Identification of novel plasmid-mediated colistin resistance gene mcr-11.1 in Enterobacter mori isolated from seafood

BACKGROUND: Mobile colistin resistance determinants (mcr) in ESBL-producing Enterobacterales represent a public health concern, given the renewed reliance on colistin as a last-resort agent for multidrug-resistant Gram-negative infections. OBJECTIVES: To resolve the genetic context and transferability of colistin resistance in an ESBL-producing Enterobacter mori isolate (SH93I1) recovered from seafood (MIC of 8 mg/L). METHODS: We characterized the location, genetic environment and mobility of the colistin resistance determinant in SH93I1 using hybrid whole-genome sequencing (short- and long-read data), comparative genomics, in vitro conjugation to Escherichia coli MG1655 and gene cloning in E. coli DH5α. RESULTS: E. mori SH93I1 carried a novel mcr allele, designated mcr-11.1 according to current classification criteria. The allele shared 91% nucleotide identity and 93% predicted amino acid identity with mcr-9.1. mcr-11.1 was located on a 328 kb non-typeable conjugative plasmid that also carried blaCTX-M-14. The mcr-11.1 region formed a 25.5 kb segment bracketed by IS3-family elements and included wbuC-qseBC-vapBC and the arnBCADTEF operon. Conjugation experiments confirmed plasmid transfer, with co-transfer of resistance phenotypes; however, colistin MICs were not increased in the E. coli MG1655 transconjugants or in E. coli DH5α strains expressing mcr-11.1, alone or with wbuC and qseBC. CONCLUSIONS: We describe mcr-11.1 on a large conjugative plasmid in E. mori from seafood, with evidence consistent with mobilization of a chromosomal fragment from L. adecarboxylata. Although mcr-11.1 did not increase colistin MICs in E. coli, its presence on a multidrug-resistance plasmid highlights the potential for dissemination under co-selection and future adaptation in clinically relevant hosts.