KPC-producing Klebsiella pneumoniae isolates have emerged as important pathogens of
nosocomial infections, and
tigecycline is one of the
antibiotics recommended for severe
infections caused by KPC-producing K. pneumoniae. To identify the susceptibility profile of KPC-producing K. pneumoniae to
tigecycline and investigate the role of efflux pumps in
tigecycline resistance, a total of 215 KPC-producing K. pneumoniae isolates were collected. The minimum inhibitory concentration (MIC) of
tigecycline was determined by standard broth microdilution tests. Isolates showing resistance to
tigecycline underwent susceptibility test with efflux pump inhibitors. Expression levels of efflux pump genes (acrB and oqxB) and their regulators (ramA, marA, soxS and rarA) were examined by real-time PCR, and the correlation between
tigecycline MICs and gene expression levels were analysed. Our results show that the
tigecycline resistance rate in these isolates was 11.2%. Exposure of the
tigecycline-resistant isolates to the efflux pump inhibitor NMP resulted in an obvious decrease in MICs and restored susceptibility to
tigecycline in 91.7% of the isolates. A statistically significant association between acrB expression and
tigecycline MICs was observed, and overexpression of ramA was found in three
tigecycline-resistant isolates, further analysis confirmed ramR mutations existed in these isolates. Transformation of one mutant with wild-type ramR restored susceptibility to
tigecycline and repressed overexpression of ramA and acrB. These data indicate that efflux pump AcrAB, which can be up-regulated by ramR mutations and subsequent ramA activation, contributed to
tigecycline resistance in K. pneumoniae clinical isolates.