Molecular Epidemiology and Genomic Insights into Enterobacter cloacae Complex Co-Harboring bla and mcr: Implications for Resistance Gene Transmission.

Objective: This study aims to systematically investigate the molecular epidemiology and genomic characteristics of Enterobacter cloacae complex (ECC) strains globally harboring bla and mcr, as well as the co-existence of drug resistance genes. The goal is to provide insights and recommendations for monitoring clinical drug-resistant strains and super-resistant plasmids.

Methods: This study analyzed 281 ECC isolates harboring both bla and mcr, obtained from NCBI GenBank database (2003-2024) with whole-genome sequencing data. We constructed a phylogenetic tree of ECC strains for phylogenetic analysis. Resistance genes were identified using the ABRicate and CARD databases, and their distribution was examined. Plasmid replicon types for bla and mcr were analyzed with PlasmidFinder, including upstream and downstream genetic environments of these genes.

Results: The predominant genotype combinations harboring both bla and mcr are bla and mcr-9.1, followed by bla and mcr-9.1, with the dominant strain being E. hormaechei ssp. xiangfangensis. The IncHI2(2A) plasmid co-harboring bla and mcr-9 was mainly detected in genomes from the United States. Phylogenetic analysis indicated that the bla-mcr-9-IncHI2(2A) plasmids in ECC strains have a high genetic similarity to mcr-9-IncHI2(2A), suggesting that the latter may have acquired the additional highly transferable bla.

Conclusions: ECC strains have become an important reservoir cluster for bla and mcr-9, and the IncHI2(2A) plasmid is a potential vector for the horizontal co-transmission of carbapenem and colistin resistance genes. Effective monitoring should be implemented to assess the prevalence of co-harboring bla and mcr-9 in individual ECC isolate and even in single plasmid.