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Group A Strep A Disk/SXT

In follow up to the previous question, the upper image again illustrates the colonies recovered from the blood culture bottle. The colonies are small, transluscent, gray-yellow, and surrounded by a wide zone of beta hemolysis. The size of the colonies compared to the zones of hemolysis suggests a group A Streptococcus.
The susceptibility to bacitracin (zone of inhibition around the "A" disk in the lower image) is virtually diagnostic of a group A Streptococcus.
The absence of a zone of inhibition around the sulfamethoxazole/trimethoprim (SXT) disk indicates resitance to sulfamethoxazole/trimethoprim. SXT resistance is also shared by group B streptococci, which are, however, resistant to bacitracin. The resistance to SXT is used for the primary recovery of groups A and B streptococci from specimens with mixed culture. Their resistance allows them to selectively grow out from contaminating bacteria that are inhibited by this antibiotic.