Properties of group B streptococci with protein surface antigens X and R.

A total of 128 bovine and 134 human group B streptococci were serotyped by conventional methods. Among the bovine cultures, 60 (47%) had type antigen X, and among the human cultures, 53 (39%) had type antigen R. The occurrence of type antigens X and R was significantly related to the growth pattern of the bacteria in fluid media. Type X- and R-positive cultures and most of the nontypeable cultures predominantly formed long chains and grew as granular sediment with clear supernatant. In addition, group B streptococci with surface antigen X or R showed compact colony formation in soft agar and reacted positively in the salt aggregation test. These properties, possibly caused by the surface charges of the X- and R-positive cultures, might be related to bacterial pathogenicity.