Which scenario would cause the disk diffusion result to falsely indicate resistance?

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Multiple Choice

Which scenario would cause the disk diffusion result to falsely indicate resistance?

In disk diffusion, the size of the zone of inhibition depends on how much active antibiotic diffuses from the disk into the surrounding agar. When an antibiotic disk is expired, its potency can decline, so the effective concentration around the disk is lower than intended. This reduces or eliminates the inhibition zone, making the test read as resistant even if the organism would be susceptible with a fresh disk. Other scenarios don’t specifically produce a false resistant reading in the same way: a too light inoculum tends to enlarge zones (risking a false susceptible result), delaying incubation affects growth timing and may cause inconsistent results but not a systematic false resistance, and forgetting to add the disk removes the antibiotic entirely rather than reflecting degraded potency.

In disk diffusion, the size of the zone of inhibition depends on how much active antibiotic diffuses from the disk into the surrounding agar. When an antibiotic disk is expired, its potency can decline, so the effective concentration around the disk is lower than intended. This reduces or eliminates the inhibition zone, making the test read as resistant even if the organism would be susceptible with a fresh disk.

Other scenarios don’t specifically produce a false resistant reading in the same way: a too light inoculum tends to enlarge zones (risking a false susceptible result), delaying incubation affects growth timing and may cause inconsistent results but not a systematic false resistance, and forgetting to add the disk removes the antibiotic entirely rather than reflecting degraded potency.

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