Rifampin and penicillin for the elimination of group B streptococci in nasally colonized infant rats

Although multiple antibiotic strategies to eradicate group B streptococci (GBS) from colonized infants and women have been utilized, no regimen has been successful in eliminating GBS carriage reliably. Because rifampin has been successful in terminating nasopharyngeal colonization with other bacteria, we tested both the in vitro sensitivity of GBS to rifampin and the in vivo efficacy of rifampin in eliminating GBS from a new animal model of nasally colonized infant rats. The minimal inhibitory concentration of rifampin for 18 clinically derived strains of type III GBS ranged from 0.1 to 0.4 μg/ml. Atraumatic nasal inoculation of infant rats with 10⁶-10⁷ colony forming units of GBS twice daily for 4 days resulted in heavy asymptomatic carriage for at least 10 days. Colonized animals were divided into four treatment groups: 1) saline, 2) oral rifampin, 3) intraperitoneal penicillin, or 4) oral rifampin plus intraperitoneal penicillin. Treatment was administered every 12 h for 4 days. All 78 saline-treated controls and 47 of 52 (90.4%) penicillin-treated animals had continued GBS carriage 36 h after completion of therapy. In contrast, only 18 of 52 (34.6%) rifampin-treated animals and seven of 54 (13.0%) rifampin plus penicillin-treated animals remained GBS-positive. No ri-fampin-resistant GBS were detected. Combination rifampin plus penicillin therapy was significantly more effective in terminating GBS carriage compared to saline or penicillin alone (p < 0.0001) or to rifampin (p < 0.01). Nasal cultures taken from a subgroup of animals 6-7 days after completion of therapy showed that 20 of 30 (66.7%) animals treated with rifampin and 43 of 54 (79.6%) animals treated with rifampin plus penicillin remained GBS-free (p = NS). These data demonstrate that, unlike penicillin alone, rifampin alone is effective, while combination rifampin plus penicillin therapy is most effective in eliminating GBS from nasally colonized infant rats.