# Co-occurrence of mcr-9 and blaNDM-1 in carbapenem-resistant Enterobacter hormaechei from burn patients

## TL;DR

This study identifies carbapenem-resistant Enterobacter hormaechei strains from burn patients that carry two resistance genes, posing a risk of spreading antibiotic resistance.

## Contribution

The co-occurrence of mcr-9 and blaNDM-1 in Enterobacter hormaechei is reported, with insights into plasmid mobility and genetic diversity.

## Key findings

- Three CR-E. hormaechei strains co-harboring blaNDM-1 and mcr-9 were isolated from burn patients.
- The mcr-9 gene was found on IncHI2 plasmids, and blaNDM-1 on IncX3 or IncHI2 plasmids.
- All strains belonged to sequence type 97 and showed high genetic relatedness.

## Abstract

Carbapenem-resistant Enterobacter (CRE) has emerged as a critical clinical concern due to its broad multidrug resistance. This study aims to characterize the features of clinical CRE strains co-harboring the blaNDM-1 and mcr-9 genes from three burn patients.

This study collected 110 non-repetitive carbapenem-resistant Enterobacteriaceae (CRE) from clinical settings. blaNDM-1 and mcr-9 genes were identified by PCR, and strains were identified via MALDI-TOF MS and 16S rRNA sequencing. Minimum inhibitory concentrations (MICs) of common antimicrobial agents were determined by the broth microdilution method. The conjugation experiment was used to verify the transfer of resistance plasmids. Whole-genome sequencing (WGS) was performed using NovaSeq and PacBio_HIFI platforms, and analyzed through bioinformatics to characterize the resistance genes, virulence factors, plasmid profiles, and genetic relatedness of the bacterial strains.

Three carbapenem-resistant Enterobacter hormaechei (CR-E. hormaechei) strains co-harboring blaNDM-1 and mcr-9 were isolated from burn patients, and exhibited broad multidrug resistance with 100% conjugation efficiency. Whole-genome sequencing (WGS) showed that CR-1025 and CR-1050 carried 12 additional resistance genes (targeting aminoglycosides, β-lactams, etc.), while CR-1051 harbored 9. The mcr-9 gene was localized on IncHI2-type plasmids, and the blaNDM-1 gene was localized on IncX3-type plasmids in CR-1025 and CR-1050 and on an IncHI2-type plasmid in CR-1051. Multilocus sequence typing confirmed all strains as sequence type 97 (ST97), and Venn diagram analysis showed close genetic relatedness among the strains.

In conclusion, CR-E. hormaechei co-harboring blaNDM-1 and mcr-9 exhibits genetic diversity and plasmid mobility, posing a risk of cross-species transmission and clonal spread mediated by mobile genetic elements. Urgent measures are required to curb the dissemination of such multidrug-resistant strains in clinical settings.