Polymyxin resistance in carbapenem-resistant Enterobacteriaceae isolates from patients without polymyxin exposure: a multicentre study in China.

Polymyxins were recently approved for the clinical treatment of carbapenem-resistant Enterobacteriaceae (CRE) infections in China. The aim of this study was to determine the prevalence and molecular mechanisms of polymyxin-resistant CRE prior to the clinical application of polymyxin and to evaluate the potential for emerging polymyxin resistance in China. A total of 504 unique CRE isolates were collected from six tertiary-care hospitals in China between October 2016 and September 2017. All isolates underwent antimicrobial susceptibility testing. Clinical, demographic, antimicrobial exposure and infection data were collected from patients' medical charts. PCR detection, Sanger sequencing and reverse transcription real-time fluorescence quantitative PCR (RT-qPCR) were used to investigate the molecular mechanism of polymyxin resistance. A total 19 (3.8%) polymyxin-resistant isolates were identified, including Klebsiella pneumoniae, Escherichia coli, Klebsiella aerogenes and Enterobacter cloacae. Genetic analysis in K. pneumoniae strains identified insertion sequence (IS) elements (n = 3), a stop codon (n = 1) and gene deletion (n = 2) in mgrB and a pmrB missense mutation (T157P) (n = 1). Two E. coli isolates contained mcr-1 and an E. cloacae strain harboured a frameshift in mgrB. Further transcriptional analysis showed that pmrA, pmrB, pmrC and pmrK were significantly upregulated in polymyxin-resistant isolates. Despite the lack of polymyxin exposure, 3.8% of CRE were resistant to polymyxin in China. Both chromosomal and plasmid-encoded mechanisms were identified. Our study suggests that clinical practice should be alert to pre-existing polymyxin resistance among CRE isolates to avoid further dissemination of polymyxin resistance.
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