In the bacitracin disk test, what indicates a positive result?

In the bacitracin disk test, a positive result is indicated by a zone of inhibition greater than 10 mm surrounding the disk. This result demonstrates that the bacteria in question are sensitive to bacitracin, as the antibiotic prevents bacterial growth in its vicinity. When a clear zone of inhibition is observed, it indicates that the bacitracin has successfully inhibited the growth of the bacteria, thus confirming their susceptibility.

In laboratory practice, measuring the diameter of the zone of inhibition is a critical step for accurately assessing bacterial sensitivity. If the zone is greater than the standard threshold, it confirms that bacitracin is effective against the tested organism. This test is particularly useful in differentiating between species of Group A Streptococcus, which is bacitracin-sensitive, and other bacteria that are not.