Enterobacter cloacae complex (ECC), as one of the common pathogens in Carbapenem-resistant Enterobacteriaceae (CRE), can cause various infections such as sepsis, pneumonia, and urinary tract infections, accounting for approximately 65%−75% of cases [1]. ECC is a diverse bacterial population, primarily consisting of six species, including E. cloacae, E. asburiae, E. hormaechei, E. kobei, E. ludwigii, and E. nimipressurali [2]. According to the hsp60 gene typing method developed by Hoffmann and Roggenkamp [3], ECC is further divided into Clade A-V, including 12 genetic clusters (I−XII) and one unstable sequence cluster (XIII). According to this classification, E. hormaechei has been confirmed as the dominant species in both clinical and environmental source [4]. On May 17, 2024, the World Health Organization (WHO) released its latest bacterial pathogen warning list, and CRE remains classified as a critical priority pathogen in the Bacterial Priority Pathogen List (BPPL) [5]. The emergence of multi-drug resistance (MDR) in ECC has become a significant public health issue [6].

Most ECC isolates produce chromosomally encoded AmpC β-lactamase, which gives them intrinsic resistance to penicillins and first- and second-generation cephalosporins [7]. Due to antibiotic selection pressure in clinical settings, an increasing number of ECC isolates harbouring various acquired resistance genes have been detected [8]. Clinical and genomic studies suggest that the spread of CRE ECC is mainly associated with Klebsiella pneumoniae carbapenemase (KPC), a type A β-lactamase. The gene blaKPC is typically carried on plasmids, commonly found on IncF family plasmids, which can move between species within the Enterobacteriaceae family, thus facilitating its spread [9−13]. At the same time, ECC may acquire colistin resistance through plasmid-mediated mcr (mobile colistin resistance) genes. Since colistin is one of the last-line treatments for MDR Gram-negative bacterial infections [14], these genes superimposed intrinsic β-lactam resistance conferred by chromosomal ampC genes pose a significant challenge to the control of resistant bacterial infections. Furthermore, the resistance mechanisms mediated by mobile genetic elements such as plasmids increase the risk of transmission. Although ECC isolates carrying blaKPC and ECC isolates carrying mcr have been widely identified, the co-existence of blaKPC and mcr in ECC isolates has been reported sporadically. To our knowledge, nine ECC strains co-harbouring blaKPC and mcr in six studies have been identified in China and South Korea. However, each study presents the antimicrobial resistance profile and genomic characteristics of individual ECC strain co-harbouring blaKPC and mcr, with some strains showing high levels of resistance [[15], [16], [17], [18], [19], [20]]. However, these findings are insufficient to fully reflect the complete situation of ECC isolates co-harbouring blaKPC and mcr. In addition, the genetic evolution process of the ECC isolate or plasmid harbouring both blaKPC and mcr genes is still unknown, as systematic data analysis and evidence are lacking. Here, we discuss the molecular epidemiological features of MDR ECC, primarily elucidating their population dynamics, and providing deeper genomic insights into ECC isolates. We analysed the plasmid characteristics of the co-harboured blaKPC and mcr, clarifying the formation pathway of blaKPC-mcr-9-IncHI2(2A). Our study represents the largest number of ECC isolates harbouring both blaKPC and mcr, as well as the highest number of blaKPC-mcr-9-IncHI2(2A) plasmids. This provides a unique opportunity for further prevention and control of the spread of such strains and multidrug-resistant plasmids.