In antibiotic susceptibility testing (AST), if the growth medium is too thick, what is the most likely result?

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Multiple Choice

In antibiotic susceptibility testing (AST), if the growth medium is too thick, what is the most likely result?

Diffusion of the antibiotic through the agar is what determines whether inhibitory concentrations reach the bacteria around the disk. When the growth medium is too thick, the diffusion path is longer and the antibiotic has a harder time reaching effective levels at the bacterial surface. That limited penetration makes the test read as if the organism is not inhibited by the drug, producing a false interpretation of resistance. Standardized methods use a specific agar thickness to ensure zone sizes accurately reflect true susceptibility. While slower diffusion can contribute to smaller zones, the fundamental effect of a too-thick medium is that the antibiotic cannot reach inhibitory concentrations, leading to a false resistant result.

Diffusion of the antibiotic through the agar is what determines whether inhibitory concentrations reach the bacteria around the disk. When the growth medium is too thick, the diffusion path is longer and the antibiotic has a harder time reaching effective levels at the bacterial surface. That limited penetration makes the test read as if the organism is not inhibited by the drug, producing a false interpretation of resistance. Standardized methods use a specific agar thickness to ensure zone sizes accurately reflect true susceptibility. While slower diffusion can contribute to smaller zones, the fundamental effect of a too-thick medium is that the antibiotic cannot reach inhibitory concentrations, leading to a false resistant result.

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