Evidence for transfer of CMY-2 AmpC beta-lactamase plasmids between Escherichia coli and Salmonella isolates from food animals and humans

Escherichia coli is an important pathogen that shows increasing antimicrobi al resistance in isolates from both animals and humans. Our laboratory rece ntly described Salmonella isolates from food animals and humans that expres sed an identical plasmid-mediated, AmpC-like beta -lactamase, CMY-2. In the present study, 59 of 377 E. coli isolates from cattle and swine (15.6%) an d 6 of 1,017 (0.6%) isolates of human E. coli from the same geographic regi on were resistant to both cephamycins and extended-spectrum cephalosporins. An ampC gene could be amplified with CMY-2 primers in 94.8% of animal and 33% of human isolates. Molecular epidemiological studies of chromosomal DNA revealed little clonal relatedness among the animal and human E. coli isol ates harboring the CMY-2 gene. The ampC genes from 10 animal and human E. c oli isolates were sequenced, and all carried an identical CMY-2 gene. Addit ionally, all were able to transfer a plasmid containing the CMY-2 gene to a laboratory strain of E. coli. CMY-2 plasmids demonstrated two different pl asmid patterns that each showed strong similarities to previously described Salmonella CMY-2 plasmids. Additionally, Southern blot analyses using a CM Y-2 probe demonstrated conserved fragments among many of the CMY-2 plasmids identified in Salmonella and E. coli isolates from food animals and humans . These data demonstrate that common plasmids have been transferred between animal-associated Salmonella and E. coli, and identical CMY-2 genes carrie d by similar plasmids have been identified in humans, suggesting that the C MY-2 plasmid has undergone transfer between different bacterial species and may have been transmitted between food animals and humans.