Taxo A discs are for the presumptive identification of group A beta-hemolytic streptococci based on susceptibility to a low level of bacitracin. Discs are intended for use with pure cultures. Taxo A discs are not for use directly with clinical specimens or other sources containing mixed flora. The organism to be presumptively identified must first be isolated as separate colonies by streaking the specimen onto appropriate culture media; e.g., Trypticase Soy Agar with 5% Sheep Blood (TSA II). However, the BBLGroup A Selective Strep Agar with 5% Sheep Blood (ssA) plate, which may be used for the primary isolation of group A streptococci from throat specimens, was designed specifically for use with the Taxo A disc.
Taxo A discs are impregnated with a low level of bacitracin. According to the work of Maxted, of Levinson and Frank, and others, the group A streptococci may be differentiated from the other Lancefield groups of hemolytic streptococci by the formation of a zone of inhibition around the disc.
Group A beta-hemolytic streptococci are sensitive to small amounts of bacitracin, while betahemolytic streptococci of other serologic groups are more resistant. The Taxo A disc on a blood agar plate can be used for presumptive identification of group A beta-hemolytic streptococci after overnight incubation.
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