The effect of intrapartum penicillin on vaginal group B streptococcus colony counts

Objective The objective of the study was to determine the temporal relationship between intrapartum penicillin-G (PCN-G) and vaginal group B streptococcus (GBS) counts. Study Design In 50 women with GBS-positive antenatal cultures, intrapartum cultures were collected just before the first PCN-G dose and every 2 hours × 4 or until delivery. Colony counts were quantified using serial dilution. Results were standardized as percent of initial colony count and analyzed using sequential Wilcoxon tests. Results Of 50 subjects with GBS-positive antepartum cultures, 35 (70%) had positive intrapartum vaginal cultures, of which 27 received intrapartum PCN-G. Degree of vaginal colonization varied greatly between subjects, and counts (percents) were not normally distributed. From the T0 (time = zero) colony count standardized to 100%, counts fell rapidly to means ± SE and medians of 18.2 ± -7.5% and 0.5% at T2 ( P < .0001), 2.5 ± 1.7% and 0.02% at T4 ( P = .006), and less than 0.2% and 0.0% at T6 and 8 ( P = .07 and P = .46, respectively). Conclusion Vaginal GBS colony counts fall rapidly after intrapartum PCN-G administration, which may partly explain the effectiveness of chemoprophylaxis.