Commentary on genetic mechanisms of antimicrobial resistance in bacteria from U.S. food animals: ESBLs are here

While Frye and Jackson do a good job describing the common mechanisms of resistance found in food animals in the U.S., they err in stating that in the U.S. extended spectrum β-lactamases (ESBLs) “thus far have only been found in human and not food animal isolates.” In fact, Wittum et al., in 2010 first reported the collection of Escherichia coli expressing CTX-M-type ESBLs in fecal isolates from healthy cattle at a livestock market and from a diagnostic isolate submitted to the Ohio Animal Disease Diagnostic Laboratory. Since then, CTXM-type ESBLs have been found in E. coli collected from healthy dairy calves in the western U.S. (Davis et al., 2011) and from 5 of 20 dairy farms in Ohio (Mollenkopf et al., 2012); in clinical Salmonella enterica isolates from swine in Minnesota and from turkeys in 4 states (Wittum et al., 2012); in E. coli from swine finishing barns in Michigan and Ohio and in Klebsiella pneumoniae from swine in Illinois (Mollenkopf et al., 2013). We are not aware of studies finding CTX-M-type ESBLs in isolates from broiler chickens, but a retail chicken meat E. coli isolate expressing CTX-Mtype ESBLs from Pennsylvania has been reported (Doi et al., 2010). Given the situation in other livestock species and in other countries, we expect the lack of detection of CTX-M-type ESBLs in U.S. chicken isolates is more the result of lack of appropriate studies than a real absence of these on chicken farms. In addition to CTX-Mtype ESBLs, the study of dairy calves in the western U.S. also found E. coli expressing OXA-type ESBLs (Davis et al., 2011).