The multiplication of three different isolates of group B streptococci in pregnant mice.

The ability of three streptococcal isolates of different pathogenicity to multiply in the tissues of pregnant mice was investigated following intravenous injection. The highly pathogenic isolate multiplied most rapidly whilst the isolate of low pathogenicity showed the least rapid growth within the observation period of 24 hours. The greatest concentration of streptococci per gram of tissue was found in the placentae of mice injected with the highly pathogenic and moderately pathogenic isolates when compared with the concentration of livers and spleen. Fetal infection did not occur within 24 hours of intravenous injection of the isolates but retarded fetal growth and infection was demonstrable five days after the injection. The isolate of moderate pathogenicity infected a greater proportion of the fetuses than the isolates of low pathogenicity. It is suggested that the outcome of streptococcal infections by the ability of isolates to multiply in host tissues.