Escherichia coli O157 is a major cause of food-borne illness. Plasmids are genetic elements that mobilise antimicrobial resistance determinants, including bla(CMY) beta-lactamases that confer resistance to extended-spectrum cephalosporins (ESCs). ESCs are important for treating a variety of infections. IncA/C plasmids are found among diverse sources, including cattle, the principal source of E. coli 0157 infections in humans. IncI1 plasmids are common among E. coli and Salmonella from poultry and other avian sources. To broaden our understanding of the reservoirs of bla(CMY), the types of plasmids carrying bla(CMY) among E. coli O157 were determined. From 1996 to 2009, 3742 E. coli O157 isolates were tested. Eleven isolates (0.29%) were ceftriaxone-resistant and had a bla(CMY)-2-containing plasmid. All four isolates submitted before 2001 as well as a single 2001 isolate had bla(CMY) encoded on IncA/C plasmids, whilst all five isolates submitted after 2001 and a single 2001 isolate had bla(CMY) carried on IncI1 plasmids. The IncI1 plasmids were ST2, ST20 and ST23. We conclude that cephalosporin resistance among E. coli O157:H7 is due to plasmid-encoded bla(CMY) genes and that plasmid types appear to have shifted from IncA/C to Inch. This shift suggests either a change in plasmid type among animal reservoirs or that the organism has expanded into avian reservoirs. More analysis of human, retail meat and food animal isolates is necessary to broaden our understanding of the antimicrobial resistance determinants of ESC resistance among E. coli O157. Published by Elsevier Ltd on behalf of International Society for Chemotherapy of Infection and Cancer.
