Which group of β-hemolytic streptococci is identified using a bacitracin A disk?

The identification of β-hemolytic streptococci using a bacitracin A disk is specifically utilized for Group A streptococci, which is primarily represented by Streptococcus pyogenes. When a sample of the organism is inoculated on a blood agar plate and a bacitracin disk is placed on the surface, it inhibits the growth of Group A streptococci. This characteristic allows for a straightforward differentiation between Group A and other groups of streptococci.

Group B streptococci, typically Streptococcus agalactiae, and other groups such as C and D do not show sensitivity to bacitracin in the same way that Group A does. This differential sensitivity to bacitracin is a key diagnostic tool in determining the presence of Group A streptococci in clinical specimens, making it an essential aspect of microbiological testing in the lab.