Molecular Epidemiology of Dairy Cattle-AssociatedEscherichia coliCarryingblaCTX-MGenes in Washington State
ABSTRACTAn increase in the prevalence of commensalEscherichia colicarryingblaCTX-Mgenes among dairy cattle was observed between 2008 and 2012 in Washington State. To study the molecular epidemiology of this change, we selected 126blaCTX-M-positive and 126blaCTX-M-negative isolates for determinations of the multilocus sequence types (MLSTs) and antibiotic resistance phenotypes fromE. coliobtained during a previous study. For 99 isolates, we also determined theblaCTX-Malleles using PCR and sequencing and identified the replicon types ofblaCTX-M-carrying plasmids. TheblaCTX-M-negativeE. coliisolates comprised 76 sequence types (STs) compared with 32 STs inblaCTX-M-positiveE. coliisolates. TheblaCTX-M-positiveE. coliisolates formed three MLST clonal complexes, accounting for 83% of these isolates; 52% ofblaCTX-M-negativeE. coliisolates clustered into 10 clonal complexes, and the remainder were singletons. Overall,blaCTX-M-negativeE. coliisolates had more diverse genotypes that were distinct to farms, whereasblaCTX-M-positiveE. coliisolates 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.blaCTX-M-15was the predominant CTX-M gene allele, followed byblaCTX-M-27andblaCTX-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.IMPORTANCEInfections caused by extended-spectrum β-lactamase (ESBL)-producingEscherichia colioccur 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 ofblaCTX-Mgenes in commensal bacteria will inform future approaches for the prevention of antibiotic-resistant pathogen emergence.