If you forget to add an antibiotic disk and then place it 30 minutes after incubation starts, what happens?

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The test relies on diffusion of the antibiotic from the disk into a uniformly swabbed lawn of bacteria, so the zone of inhibition reflects how well the bacteria are inhibited by the drug. If the disk is placed 30 minutes after incubation starts, the bacteria have already begun to grow and form a denser lawn. That thicker growth presents more resistance to diffusion, and the antibiotic has less time to diffuse outward through that layer. The result is a smaller or even absent zone of inhibition, which makes the organism appear resistant when it might actually be susceptible if the disk had been placed promptly. This is why delaying disk placement can yield a falsely resistant interpretation.

In contrast, a delay would not produce a larger zone, and it would not produce a false susceptible reading—the issue is the reduced zone size from late placement, not increased diffusion.