Bacitracin and coagglutination for grouping of beta-hemolytic streptococci

Bacitracin may be used 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 1,161 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 have correlated exactly with results of conventional grouping by a precipitin method.