In vitro interactions between rifampin and ampicillin or chloramphenicol against Haemophilus influenzae.

Twenty clinical isolates of Haemophilus influenzae type b were used to determine the in vitro interactions of rifampin with ampicillin or chloramphenicol. Potential interactions of the drugs were evaluated by calculating the fractional inhibitory concentration index and fractional bactericidal concentration index for each strain after treatments with the drugs alone and in combination. There was no evidence of synergy or antagonisms between ampicillin and rifampin or between chloramphenicol and rifampin. With ampicillin-susceptible H. influenzae type b strains, an additive effect was observed with seven strains, and an indifferent effect was observed with three strains. Similarly, with chloramphenicol-susceptible H. influenzae type b strains, an additive effect was observed with eight strains, and an indifferent effect was observed with two strains.