Perinatal antibiotic usage and changes in colonization and resistance rates of group B streptococcus and other pathogens.

OBJECTIVE: To quantify current antibiotic usage during the perinatal period and impact on vaginal-rectal colonizing organism resistance rates.
METHODS: Swabs were obtained for culture of group B streptococcus and other bacteria from a cohort of 1207 pregnant women in Calgary, Alberta, at 36 weeks' gestation. Those women who received antibiotics during labor or after pregnancy and a 10% subset who received no antibiotics had repeat cultures at 6 weeks postpartum. Cultured organisms were tested for sensitivity to several antibiotics.
RESULTS: Group B streptococcus was identified in 235 women (19.5%) in the antepartum period. Fifty-one percent of all participants received antibiotics (31.4% intrapartum). Group B streptococcus prophylaxis was given to 215 (17.8%), whereas 83 (6.9%) group B streptococcus-negative women without fever during labor received antibiotics. Ampicillin (49%), cefazolin (28%), and penicillin (18%) were the most frequently used antibiotics. Resistance rates among group B streptococcus to erythromycin and clindamycin were 5.6% and 3.0%, respectively, whereas 20.6% of Escherichia coli were ampicillin resistant. Among antibiotic recipients, 6.3% of all bacteria that were initially sensitive on prenatal cultures to a specific antibiotic became resistant in the postnatal period, whereas 6.5% that were initially resistant became sensitive.
CONCLUSION: Current prevention practices in our region were associated with perinatal antibiotic administration in over half of pregnant women. Ampicillin was the most common antibiotic administered. Some physicians are treating women who are group B streptococcus culture negative at term, a practice that is of no proven value. However, this was not associated with increased resistance for group B streptococcus or other organisms identified from maternal vaginal-rectal tracts.