In a bacitracin differentiation test for Group A beta-hemolytic streptococci, disks should contain how many units?

The bacitracin differentiation test is utilized primarily for identifying Group A beta-hemolytic streptococci, specifically Streptococcus pyogenes, from other beta-hemolytic streptococci. The correct concentration of bacitracin in the disk used for the test is crucial for effective differentiation.

In this test, disks that contain between 0.02 and 0.04 units of bacitracin are effective in demonstrating susceptibility of Group A streptococci and are therefore the standard used in clinical laboratories. The selection of this specific unit range allows for optimal inhibition of growth of the bacteria, confirming their identity as Group A streptococci. A concentration lower than this range may not provide sufficient inhibition, while a concentration higher than this could result in too much inhibition, making interpretation difficult.

Understanding the appropriate concentration of bacitracin in the testing disks is essential for accurate diagnosis and treatment decisions.