What effect does excessive moisture on a Mueller-Hinton plate have during the Kirby-Bauer susceptibility test?

During the Kirby-Bauer susceptibility test, excessive moisture on a Mueller-Hinton plate can influence the outcome of the test by causing a smaller zone of growth around the antibiotic disc. When there is too much moisture, it can lead to increased diffusion of the antibiotic, which may result in an uneven concentration gradient. This diffusion can enhance the antibacterial effect in some areas of the plate while diminishing it in others, potentially leading to a narrower zone of inhibition or misinterpretation of resistance or susceptibility. Therefore, maintaining appropriate moisture levels is crucial for obtaining accurate results in this standardized test, making smaller zones of growth a logical outcome when excess moisture is present.