BACKGROUND: Genital group B streptococcus (GBS) may be transmitted from a colonized mother to her infant if appropriate intrapartum antibiotic prophylaxis is not given. A recent case of GBS neonatal sepsis occurred due to an erythromycin-intermediate strain after empirical use of this drug as intrapartum prophylaxis.OBJECTIVE: To determine the regional antibiotic resistance rates of genital GBS isolates to penicillin, erythromycin and clindamycin.METHODS: A total of 309 genital GBS strains cultured from vaginal/rectal swabs were prospectively isolated and randomly selected between March and May 2011. Etest strips (bioMèrieux, France) were used to determine the minimum inhibitory concentrations to penicillin, erythromycin and clindamycin according to standard methods. All isolates that either demonstrated intermediate or full resistance to erythromycin had a D-test performed to detect inducible resistance to clindamycin. The resistance mechanism for each isolate was inferred from its antibiogram phenotype.RESULTS: All genital GBS isolates were susceptible to penicillin, but high rates of resistance were found to both erythromycin (25%) and clindamycin (22%), mainly due to acquisition of erythromycin ribosomal methylation genes (erm) that result in the MLSB resistance phenotype. Most often the MLSB resistance phenotype was constitutive (MLSB-C; 14.2%) rather than inducible (MLSB-I; 8.1%), and an efflux mechanism (msrA; 3%) was much less common.DISCUSSION: The present article is the first point prevalence study of genital GBS antibiogram profile that has been reported from a Canadian health care region. The high rates of resistance of genital GBS to both erythromycin and clindamycin is mainly due to the acquisition and spread of erm genes conveying the MSLB phenotype.CONCLUSION: Changes to clinical and laboratory practice in the Calgary, Alberta, region were made to prevent additional cases of neonatal GBS sepsis due to inappropriate intrapartum antibiotic prescription.