Effect of rifampin on ampicillin killing of group B streptococci

The activity of rifampin against group B streptococci as a single agent and in combination with ampicillin was examined. The minimum concentration of rifampin required to inhibit 90% of the isolates of group B streptococci (18 of 20 isolates) was 0.6 microgram/ml. The checkerboard double-dilution studies of these two antibiotics and the time-killing studies both indicate that rifampin inhibits the bactericidal activity of ampicillin for group B streptococci.