Mouse protection test for group B Streptococcus type III

The mouse model has been used previously for the study of protection against challenge with strains of group B Streptococcus types Ia, Ib, and Ic, but investigators have not yet been successful in causing lethal disease when type III strains are inoculated. In this study, six clinical isolates belonging to serotype III were lethal in seven-week-old outbred albino mice; the 50% lethal dose (LD50) for mice tested ranged from 1.8 x 10(4) to 4.6 x 10(6). In contrast, a laboratory prototype strain of serotype III was not lethal. The lethality observed with this protection test depended on both the volume of the inoculum given and the medium in which the streptococci were suspended. LD50 values decreased as the volume of the inoculum, given intraperitoneally, increased, up to a volume of 1.5 ml. Hyperimmune rabbit antiserum to a clinical isolate of type III protected mice against an estimated 90% lethal dose of the isolate. Normal rabbit serum and heterologous antisera, except for one of two antisera to type Ia organisms, were not protective. Absorption of type III antiserum with whole organisms of serotype III or with purified, type III-specific polysaccharide inhibited protection of mice against type III organisms.