Background: Colistin has been considered as one of the most effective treatments in clinical infections, especially multidrug-resistant (MDR) bacteria-infected patients. The mcr-1 gene leads to polymyxin resistance in China. The present study investigated the prevalence of mcr-1  in a Chinese teaching hospital, and the molecular phenotypes of the positive strains were analyzed.
Methods: A total of 1,112 Escherichia coli  strains were collected from a Chinese University Hospital from January 2015 to January 2016. The mcr-1  gene was detected by PCR. All positive specimens were subjected to susceptibility testing, clinical analysis, phylogenetic analysis, DNA Southern blot hybridization, and gene sequencing.
Results: Six (0.6%) strains of mcr-1 -positive E. coli  were susceptible to imipenem, meropenem, and tigecycline, except for one that presented moderate levels of tigecycline resistance. The six isolates were resistant to cefotaxime and cefepime and divided into six types of sequences. These positive strains carried a total of three plasmids: approximately 33, 61, and >92 kb plasmids. All patients were eventually cured using different types of antibiotics and discharged.
Conclusion: The current study showed that the mcr-1  gene was responsible for the majority of colistin resistance in clinical isolates of E. coli . The gene can be transferred into plasmids containing other drug resistance genes by plasmid–DNA conjugation, which might cause severe consequences in drug-resistant strains. Thus, the widespread popularity of mcr-1  gene should be prevented.
Keywords: mcr-1 , plasmid, colistin, clinical infections