Occurrence of plasmidic AmpC type beta-lactamase-mediated resistance in Escherichia coli: report from the SENTRY Antimicrobial Surveillance Program (North America, 2004).

Among 1429 Escherichia coli isolates collected as part of the SENTRY Antimicrobial Surveillance Program (2004) from 30 North American medical centres, 65 (4.5%) were screen-positive for an extended-spectrum beta-lactamase (ESBL). Among the strains with a negative ESBL confirmatory test (n=26; clavulanic acid inhibition), a CMY-2 enzyme was detected in 13 isolates (50.0%), FOX-5 in 3 isolates (11.5%) and DHA-1 in 1 isolate (3.8%). These AmpC-producing E. coli were cephamycin (cefoxitin)-resistant but susceptible to cefepime (minimum inhibitory concentration (MIC) < OR =0.12-4 mg/L). Clearly, the ESBL tests recommended by the Clinical and Laboratory Standards Institute identify only a fraction of E. coli with elevated beta-lactam MIC values as ESBL-producing strains; the majority of the remaining strains would be potentially responsive to some other beta-lactams, directed by accurately performed and interpreted susceptibility methods.