Typing of Streptococcus agalactiae (Lancefield group B).

A two-stage typing scheme in routine use in this laboratory is described. The strains of group B streptococci (GBS) are first serotyped and then, if necessary, phage-typing is performed. Serotyping lacks discrimination because almost 40% of strains handled carry either the Ia/c or III/R antigens. On the other hand, the typability rate with phages is less than 80%. The evidence from the combined scheme is consistent with the view that GBS infections occurring within the first five days of life are, with few exceptions, due to acquisition of the mother's flora. After this time the infecting strain may come from other sources. Human and bovine strains of GBS belong predominantly to two recognizably different populations that can be distinguished by antigenic and biochemical differences. There are no patterns of lysis by phages characteristic of either human or bovine strains.