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

Leclercia adecarboxylata is an emerging clinical pathogen within the Enterobacteriaceae family and causes infections in immunocompromised and immunocompetent patients. It frequently exhibits resistance to a broad spectrum of β-lactams as well as to colistin, which is 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. 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): Tn6572 and Tn6573. mcr-9.1 was carried by a chromosomally located Tn7-family/Tn6230-subfamily transposon (Tn6574) 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 Tn6573-family IMEs as well as IS903B–mcr-9.1–wbuC–qseCB–exeA–int–IS26 and its variants within the Tn6230-related regions of Tn6574 and IncHI2 plasmids. Tn1696-related regions carried multiple mobile genetic elements (MGEs), further facilitating the dissemination of various antibiotic resistance genes (ARGs). In clinical Leclercia spp. isolates, this study identified a novel colistin resistance gene (mcr-11.1), three novel chromosomal MGEs (including Tn6572–Tn6574), as well as evidence for 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 transfer, providing an urgent indication for ongoing resistance surveillance.