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

An increase in the prevalence of commensalEscherichia coli carryingbla 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 126bla CTX-M -positive and 126bla CTX-M -negative isolates for determinations of the multilocus sequence types (MLSTs) and antibiotic resistance phenotypes fromE. coli obtained during a previous study. For 99 isolates, we also determined thebla CTX-M alleles using PCR and sequencing and identified the replicon types ofbla CTX-M -carrying plasmids. Thebla CTX-M -negativeE. coli isolates comprised 76 sequence types (STs) compared with 32 STs inbla CTX-M -positiveE. coli isolates. Thebla CTX-M -positiveE. coli isolates formed three MLST clonal complexes, accounting for 83% of these isolates; 52% ofbla CTX-M -negativeE. coli isolates clustered into 10 clonal complexes, and the remainder were singletons. Overall,bla CTX-M -negativeE. coli isolates had more diverse genotypes that were distinct to farms, whereasbla CTX-M -positiveE. 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 bybla CTX-M-27 andbla 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)-producingEscherichia coli occur globally and present treatment challenges because of their resistance to multiple antimicrobial drugs. Cattle are potential reservoirs of ESBL-producingEnterobacteriaceae , and so understanding the causes of successful dissemination ofbla CTX-M genes in commensal bacteria will inform future approaches for the prevention of antibiotic-resistant pathogen emergence.