Molecular Epidemiology of Dairy Cattle-Associated Escherichia coli Carrying bla CTX-M Genes in Washington State.

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.