Characterization of β-lactamases responsible for resistance to extended-spectrum cephalosporins in Escherichia coli and Salmonella enterica strains from food-producing animals in the United Kingdom

Nine epidemiologically unrelated isolates {1 Salmonella Bredeney from turkeys, and 8 Escherichia coli [3 environmental isolates (2 from chickens, 1 from pigs), and 5 isolates from cattle with neonatal diarrhea]) were examined both pheno- and genotypically for extended-spectrum beta-lactam (ESBL) resistance. Resistance phenotypes (ampicillin, aztreonam, cefotaxime, cefpodoxime, ceftazidime, and ceftriaxone) suggested the presence of an ESBL enzyme, but cefoxitin MICs (greater than or equal to 32 mg/L) suggested the presence of an AmpC-like enzyme. Synergism experiments with benzo(b)thiophene-2-boronic acid (BZBTH2B) and isoelectric focusing (IEF) revealed the presence of an AmpC beta-lactamase with a pI greater than or equal to ampC multiplex PCR, sequence, and Southern analyses indicated that only the Salmonella isolate had a plasmid-encoded AmpC beta-lactamase CMY-2 on a nonconjugative 60-MDa plasmid. PCR and sequence analysis of the E. coli ampC promoter identified mutations at positions -88(T), -82(G), -42(T), -18(A), -1(T) and +58(T) in all the isolates. In addition one strain had two extra-mutations at positions +23(A) and +49(G), and another strain had one extra-mutation at position +32(A). DNA fingerprinting revealed that all the E. coli isolates were different clones. It also showed that the U.K. Salmonella isolate was indistinguisable from a Canadian Salmonella isolate from turkeys; both had identical resistance phenotypes and produced CMY-2. This is the first report of a CMY-2 Salmonella isolate in the United Kingdom. These data imply that beta-lactam resistance in animal isolates can be generated de novo as evidenced by the E. coli strains, or in the case of the Salmonella strains be the result of intercontinental transmission due to an acquired resistance mechanism.