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

Appl Environ Microbiol. 2018 Mar 1;84(6):e02430-17. doi: 10.1128/AEM.02430-17. Print 2018 Mar 15.


An increase in the prevalence of commensal Escherichia coli carrying blaCTX-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 blaCTX-M-positive and 126 blaCTX-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 blaCTX-M alleles using PCR and sequencing and identified the replicon types of blaCTX-M-carrying plasmids. The blaCTX-M-negative E. coli isolates comprised 76 sequence types (STs) compared with 32 STs in blaCTX-M-positive E. coli isolates. The blaCTX-M-positive E. coli isolates formed three MLST clonal complexes, accounting for 83% of these isolates; 52% of blaCTX-M-negative E. coli isolates clustered into 10 clonal complexes, and the remainder were singletons. Overall, blaCTX-M-negative E. coli isolates had more diverse genotypes that were distinct to farms, whereas blaCTX-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. blaCTX-M-15 was the predominant CTX-M gene allele, followed by blaCTX-M-27 and blaCTX-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 blaCTX-M genes in commensal bacteria will inform future approaches for the prevention of antibiotic-resistant pathogen emergence.

Keywords: Escherichia coli; dairy cattle; extended-spectrum β-lactamases; molecular epidemiology; plasmids; sequence types.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Escherichia coli / enzymology
  • Escherichia coli / genetics*
  • Escherichia coli Infections / epidemiology*
  • Escherichia coli Infections / microbiology
  • Escherichia coli Proteins / genetics*
  • Molecular Epidemiology
  • Multilocus Sequence Typing
  • Plasmids / genetics
  • Prevalence
  • Washington / epidemiology
  • beta-Lactamases / genetics*


  • Escherichia coli Proteins
  • beta-Lactamases