Carriage of the mcr-9 and mcr-10 genes in clinical strains of the Enterobacter cloacae complex in China: a prevalence and molecular epidemiology study.

OBJECTIVES

Enterobacter cloacae complex (ECC) is among the most common carbapenem-resistant Enterobacteriaceae (CRE) in China. The emergence of mcr has rendered CRE strains resistant to the last-line antibiotic colistin. This study investigated the prevalence of mcr-9 and mcr-10 in carbapenem-resistant ECC (CRECC) and carbapenem-susceptible ECC (CSECC) in China.

METHODS

The CRECC and CSECC strains were collected from different regions of China. Antimicrobial susceptibility tests, conjugation experiments, whole genome sequencing, bioinformatic analysis, and quantitative RT-PCR were performed to understand the mechanisms of resistance and transmission of mcr in ECC.

RESULTS

A total of 534 ECC were collected, among which 57 (10.7%) and 23 (4.3%) were positive for mcr-9 and mcr-10, respectively. The prevalence of mcr-9 in CRECC was significantly higher than that in CSECC (31.8% vs. 3.7%; P < 0.001), while the prevalence of mcr-10 in CRECC was significantly lower (0.8% vs. 5.5%; P < 0.05). Most mcr-9-positive strains (n = 45, 78.9%) exhibited multidrug-resistant phenotype, and four (17.4%) of the mcr-10-positive strains exhibited multi-drug resistance. Coexistence of mcr and carbapenemase genes was commonly observed, including 41 (71.9%) mcr-9-positive strains and one (4.3%) mcr-10-positive strain, and the possibility of co-transfer was confirmed by conjugation experiments. The mcr-positive ECC were highly diverse, while most mcr genes were plasmid-encoded, indicating the important role of plasmids in the transmission of mcr in ECC. Furthermore, the expression of mcr-9 was increased after induction by colistin.

CONCLUSIONS

The widespread mcr genes and co-transfer with carbapenemase genes among ECC strains pose an urgent threat to public health.