Extracellular products of type IIIStreptococcus agalactiae and their relationship to virulence

Various human isolates of type III group B streptococci (GBS) orStreptococcus agalactiae could be divided into two distinct groups (high and low producers) on the basis of their in vitro production of extracellular type-specific antigen (ETSA). The high ETSA producers were shown to be significantly more virulent in mice than were the low producers. In an effort to examine the possibility that purified extracellular products (ETSA, neuraminidase, or protease_ had a significant effect on GBS virulence in the mouse model, mice received either 1.0 ml of organisms intraperitoneally (IP) or 1.0 ml of organisms IP plus 0.1 ml IP of the appropriate purified extracellular products. Only purified ETSA demonstrated a substantial decrease (1.0 log₁₀) in the 50% lethal dose (LD₅₀) values and only for a high ETSA producing strain. Serum from mice infected with a high ETSA producer contained approximately 12.5 μg/ml of the ETSA, whereas serum from mice infected with a low ETSA producer contained no detectable ETSA. These data imply that the two different types of organisms (high and low ETSA producers) have somewhat different mechanisms of pathogenicity in the mouse model.