Multidrug resistance in group B Streptococcus causing urinary tract infection exposes an erythromycin-driven protective effect against oxidative stress

Multidrug resistance has been reported in group B <i>Streptococcus</i> (GBS) from various origins, but rates among urinary tract infection (UTI) isolates are largely unknown. Erythromycin, a second-line antibiotic for GBS for which high rates of resistance have been reported, was recently shown to support the resistance of <i>Staphylococcu</i>s to oxidative stress. To survey multidrug-resistant (MDR) GBS from UTI and to investigate the effect of erythromycin exposure on the bacteria's ability to resist oxidative stress, we determined the antibacterial activity of 18 antibiotics against 292 GBS UTI isolates by disc diffusion and used i<i>n vitro</i> growth assays of MDR GBS exposed to erythromycin to examine relative resistance to oxidative stress in the form of H₂O₂. A high proportion of all 292 GBS isolates (33.6%) were MDR, reflecting high rates of resistance to four antibiotics: azithromycin (44.5%), clindamycin (26%), erythromycin (36.3%) and tetracycline (81.5%); however, no resistance was detected for any other antibiotics tested. Rates of resistance were not significantly different when analysed according to clinical origins (acute and recurrent UTI, asymptomatic bacteriuria). The growth of MDR GBS was attenuated and severely inhibited by exposure to erythromycin and H₂O₂, respectively. Surprisingly, exposure of MDR GBS to erythromycin significantly relieved the severe growth inhibitory effect of H₂O₂, signifying a partial rescue effect of the antibiotic. The GBS isolates in this study exhibit high levels of multidrug resistance without an association between resistance and clinical origin. Exposure of MDR GBS to erythromycin can partially counteract the severe growth inhibitory effect from H₂O₂.