[Inhibitory activity of ampicillin and mecillinam on Haemophilus influenzae]

Mecillinam, an antibiotic of the beta-lactam group, is active against Gram negative bacilli, with the exception of a few species such as Haemophilus influenzae. The activities of ampicillin, mecillinam, and ampicillin-mecillinam combinations at ratios of 1/1, 2/1, 4/1, 1/2 and 1/4, on 21 Haemophilus influenzae strains, were studied by agar dilution and determination of minimal inhibitory concentrations (MIC). Ampicillin at concentrations of 0.12 to 0.25 mg/l was active on susceptible strains. Beta-lactamase-producing strains were inhibited by 4 and 8 mg/l ampicillin. MICs of mecillinam were above 128 mg/l for 19 strains; two strains were susceptible to 2 mg/l. In every case, antibacterial activity was greater with the ampicillin-mecillinam combination than with either one of the antibiotics alone. The maximum antibacterial effect was obtained at ratios of 1/1 and 1/2; in this case, susceptible strains were inhibited by 0.06 mg/l ampicillin. Kinetic evaluation (killing curves) showed that ampicillin has a bactericidal effect at concentrations under the MIC when it is combined with mecillinam at a 1/1 ratio.