Serological differentiation of strains of group B streptococci by the soft-agar technique

Soft-agar technique was applied for serological determination of strains of group B streptococci. With representative type strains, their colonial morphology was converted from diffuse- to compact-type growth only by the addition of homologous rabbit anti-serum in the medium and no conversion of colonial morphology was observed by heterologous rabbit antisera. Twenty seven out of 30 fresh isolates obtained from human clinical specimens showed diffuse-type growth in soft-agar medium and were subjected to this examination. Twenty-one strains reacted with a single antiserum, 5 strains showed reactivities to two different antisera although reaction to an antiserum were significantly higher than those of the other antiserum and no reaction was observed with a strain. Twenty single and 5 major serotypes determined by this technique were coincided with those differentiated by Lancefield's precipitin method. From these experimental results, soft-agar technique was regarded being available for serological typing of strains of group B streptococci.