Epidemiology and mechanisms of tigecycline- and carbapenem- resistant Enterobacter cloacae in Southwest China: a five-year retrospective study

• This study firstly reported the epidemiology and mechanisms of tigecycline- and carbapenem- resistant Enterobacter cloacae (TCREC) in mainland China. • A serious situation of TCREC was found with a high incidence rate of 21.7%. • Its high prevalence may be ascribed to potential clonal spread of ST88 isolates harboring bla NDM-1 , which confer a modest level of resistance to tigecycline and high level of resistance to carbapenems, surveillance of TCREC is needed. • Over-expressions of AcrAB-TolC and OqxAB efflux pump contributed to tigecycline resistance in clinical TCREC isolates but not high tigecycline MICs. The prevalence and molecular epidemiology of tigecycline resistance in carbapenem-resistant Enterobacter cloacae (CREC) in mainland China is unknown. In this study, we aimed to investigate the molecular characteristics and resistance mechanism of tigecycline-resistant CREC (TCREC) in Southwest China. We conducted a five-year retrospective study. TCREC isolates were subjected to antimicrobial susceptibility testing, pulsed-field gel electrophoresis, and multilocus sequence typing. We determined the presence of genes, deficiency of outer membrane proteins, and expression of efflux pumps using polymerase chain reaction (PCR), reverse transcription-polymerase chain reaction (RT-PCR), and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). We found that a high incidence rate of 21.7% (36/166) of isolates were positive for TCREC. All isolates were resistant to ertapenem whereas 67% remained susceptible to imipenem and meropenem. ST88 (10/36, 27.8%) was predominant and associated with moderate resistance to tigecycline and high resistance to carbapenems, followed by ST256 (3/36, 8.3%), ST78 (2/36, 5.6%), ST577 (2/36, 5.6%), and ST102 (2/36, 5.6%). bla NDM-1 (6/36, 16.6%) carriers was the most common carbapenemase gene and ST88 (5/6, 83.3%) was the most common type, followed by bla IMP-8 (n=3/36, 8.3%). Coexistence of extensive-spectrum β- lactamase (ESBL) genes and outer membrane protein OmpF and/or OmpC loss were found in 27 out of 36 isolates, in addition, increased co-expression of efflux pump genes acrB and oqxA was identified in 25 out of 36 isolates, which may together contribute to co-resistance to carbapenem and tigecycline. Most ST88 strains carried carbapenemases, especially New Delhi metallo-β-lactamase 1 (NDM-1). Overexpression of efflux pumps contributed to tigecycline resistance. The presence of carbapenemase and/or ESBL genes and lack of outer membrane proteins, but not overexpression of efflux pumps, may confer carbapenem resistance. Reasonable supervision and management the epidemic of TCREC will help to stem the transmission of the isolates.