# The Co-Existence of mcr-1.1 and mcr-3.5 in Escherichia coli Isolated from Clinical Samples in Thailand

## TL;DR

This study reports the co-occurrence of two colistin resistance genes in E. coli from Thailand, raising concerns about the spread of antibiotic resistance.

## Contribution

The co-existence of mcr-1.1 and mcr-3.5 on different plasmids in E. coli isolates is newly documented in this region.

## Key findings

- Six E. coli isolates carried mcr genes, with two isolates carrying both mcr-1.1 and mcr-3.5.
- mcr-1.1 was found on IncX4 plasmids, while mcr-3.5 was on IncFIB and IncFII plasmids.
- The isolates belonged to ST410 and ST617 lineages, suggesting potential for widespread dissemination.

## Abstract

The emergence of colistin resistance poses a significant threat to its efficacy as a last-line treatment against multidrug-resistant Gram-negative bacterial infections. In this study, 178 multi-drug resistant (MDR) Escherichia coli isolates collected from clinical samples at Queen Sirikit Naval Hospital, Chonburi, Thailand, were evaluated for colistin resistance. Of these, six were identified as mcr gene carriers, mediating colistin resistance. Specifically, mcr-1 was detected in three E. coli isolates, mcr-3 was detected in one E. coli isolate, and mcr-1 and mcr-3 were detected in two E. coli isolates, designated AMR-0220 and AMR-0361. Whole-genome sequencing and bioinformatics analysis revealed that AMR-0220 and AMR-0361 belonged to ST410 and ST617 lineages, respectively. Both isolates carried multiple plasmids, with mcr-1.1 located on an IncX4-type plasmid that is closely related to previously reported mcr-1.1-carrying IncX4 plasmids. In contrast, mcr-3.5 was identified on distinct plasmid backbones: an IncFIB-type plasmid in AMR-0220 and an IncFII-type plasmid in AMR-0361. Overall, our findings demonstrate that the mcr genes found in E. coli isolates in this region are located on different mobile genetic elements, indicating the potential for a widespread dissemination of colistin resistance among Gram-negative bacteria throughout Thailand’s healthcare system.