Co-occurrence of Plasmid-Mediated Colistin Resistance (mcr-1) and Extended-Spectrum β-Lactamase Encoding Genes in Escherichia coli from Bovine Mastitic Milk in China

Both mcr-1 phosphoethanolamine transferase enzymes and extended-spectrum beta-lactamases (ESBLs) are the main plasmid-mediated mechanisms of resistance to colistin and third-generation cephalosporins, respectively, and currently considered a major concern to humans and food animals. Prevalence of mcr-1 gene in Escherichia coli from dairy cattle has rarely been reported. Our objective was to determine prevalence and characteristics of mcr-1 carrying E. coli isolated from clinical mastitis cases in large dairy farms (>500 cows) in 16 provinces of China. A total of 249 E. coli was isolated from 2,038 mastitic milk samples. Among these isolates, 2.0% (n = 5) and 19.7% (n = 49) were colistin resistant mcr-1-positive and ESBL-producing isolates, respectively. All mcr-1-positive isolates that produced ESBLs also carried the bla(CTX-M-15) gene and belonged to phylogroup-A. Most mcr-1 and bla(CTX-M-15) genes were located on conjugative plasmids (IncP and IncF, respectively) that were successfully transferred to transconjugants in conjugation experiments. All mcr-1-positive E. coli isolates were multidrug resistant, exhibiting resistance to common antimicrobials. Multilocus sequence typing of these mcr-1-carrying E. coli isolates revealed four sequence types, reflecting substantial diversity. Multilocus sequence analysis detected evolutionary connection of mcr-1 carrying isolates with our recently reported ESBL-producing E. coli isolates, raising concerns regarding fast dissemination between bacteria. To our knowledge, this was the first nation-wide report describing isolates of E. coli from mastitic milk samples collected on large dairy farms in China, carrying mcr-1 and bla(CTX-M-15) genes on conjugative plasmids. We concluded that dairy cattle are a potential source of mcr-1-carrying and ESBL-producing E. coli.