Dynamics of Streptococcus agalactiae colonization in women during and after pregnancy and in their infants

Abstract

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 1500 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.

FIG. 1.

Carriage rates of GBS in a cohort of 77 women during their pregnancy and 1 year after delivery. Figures indicate the number of women (carriers/total examined). There was no association between gestational age and carriage rate (mean, 36%; P = 0.83 [logistic regression]).

FIG. 2.

Distribution of vaginal and rectal samples according to the density of GBS growth. Bars: □, negative samples; ▨, light growth of GBS; ▪, moderate to heavy growth of GBS. Samples were obtained from 129 GBS positive sample pairs, i.e., one or both samples (vaginal and rectal) in the pair exhibited growth for GBS. GBS-negative samples were included since the corresponding vaginal or rectal samples were positive.

FIG. 3.

Distribution of carriers and noncarriers of GBS one year after delivery compared to the carriage status during pregnancy. Follow-up samples from 44 women were examined. The carriage rate was 34% (15 of 44). Figures indicate the number of women per subgroup. Carriage status during pregnancy: □, noncarriers; ▩, intermittent carriers; ▪, carriers. Persistent carriers, for whom all sample pairs were positive both during pregnancy and at follow-up, were considered chronically colonized (16%). Of the women in the follow-up group, 48% were found to be negative for GBS at all samplings occasions both during pregnancy and at follow-up, i.e., they were permanently noncolonized.

FIG. 4.

PFGE gel band patterns for individual GBS isolates. The restriction enzyme SmaI was used for digestion of the DNA. (A) Twenty-four GBS isolates isolated from one sample pair obtained from one participant (subject 38). Only one clone was seen among the rectal isolates (R1 to R13). This clone was also seen among the vaginal isolates (V1 and V6), whereas the rest of the vaginal isolates (V2 to V5 and V7 to V11) belong to a different clone. (B) GBS isolates isolated from 11 different women. None of the 11 isolates exhibited an identical gel band pattern, i.e., they belonged to different clones.