To the Editor: Plasmid-mediated, colistin-resistance mechanism gene mcr-1 was first identified in Escherichia coli isolates from food, food animals, and human patients in November 2015 (1). Reports on detection of mcr-1 in Enterobacteriaceae from humans and food animals soon followed from ≈12 countries (2–5). Here we report detection of mcr-1 in colistin-resistant E. coli isolated from companion animals and the possible transmission of mcr-1–harboring E. coli between companion animals and a person.

Three mcr-1–harboring E. coli clinical isolates were identified from specimens of 3 patients admitted to a urology ward of a hospital in Guangzhou, China. E. coli isolate EC07 was identified in the urine of a 50-year-old male patient with glomerulonephritis in October 2015. Isolate EC08 was cultured from the urine of a 48-year-old male patient with prostatitis in December 2015. Isolate EC09 was identified in the blood of an 80-year-old male patient with bladder cancer 3 weeks after EC08 was identified.

Review of medical records identified the patient carrying E. coli isolate EC07 as a worker at a pet shop. In light of this finding, we collected a total of 53 fecal samples from 39 dogs and 14 cats in the pet shop where the man worked. We isolated and identified colonies consistent with E. coli from fecal samples on MacConkey agar plates (Thermo Fisher, Beijing, China) and API 20E system (bioMérieux, Durham, NC, USA). We prepared crude DNA samples of isolates for PCR testing by boiling cells in water. Among them, 6 were positive for mcr-1 by PCR and sequencing (4 from dogs and 2 from cats). All 6 isolates were resistant to colistin, polymyxin B, cephalosporin, gentamicin, and ciprofloxacin by using the agar dilution method, in accordance with the European Committee on Antimicrobial Susceptibility Testing (http://www.eucast.org) for colistin and polymyxin B and Clinical and Laboratory Standards Institute guidelines (http://www.clsi.org) for the other antimicrobial drugs. We identified various resistance genes accounting for the multidrug resistance in these 9 mcr-1–positive isolates (6,7) (Table). We noted that E. coli isolate EC09 was also resistant to carbapenems and positive for bla_IMP-4. We observed co-production of mcr-1 and IMP-type metallo-β-lactamase in E. coli.

We subjected all isolates to multilocus sequence typing, in accordance with the protocol described at http://mlst.warwick.ac.uk/mlst/dbs/Ecoli, and pulsed-field gel electrophoresis as described previously (8–10). We identified 5 mcr-1–positive isolates from 4 dogs (PET02–04 and PET06) and isolate EC07 as sequence type (ST) 354. Isolates PET01 and PET05, identified from cats, belonged to ST93 and a new ST strain, respectively. Isolates EC08 and EC09, from the patients who shared the same hospital room with the pet shop worker, were ST156 (Table). Results of pulsed-field gel electrophoresis were consistent with multilocus sequence typing results and showed that isolates consisted of 5 types (types I to V; Technical Appendix). Isolate EC07 was clonally related to 4 E. coli strains from dogs, according criteria described by Tenover et al. (10), suggesting possible transmission of mcr-1–harboring E. coli between dogs and the patient. Colistin resistance was successfully transferred to E. coli C600 through conjugation in all isolates, suggesting that mcr-1 was located on transferable plasmids.

These findings suggest that mcr-1–producing E. coli can colonize companion animals and be transferred between companion animals and humans. The findings also suggest that, in addition to food animals and humans, companion animals can serve as a reservoir of colistin-resistant E. coli, adding another layer of complexity to the rapidly evolving epidemiology of plasmid-mediated colistin resistance in the community.

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