Bacitracin and coagglutination for grouping of beta-hemolytic streptococci

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Abstract
Bacitracin may be use for presumptive differentiation of group A from other .beta.-hemolytic streptococci. Whatever criteria are used for the test, a small number of erroneous results will be obtained. With the 1161 strains studied in this laboratory, using a zone of 10 mm or greater as indicative of group A was better than using Maxted''s original criterion of any zone of inhibition for group A. Co-agglutination is a preferable alternative to bacitracin testing, providing a grouping result as quickly as the bacitracin test but with the advantage of giving a definite grouping result. With the 247 strains studied so far, coagglutination results correlated exactly with results of conventional grouping by a precipitin method.