Grouping of beta-haemolytic streptococci by using coagglutination, precipitation or bacitracin sensitivity

Abstract

This study was made to compare coagglutination to precipitin test in grouping beta-haemolytic streptococci from clinical specimens and to investigate the accuracy of the bacitracin test in identification of group A streptococci. Results of grouping 126 strains with coagglutination and precipitation were identical in all except two cases. These two 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 it was found that group B strains were the most frequently (41 per cent) isolated streptococci and even in isolates from pharyngeal swabs their proportion was 33 per cent. The accuracy of the bacitracin test in identification of group A streptococci was unsatisfactory as 26/62 (42 per cent) 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 incidence of false positive in the bacitracin test it is suggested that this test should be replaced by a more efficient method of serological grouping.