In vitro activity of mecillinam and amoxicillin/clavulanic acid against strains of Escherichia coli producing TEM-1, Oxa-1 and chromosomal beta-lactamases.

The in vitro activity of mecillinam and amoxicillin/clavulanic acid against Escherichia coli strains producing beta-lactamases of the TEM-1, Oxa-1 and chromosomal type were studied using the broth and agar dilution technique. In addition the beta-lactamase activity was determined. The experiments indicated that the beta-lactamase types differed in their influence on the minimum inhibitory concentrations. Both, mecillinam and amoxicillin/clavulanic acid were considerably more active against TEM-1 strains than ampicillin alone, but, with increasing specific beta-lactamase activity, a decrease of susceptibility was found. Strains producing Oxa-1 and chromosomal beta-lactamases usually where highly susceptible to mecillinam but only moderately sensitive or resistant to the combination of amoxicillin/clavulanic acid. In vitro activity of TEM-1 E. coli to mecillinam was strongly reduced when tested in a broth dilution technique. These test systems, however, did not affect the susceptibility of Oxa-1 and chromosomal beta-lactamase producing E. coli strains to either antibiotic.