Antimicrobial susceptibility, tolerance, and synergy of groups A and B streptococci

Although group A and B streptococci have similar susceptibility to antimicrobics, group A streptococci are killed significantly faster at clinically relevant concentrations of penicillin or ampicillin. The addition of an aminocyclitol to either penicillin or ampicillin enhances the rate of killing of streptococci. Because the rate of killing of group B streptococci is significantly increased by the addition of gentamicin at concentrations of 0.1 μg and 0.5 μg/ml to either penicillin or ampicillin, the use of combined therapy may indeed be beneficial in the initial treatment of group B streptococcal meningitis. However, as meningeal inflammation subsides, gentamicin may not enter the central nervous system well enough to justify continued use. Combined therapy for the duration of treatment of penicillin-tolerant group B streptococci has been recommended on the basis of isolated case reports of relapse after single drug therapy. This approach is not supported by in vitro observations of no enhancement of the rate of killing of tolerant strains by the addition of gentamicin to penicillin. Thus, a final recommendation regarding the duration of combined therapy, and use with tolerant organisms, awaits further studies.