Synergistic action of ampicillin and nafcillin against ampicillin-resistant Haemophilus influenzae

Six strains of ampicillin-resistant Haemophilus influenzae type b were studied in vitro for synergy between ampicillin and nafcillin. The minimal inhibitory concentrations for these strains with 10⁴ colony-forming units per ml were 6.25 to 12.5 μg of ampicillin per ml and 6.25 to 25 μg of nafcillin per ml. Studies with these agents demonstrated synergism against all six strains of H. influenzae type b. Most strains were inhibited by 0.78 μg of nafcillin plus 0.78 μg of ampicillin per ml. A child with osteomyelitis caused by H. influenzae type b was successfully treated with a combination of ampicillin and nafcillin. These data suggest that further studies assessing the synergistic effect of this drug combination against H. influenzae type b are warranted.