Selective versus non-selective culture medium for group B streptococcus detection in pregnancies complicated by preterm labor or preterm-premature rupture of membranes

ABSTRACT

The objective of this study was to identify group B streptococcus (GBS) colonization rates and compare detection efficiency of selective versus non-selective culture media and anorectal versus vaginal cultures in women with preterm labor and preterm-premature rupture of membranes (PROM). A prospective cohort study of 203 women was performed. Two vaginal and two anorectal samples from each woman were collected using sterile swabs. Two swabs (one anorectal and one vaginal) were placed separately in Stuart transport media and cultured in blood-agar plates for 48 hours; the other two swabs were inoculated separately in Todd-Hewitt selective media for 24 hours and then subcultured in blood-agar plates. Final GBS identification was made by the CAMP test. A hundred thrity-two cultures out of 812 were positive. The maternal colonization rate was 27.6 percent. Colonization rates were 30 percent for preterm PROM and 25.2 percent for preterm labor. Todd-Hewitt selective medium detected 87.5 percent and non-selective medium 60.7 percent GBS-positive women. Vaginal samples and anorectal samples had the same detection rate of 80.3 percent. Anorectal selective cultures detected 75 percent of carriers; 39 percent of GBS-positive women were detected only in selective medium. A combined vaginal-anorectal selective culture is appropriate for GBS screening in this population, minimizing laboratory costs.