is the factor that most directly affects diffusion. Other factors influence the result in other ways but not the diffusion distance itself. Incubation time changes how much growth occurs and can enlarge or shrink the zones indirectly; pH can affect antibiotic activity; inoculum density changes the amount of bacteria present. None of these alter the physical distance the antibiotic must diffuse through the gel as directly as the depth of the agar.

The main idea is how the gel’s depth controls how far antibiotics must diffuse in the Kirby-Bauer assay. In this test, antibiotic from the disk spreads into the surrounding agar to create a zone where bacteria don’t grow. The distance the antibiotic molecules must travel is set by how thick the agar layer is. A shallower layer shortens the diffusion path and lets the antibiotic spread farther, producing a larger inhibition zone; a deeper layer lengthens the path and reduces diffusion, giving a smaller zone. That direct relationship is why agar depth is the factor that most directly affects diffusion.

Other factors influence the result in other ways but not the diffusion distance itself. Incubation time changes how much growth occurs and can enlarge or shrink the zones indirectly; pH can affect antibiotic activity; inoculum density changes the amount of bacteria present. None of these alter the physical distance the antibiotic must diffuse through the gel as directly as the depth of the agar.