GROUPING OF BETA‐HAEMOLYTIC STREPTOCOCCI BY USING COAGGLUTINATION, PRECIPITATION OR BACITRACIN SENSITIVITY

Abstract

Coagglutination was compared to the precipitin test in grouping .beta.-hemolytic streptococci from clinical specimens and the accuracy of the bacitracin test in identification of group A streptococci was investigated. Results of grouping 126 strains with coagglutination and precipitation were identical in all except 2 cases. These 2 strains were nongroupable with precipitation but appeared as group B and C by coagglutination. When the distribution of group A, B, C and G streptococci in various clinical sources was investigated, group B strains were the most frequently (41%) isolated streptococci and even in isolates from pharyngeal swabs their proportion was 33%. The accuracy of the bacitracin test in identification of group A streptococci was unsatisfactory as 26/62 (42%) strains reported as group A by using this test were in fact group B, C or G streptococci. One of the reasons for this high number of false positives appeared to be the medium used for the preparation of the blood agar plates. In view of the frequent occurrence of non-A-streptococci in clinical specimens and high false positive incidence in the bacitracin test it is suggested that this test should be replaced by a more efficient method of serological grouping.