Prevalence of colistin-resistant mcr-1-positive Escherichia coli isolated from diarrheal children in Shanghai, 2016-2021.

BACKGROUND: The emergence of the plasmid-mediated colistin resistance 1 (mcr-1) of Escherichia coli has become a global health concern. This study reports the prevalence of mcr-1 among E. coli isolates from diarrheal children under 5 years old in Shanghai and the genetic characterization of mcr-1-harbouring plasmid.

METHODS: A total of 1,723 E. coli strains were collected from the feces of diarrheal patients in all sentinel hospitals in Shanghai during 2016-2021. Antimicrobial susceptibility testing was performed by broth microdilution and plasmid conjunction transfer assay was carried out using E. coli C600 as the recipient. The mcr-1-positive E. coli strains (MCRPEC) were subjected to molecular characterization and bioinformatic analysis of the mcr-1-bearing plasmids that they harboured.

RESULTS: Only 5 (0.28%) strains were found to harbour the mcr-1 gene using PCR screening. Plasmid conjugation assay and whole genome sequencing indicated EC16500, one MCRPEC strain, co-exhibited mcr-1, blaTEM-1 , blaOXA-1 , qnrS1, qnrS2, arr-3, and catB3, could be conjugated to EC C600 by horizontal transfer with an average efficiency of 3.2 × 10-5 . The plasmid pEC16500 harboured the similar backbone as p70_2_15, pECGD-8-33, pNCYU-29-19-1_MCR1 and pIBMC_mcr1, and was shown to be encoded within a type IV secretion system (T4SS)-containing 32.6 kbp IncX4, next to the pap2-like membrane-associated gene to form a 2.4-kb cassette. Furthermore, sequencing and phylogenetic analyses revealed a similarity between other MCR-1-homolog proteins, indicating that the 5 E. coli isolates were colistin resistant.

CONCLUSION: Our data represents a significant snapshot of colistin resistance mcr-1 genes and highlights the need to increase active surveillance especially among the children under 5 years old in Shanghai. Great effort needs to be taken to avoid further dissemination of plasmid-mediated colistin resistance among clinically relevant Gram-negative bacterial pathogens.