Effect of mecillinam on Escherichia coli growth curves when given alone and associated with ampicillin

The effects on growth curves of strains of Escherichia coli by mecillinam, ampicillin, and combinations of the two antibiotics were investigated using the Abbott-MS-2 system. Maximal increase in optical density in the residual growth phase was found to be concentration dependent for ampicillin and nearly concentration independent for mecillinam. The addition of ampicillin potentiated the effect on mecillinam. However, when mecillinam concentrations were more than 100 times the MIC or when ampicillin was added with a lag time, the growth curves for combinations of mecillinam plus ampicillin showed a mecillinam profile. Killing curves confirmed that the combination of ampicillin and mecillinam acts strongly synergistic and exerts a much faster bactericidal effect.