The upper image again illustrates the colonies recovered from the blood culture bottle. The colonies are small, translucent, gray-yellow, and surrounded by a broad 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 resistance to this antibiotic. Group B streptococci, which are resistant to bacitracin, also share SXT resistance. SXT resistance is used primarily to recover groups A and B streptococci from specimens with mixed culture. Their resistance allows them to grow from contaminating bacteria selectively inhibited by this antibiotic.
