Differentiation of group A from other beta hemolytic streptococci with bacitracin.

In the study of upper respiratory infections, nose and throat cultures for beta hemolytic streptococci3 are essential. Since group A streptococci are implicated in human respiratory infections more than other groups, it is important that streptococci recovered on culture be identified as belonging to group A or not. The finding of streptococci which belong to groups other than A in the nose or throat usually is interpreted as being of little or no clinical or epidemiological importance. The only satisfactory means for differentiation of group A streptococci has been a serological method. The supply of necessary antisera has been limited, and the method requires considerable skill and technical experience if any high degree of accuracy is to be obtained. In epidemiological studies in large populations during epidemics of streptococcal infections, the number of streptococci isolated may be large and the serological identification burdensome and costly. Maxted (1953) reported a method for differentiating streptococci belonging to group A from other beta hemolytic streptococci by their sensitivity to bacitracin. Filter paper squares were soaked in the antibiotic and rapidly dried. The strain of streptococcus to be tested was streaked on the surface of a blood agar plate and a bacitracin impregnated filter paper square placed on the