The role of type-IIIStreptococcus agalactiae extracellular type-specific antigen in mouse virulence enhancement

The adult mouse model was employed to examine the mechanism of virulence enhancement by the extracellular type-specific antigen (ETSA) of type-IIIStreptococcus agalactiae. Intraperitoneal injection of ETSA along with the infecting strain lowered the LD₅₀ value forS. agalactiae and allowed for a more rapid proliferation of the organisms as opposed to that caused by the presence of a nonspecific dextran. The ETSA did not hinder recruitment of peritoneal exudate cells (PEC) into the peritoneal cavity as has been shown with capsular antigens of other bacteria. Chemiluminescent responses of mouse resident PEC demonstrated that these cells could render a respiratory burst when exposed to type-IIIS. agalactiae in the absence of complement and/or type-specific antibody. If the mouse PEC were exposed to ETSA before phagocytosis was attempted, however, ingestion of the type-III group-B streptococci was reduced. ETSA did not affect the viability of the PEC as determined by trypan blue exclusion. These results indicate that the ETSA can affect mouse PEC in an unidentified manner, thus rendering them less capable of ingesting and/or kiling type-IIIS. agalactiae.