| Summary: | Background: The dissemination of mobile colistin resistance (<i>mcr</i>) genes is a serious healthcare threat because polymyxins represent “last-line” therapeutics for multi-drug-resistant Gram-negative pathogens. This study aimed to assess the prevalence of colistin resistance and <i>mcr</i> genes and characteristics of clinical <i>Escherichia coli</i> (<i>Eco</i>) and <i>Klebsiella pneumoniae</i> (<i>Kpn</i>) isolates and plasmids carrying these genes in Russia. Methods: A total of 4324 <i>Eco</i> and 4530 <i>Kpn</i> collected in the frame of sentinel surveillance in 2013–2018 were tested for susceptibility to colistin and other antibiotics using the broth microdilution method. <i>mcr</i> genes were screened by real-time PCR. Phylogeny, genomic features and plasmids of <i>mcr</i>-positive isolates were assessed using whole-genome sequencing and subsequent bioinformatic analysis. Results: Colistin resistance was detected in 2.24% <i>Eco</i> and 9.3% <i>Kpn</i>. Twenty-two (0.51%) <i>Eco</i> and two (0.04%) <i>Kpn</i> from distant sites carried <i>mcr-1.1</i>. Most <i>mcr-</i>positive isolates co-harbored ESBLs and other resistance determinants to various antibiotic classes. The <i>mcr</i>-positive <i>Eco</i> belonged to 16 MLST types, with ST359 being most common; <i>Kpn</i> belonged to ST307 and ST23. <i>mcr-1.1</i> was carried mainly in IncI2 (<i>n</i> = 18) and IncX4 (<i>n</i> = 5) plasmids highly similar to those identified previously in human, animal and environmental isolates. Conclusion: This study demonstrated a dissemination of “typical” <i>mcr</i>-bearing plasmids among diverse <i>Eco</i> and <i>Kpn</i> genotypes and across a wide geographic area in Russia. Given the frequent association of <i>mcr</i> with other resistance determinants and potential clinical impact, the continual surveillance of this threat is warranted.
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