Susceptibility testing of urinary isolates of Escherichia coli to mecillinam using NCCLS methodology

Criteria for susceptibility testing of mecillinam against 533 isolates of Escherichia coli and a further 309 Enterobacteriaceae, according to NCCLS methodology, were determined. Correlation of MIC to inhibition zones was good for all species. For urinary isolates of E. coli, the following agar dilution breakpoints and corresponding interpretive zone diameters seem appropriate: ≤8 mg/L/≥15 mm for susceptible; 16 mg/L/12-14 mm for intermediate susceptible and ≥32 mg/L/≤11 mm for resistant. The appearance of isolated colonies within the inhibition zone was sometimes noted with disc diffusion, particularly for non-E. coli Enterobacteriaceae. The relevance of these colonies to clinical (bacteriological) efficacy was determined and the results suggested that they could be ignored when testing urinary E. coli.