What does the size of the zone of growth inhibition correlate with if the correct procedure has been used?

The size of the zone of growth inhibition in antimicrobial susceptibility testing correlates with the minimum inhibitory concentration (MIC) of the antimicrobial agent being tested. The MIC is the lowest concentration of an antibiotic that prevents the visible growth of a bacterium.

When an antibiotic is applied to a culture of bacteria, it diffuses into the surrounding agar medium. If the antibiotic effectively inhibits bacterial growth, it will create a clear area around the disk known as the zone of inhibition. The larger the zone of inhibition, the more susceptible the bacterium is to the antibiotic, indicating a lower MIC.

In contrast, other measures like the minimum lethal concentration (MLC), which indicates the concentration at which the bacteria are killed, or the minimum bactericidal concentration (MBC), which measures the lowest concentration that will kill a specific bacterium, do not directly relate to the size of the inhibition zone. They focus more on bacterial death rather than just inhibition of growth. Thus, the relationship between the zone of inhibition and MIC directly underscores why the proper interpretation of this zone is essential in determining the effective concentrations of antibiotics in practice.