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

• Colistin resistance rate of Enterobacter spp. in a tertiary hospital in China (2011–2020) was very high (22.2%; 190/854). • mcr-9 and mcr-10 genes were detected both in colistin-resistant and non-resistant Enterobacter spp. isolates. • mcr-9 -positive E. cloacae complex isolates usually co-produced extended-spectrum β-lactamases (ESBLs) or carbapenemases. • mcr-10 -positive E. cloacae complex isolates produced neither ESBLs or carbapenemases. • The backbone of mcr-9 -harbouring plasmids was conserved, while that of mcr-10 -harbouring plasmids was diverse. 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 β-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.