# Dual resistance to carbapenems and colistin in Enterobacter: Taiwan surveillance of antimicrobial resistance, 2010–2020

## TL;DR

A study in Taiwan found that a significant portion of Enterobacter bacteria are resistant to both carbapenems and colistin, highlighting a growing threat to public health.

## Contribution

The study reveals the prevalence and genetic mechanisms of dual carbapenem and colistin resistance in Enterobacter isolates in Taiwan over a decade.

## Key findings

- 35 out of 41 carbapenem-resistant Enterobacter isolates exhibited dual resistance to colistin.
- Colistin resistance was mainly due to activation of the arnBCADTEF operon, not mcr genes.
- A conserved IncHI2 plasmid carrying blaIMP-8 and mcr-9 circulated across Enterobacter species for over 10 years.

## Abstract

Extensively drug-resistant gram-negative bacteria harbouring dual resistance to carbapenems and colistin represent a critical global health threat. A total of 929 population-representative Enterobacter isolates were systematically collected from 29 hospitals across four regions of Taiwan between 2010 and 2020. Forty-one isolates (4.4%) were nonsusceptible to carbapenems and underwent whole-genome sequencing, resistance gene profiling, plasmid analysis, and antimicrobial susceptibility testing (AST). Among them, 35 isolates (85.4%) exhibited dual resistance to carbapenems and colistin; however, only half (17/35) were detectable by standard phenotypic AST. Colistin resistance was primarily mediated by activation of the chromosomal arnBCADTEF operon, which was frequently inducible and often undetected by standard testing, rather than by mcr-9 or mcr-10. A conserved IncHI2 plasmid carrying blaIMP-8 and mcr-9 persisted and circulated across Enterobacter species for over a decade. Species-specific resistance patterns were observed: E. roggenkampii typically exhibited colistin resistance despite lacking carbapenemases, whereas E. hormaechei commonly carried blaIMP-8 and occasionally lacked the arn operon. Both species exhibited comparable imipenem nonsusceptibility, complicating therapeutic decision-making. The convergence of carbapenem and colistin resistance in a substantial proportion of Enterobacter isolates at the population level makes this genus an emerging priority for hospital infection control and antimicrobial resistance surveillance. These findings underscore the urgent need for improved diagnostics, strengthened antimicrobial resistance surveillance, and optimized treatment strategies.