Activity of moxalactam and cefotaxime alone and in combination with ampicillin or penicillin against group B streptococci

The activities of moxolactam and cefotaxime, alone and combined with ampicillin or penicillin, against 40 isolates of group B streptococci were assessed by using the microtiter broth dilution, checkerboard, and time-kill techniques. Penicillin and cefotaxime were bactericidal for all isolates at concentrations of 0.06 μg/ml or less. Ampicillin was slightly less active. Moxalatam was bactericidal for all strains at concentrations of 4 to 8 μg/ml. The ampicillin-moxalactam combination was partially synergistic for 60% of the isolates tested; the ampicillin-cefotaxime combination was partially synergistic for 35% of these isolates. No instances of antagonism were observed. In time-kill evaluations, ampicillin (3.0 μg/ml) and penicillin (0.75 μg/ml) effected 2.5 to 3.5 log₁₀ reductions in numbers of colony-forming units. The addition of 4 μg of cefotaxime per ml or 8 to 16 μg of moxalactam per ml to penicillin or ampicillin did not alter killing kinetics. Moxalactam and cefotaxime neither enhanced nor decreased the activity of ampicillin or penicillin against group B streptococci.