Characterization of an Escherichia coli ST156 Isolate Harboring an IncHI2-Type Plasmid Co-Carrying blaNDM-5 and mcr-1.1 Genes from Urban Wastewater Treatment Plants in Fengxian, Shanghai

Background: The emergence of carbapenem-resistant enterobacteriaceae (CRE) co-harboring the mcr-1.1 gene and carbapenemase-encoding genes poses a severe threat to public health. Urban wastewater treatment plants (WWTPs) act as natural reservoirs and hotspots for the dissemination of antimicrobial resistance genes (ARGs). This study aimed to elucidate the molecular characteristics of CRE carrying mcr-1.1 in urban WWTPs. Methods: Samples were collected from the influent of urban WWTPs in Fengxian, Shanghai, from April 2024 to March 2025. mcr-1.1-positive Escherichia coli (E. coli) isolates were screened using real-time PCR, and their antimicrobial susceptibility was determined via the broth microdilution method. Plasmid conjugation assays were performed with E. coli C600 as the recipient strain. Whole-genome sequencing (WGS) was carried out to analyze the molecular characteristics of mcr-1.1-positive E. coli isolates. Results: A total of 312 samples were collected, and 5 (1.6%) mcr-1.1-positive E. coli isolates were identified. All isolates were multidrug-resistant (MDR) but susceptible to tigecycline (TIG). WGS of strain EC0176 (sequence type 156 [ST156], enteroaggregative E. coli [EAEC]) detected the presence of blaNDM-5, blaTEM-1, blaCTX-M-55, and mcr-1.1 as well as related virulence genes. Further analysis revealed that pEC0176 was an IncHI2-type plasmid co-harboring mcr-1.1, blaNDM-5, arr-3, aph(4)-Ia, aph(3′)-Ia, aac(3)-IVa, and mph(A). The plasmid pEC0176 harbored similar backbones as p20014-MCR, p2017.03.02CC_1, pSC2017167-mcr-256k, pEC17CM13_MCR and pGDE043-mcr1, including the type IV secretion system (T4SS) and IncHI-type conjugal transfer genes. Conjugation experiments confirmed that pEC0176 could be horizontally transferred into E. coli C600, with an average transfer efficiency of 3.3 × 10−2. Phylogenetic analysis showed that the MCR-1 protein of EC0176 is closely related to that of two human-derived E. coli strains from China (GenBank accession: AVR64822.1 and WP_076611062.1). Conclusions: To our knowledge, this is the first report of E. coli ST156 carrying an IncHI2-type plasmid co-harboring mcr-1.1 and blaNDM-5 from urban WWTPs in Fengxian, Shanghai. Our findings underscore the severe status of bacterial antimicrobial resistance and emphasize the necessity of enhancing antimicrobial resistance surveillance in urban WWTPs.