Demonstration and characterization of buoyant-density subpopulations of group B Streptococcus type III

Group B Streptococcus (GBS) type III (18 strains) was investigated for the presence of buoyant-density subpopulations; 10 strains originated from invasive isolates causing early-onset septicemia, and 8 were colonizing isolates from pregnant women. By repeated processing in hypotonic density gradients of Percoll, high- (HD) and low-density (LD) subpopulations were recovered from all strains. Synthesis of type-specific polysaccharide (TPS) was increased in the invasive isolates and their respective LD variants compared with the colonizing strains and their LD subpopulations. Production of capsular TPS correlated directly with synthesis of soluble TPS; there was an inverse exponential relationship between soluble TPS production and buoyant density. LD variants were more resistant to phagocytic killing by human polymorphonuclear leukocytes (PMNL) than were the original strains and HD variants. In a luminol-enhanced chemoluminescence assay, LD variants elicited a weak, slow response. In contrast, the HD subpopulations evoked a prompt, strong respiratory burst in the PMNL.