The population dynamics of Streptococcus agalactiae (group B streptococci [GBS]) colonization of the vagina and anorectal area was investigated in a cohort of 77 Danish women during and after their pregnancy by a new sensitive method. The mean carriage rate among individual observations was 36%, and the cumulative carriage rate over the entire observation period was 54%. Examination of more than 1,500 GBS isolates by pulsed-field gel electrophoresis demonstrated that the GBS population was remarkably homogeneous and stable in each carrier. Virtually all carriers were colonized by a single GBS clone on all occasions spanning up to 2 years. Repeated detection of the same clone even in women who were recorded as intermittent carriers suggests that the actual carrier rate exceeds 50% but that fluctuations in the GBS proportions of the flora occasionally preclude their detection. Newborns and young infants usually carried the same GBS clone as their mothers. However, only twice were identical clones of GBS detected in different women in contrast to the observed clonal relationships of clinical isolates. These observations strongly suggest differences in the properties and epidemiology of virulent GBS clones compared to clones commonly carried by healthy individuals.
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