In disk diffusion, which statement best describes the relationship between zone size and susceptibility?

In disk diffusion, which statement best describes the relationship between zone size and susceptibility?

• A. Larger zone indicates higher susceptibility
• B. Larger zone indicates higher resistance
• C. Zone size is unrelated to susceptibility
• D. Zone size only indicates growth rate

Answer: (A)

In disk diffusion, you place an antibiotic disk on an agar plate inoculated with the bacteria, and the drug diffuses outward. If the organism is susceptible, the antibiotic inhibits growth around the disk, creating a clear zone whose diameter grows larger as susceptibility increases. A larger zone means the bacteria are inhibited by lower concentrations of the drug, i.e., they are more susceptible. Conversely, a small or absent zone indicates resistance or reduced susceptibility. Zone size is interpreted against standardized breakpoints, though it can be influenced by factors like inoculum density and agar depth. The key idea is that a bigger inhibition zone reflects greater susceptibility, not the bacteria’s baseline growth rate.

In disk diffusion, you place an antibiotic disk on an agar plate inoculated with the bacteria, and the drug diffuses outward. If the organism is susceptible, the antibiotic inhibits growth around the disk, creating a clear zone whose diameter grows larger as susceptibility increases. A larger zone means the bacteria are inhibited by lower concentrations of the drug, i.e., they are more susceptible. Conversely, a small or absent zone indicates resistance or reduced susceptibility. Zone size is interpreted against standardized breakpoints, though it can be influenced by factors like inoculum density and agar depth. The key idea is that a bigger inhibition zone reflects greater susceptibility, not the bacteria’s baseline growth rate.