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 ,ug/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.Group B streptococci (GBS) are the leading causes of meningitis, septicemia, and other severe infections within the first few weeks of life (14). Other serious infections, such as septic arthritis, pyelonephritis, and endometritis, can also be caused by GBS in adults (3, 10). In general, GBS are susceptible to penicillin and ampicillin, and these have been the antibiotics of choice in treating the wide range of diseases caused by these organisms. Recently, clinical failures and relapses due to GBS have been reported after appropriate therapy with penicillin or ampicillin (4,16). Some failures may result from the persistence of tolerant organisms (8,15) or from infection involving a high density of organisms (1).Clinically, the combination of ampicillin or penicillin and an aminoglycoside has resulted in the improvement of several cases of endocarditis, septic arthritis, and pyelonephritis (15, 16). In vitro synergy between penicillin and gentamicin against GBS has been reported (2, 4, 7). However, this combination may prove to be clinically impractical in cases of meningitis since the aminoglycosides do not achieve high levels in the cerebrospinal fluid (5).Recently, rifampin has become important in the combination therapy of clinical and experimental staphylococcal endocarditis (12,13

No abstract