Abstract
An increase in the prevalence of commensal Escherichia coli carrying bla(CTX-M) genes among dairy cattle was observed between 2008 and 2012 in Washington State. To study the molecular epidemiology of this change, we selected 126 bla(CTX-M)-positive and 126 bla(CTX-M)-negative isolates for determinations of the multilocus sequence types (MLSTs) and antibiotic resistance phenotypes from E. coli obtained during a previous study. For 99 isolates, we also determined the bla(CTX-M) alleles using PCR and sequencing and identified the replicon types of bla(CTX-M)-carrying plasmids. The bla(CTX-M)-negative E. coli isolates comprised 76 sequence types (STs) compared with 32 STs in bla(CTX-M)-positive E. coli isolates. The bla(CTX-M)-positive E. coli isolates formed three MLST clonal complexes, accounting for 83% of these isolates; 52% of bla(CTX-M)-negative E. coli isolates clustered into 10 clonal complexes, and the remainder were singletons. Overall, bla(CTX-M)-negative E. coli isolates had more diverse genotypes that were distinct to farms, whereas bla(CTX-M)-positive E. coli isolates had a clonal population structure and were widely disseminated on farms in both regions included in the study. Plasmid replicon types included IncI1 which predominated, followed by IncFIB and IncFIA/FIB. bla(CTX-M-15) was the predominant CTX-M gene allele, followed by bla(CTX-M-27) and bla(CTX-M-14) There was no significant association between plasmid replicon types and bacterial STs, and neither clonal complexes nor major plasmid groups were associated with two discrete dairy-farming regions of Washington State.IMPORTANCE Infections caused by extended-spectrum β-lactamase (ESBL)-producing Escherichia coli occur globally and present treatment challenges because of their resistance to multiple antimicrobial drugs. Cattle are potential reservoirs of ESBL-producing Enterobacteriaceae, and so understanding the causes of successful dissemination of bla(CTX-M) genes in commensal bacteria will inform future approaches for the prevention of antibiotic-resistant pathogen emergence.