Microbiology Lab Practical Practice Exam

Which statement best describes the relationship between zone size and bacterial susceptibility in disk diffusion?

Larger zone indicates greater susceptibility Larger zone indicates greater resistance Zone size indicates nothing about susceptibility Zone size correlates with growth rate

Correct Answer:

Larger zone indicates greater susceptibility

Explanation:

Disk diffusion tests antibiotic susceptibility by placing a drug disk on an agar plate inoculated with bacteria and measuring the zone where growth is inhibited around the disk. A larger zone means the bacteria are more susceptible because the antibiotic diffuses into the surrounding agar and remains at inhibitory concentrations over a wider area, blocking growth farther from the disk. This zone diameter correlates with the MIC—the lower the MIC (higher susceptibility), the larger the zone you see. A small or absent zone signals reduced susceptibility or resistance under the test conditions. The zone size reflects how effectively the drug inhibits growth, not the bacteria’s inherent growth rate. So, a larger zone indicates greater susceptibility.

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