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 x 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 T-0 ( time = zero) colony count standardized to 100%, counts fell rapidly to means +/- SE and medians of 18.2 +/- -7.5% and 0.5% at T-2 ( P <.0001), 2.5 +/- 1.7% and 0.02% at T-4 ( P=.006), and less than 0.2% and 0.0% at T-6 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.