Analysis of mobile genetic elements carrying mcr genes in clinical Leclercia spp. isolates

Background Leclercia adecarboxylata is an emerging clinical pathogen within the Enterobacteriaceae family and causes infections in immunocompromised and immunocompetent patients. It often demonstrates resistance to broad-spectrum β-lactams and colistin, a last-resort antibiotic against multidrug-resistant gram-negative bacteria. The global spread of mcr genes poses a serious public health concern. This study aimed to examine the prevalence and genetic contexts of mcr genes in clinical Leclercia spp. isolates from China. Results This study identified three mobile colistin resistance genes, namely, mcr-9.1 , mcr-9.2 , and a novel variant mcr-11.1 , among 11 clinical Leclercia isolates. Some of these isolates were assigned to two novel species: Leclercia sp. LecN1 and Leclercia sp. LecN2. mcr-11.1 showed the highest sequence similarity to mcr-9.1 and was located within two novel chromosomal integrative mobile elements (IMEs): Tn 6572 and Tn 6573 . mcr-9.1 was carried by a chromosomally located Tn 7 -family/Tn 6230 -subfamily transposon (Tn 6574 ) and by three IncHI2 plasmids: p707804-mcr, p1106151-mcr, and pJ807-mcr. Meanwhile, mcr-9.2 was identified in plasmid pP10164-2. These mcr genes showed diverse local genetic contexts, including mcr-11.1 – wbuC – qseCB in Tn 6573- family IMEs as well as IS 903B – mcr-9.1 – wbuC – qseCB – exeA – int –IS 26 and its variants within the Tn 6230 -related regions of Tn 6574 and IncHI2 plasmids. Tn 1696 -related regions carried numerous mobile genetic elements (MGEs), further facilitating the dissemination of various antibiotic resistance genes (ARGs). Conclusions This study has identified mcr-11.1 , a novel gene, and many novel chromosomal MGEs (Tn 6572 –Tn 6574 ) in clinical Leclercia spp. isolates, including two new putative species. Along with four identified plasmids, these chromosomal MGEs demonstrated diverse mcr genetic contexts and suggested great potentials for the dissemination of ARGs. This study highlighted a concerning mechanism for mcr gene transmission and indicates the urgent need for ongoing resistance surveillance.