Characterization of Extended-Spectrum Cephalosporin–Resistant Salmonella entericaSerovar Heidelberg Isolated from Humans in the United States

AbstractDuring the past decade, extended-spectrum cephalosporin resistance has increased among human isolates of Salmonella entericaserovar Heidelberg, the fourth most common serotype in the United States. We therefore characterized 54 Heidelberg isolates with decreased susceptibility (minimum inhibitory concentrations ≥2 mg/L) to ceftriaxone or ceftiofur; 49 (90.7%) contained the CMY-type β-lactamase (blaCMY) gene. The 49 blaCMY-positive human Heidelberg isolates demonstrated a high degree of relatedness; 4 clusters (25 isolates total) had indistinguishable XbaI and BlnI patterns by pulsed-field gel electrophoresis and were indistinguishable from 42 retail meat Heidelberg isolates. Further characterization of 15 of these isolates demonstrated that all of the blagenes were blaCMY-2and plasmid-encoded, and most (11/15) of the plasmids were approximately 100 kb in size and belong to the incompatibility group I1 (IncI1). All five IncI1 plasmids tested by plasmid multilocus sequence typing analysis were ST12. This report suggests that extended-spectrum cephalosporin resistance among human Heidelberg isolates is mediated by the spread of a common IncI1 blaCMY-2plasmid, which may have a preference for a particular genetic background.