Which streptococci would most likely show a positive reaction to bacitracin?

Group A streptococci, primarily Streptococcus pyogenes, are known to show a positive reaction to bacitracin. This bacterial group is characterized by its ability to be inhibited by bacitracin, making it a key differentiating test for identifying Group A streptococci in clinical microbiology. A positive bacitracin sensitivity test suggests the presence of S. pyogenes, which is clinically significant due to its association with various infections, such as strep throat, scarlet fever, and rheumatic fever.

In contrast, other groups such as Enterococcus faecalis, Group B streptococci (S. agalactiae), and α-Hemolytic streptococci, including Streptococcus pneumoniae and viridans group streptococci, do not typically exhibit sensitivity to bacitracin. Therefore, the positive bacitracin reaction is a hallmark for identifying Group A streptococci, providing valuable information in diagnostic microbiology.