High prevalence of colistin resistance and mcr-9/10 genes in Enterobacter spp. in a tertiary hospital over a decade.

Enterobacter spp. are members of the 'ESKAPE' group of pathogens, which which are recognised as the leading cause of multidrug-resistant (MDR) hospital-acquired infections. Colistin is usually regarded as a last-line therapeutic option for MDR Gram-negative bacilli infections. However, colistin-resistant Enterobacter spp. have emerged in the last decade. Here we investigated the prevalence of colistin resistance and mcr genes in Enterobacter spp. of clinical origin between 2011 and 2020 in a tertiary hospital in China. Colistin resistance rates ranged between 17.1% and 34.5%, with an overall prevalence of 22.2% (190/854). No mcr-1 to mcr-8 genes were identified in the colistin-resistant Enterobacter spp. isolates, while mcr-9 and mcr-10 were detected at rates of 8.4% (16/190) and 12.6% (24/190), respectively. All of the mcr-9/10-positive Enterobacter isolates belonged to the Enterobacter cloacae complex (ECC). Meanwhile, 14.8% (98/664) and 6.0% (40/664) of non-colistin-resistant Enterobacter spp. isolates carried mcr-9 and mcr-10 genes, respectively. For the 40 mcr-9/10-positive colistin-resistant ECC isolates, mcr-9-positive ECC isolates usually co-produced extended-spectrum ß-lactamases (ESBLs) or carbapenemases, while mcr-10-positive ECC isolates produced neither. Most mcr-9/10 genes were located on plasmids. The backbone of mcr-9-harbouring plasmids was conserved, while that of mcr-10-harbouring plasmids was diverse. Our findings revealed a high prevalence of colistin resistance and a silent distribution of mcr-9/10 genes in clinical Enterobacter spp. isolates in China. It is urgent to take steps and interventions to control the prevalence of colistin resistance and prevent the dissemination of mcr-9/10 genes.