In-vitro activity of the combinations of ampicillin with mecillinam or with beta-lactamase inhibitors against strains resistant to ampicillin.

The in-vitro activity of ampicillin, of mecillinam and of combinations of ampicillin with mecillinam, clavulanic acid or 6 beta-bromopenicillanic acid has been studied against 126 Enterobacteriaceae resistant to ampicillin. The combination of ampicillin with mecillinam showed synergy or addition in 60% of the combinations tested. Synergy was seen especially when the strains were resistant to mecillinam, indifference when they were susceptible to mecillinam. The combination of ampicillin with mecillinam was more active than the combination with clavulanic acid against Escherichia coli, Klebsiella and Enterobacter, but not against Proteus, Morganella and Providencia. The combination of ampicillin with clavulanic acid was more active than the combination with 6 beta-bromopenicillanic acid in E. coli, Klebsiella and Enterobacter strains.