Molecular characterization and resistance mechanisms of ertapenem-non-susceptible carbapenem-resistant Klebsiella pneumoniae co-harboring ESBLs or AmpC enzymes with porin loss or efflux pump overexpression

ABSTRACT Carbapenem-resistant Klebsiella pneumoniae (CRKP) poses a significant challenge in clinical settings due to limited treatment options and high mortality rates. While carbapenemase-producing CRKP has been extensively studied, the mechanisms underlying resistance in non-carbapenemase-producing CRKP remain less understood. In this study, we investigated 30 ertapenem-resistant non-carbapenemase-producing CRKP clinical isolates. Antimicrobial susceptibility testing revealed multidrug resistance in all strains. Molecular analyses showed that all isolates carried extended-spectrum β-lactamase (ESBL) and/or AmpC β-lactamase genes. Sequencing of outer membrane porin (OMP) genes revealed mutations or deletions in ompK35 and/or ompK36 in the majority of isolates. SDS-PAGE analysis confirmed reduced or absent expression of the corresponding OMP proteins in these strains. Additionally, quantitative PCR showed that several isolates exhibited overexpression of efflux pump genes. These findings indicate that the combined effects of ESBL or AmpC production, porin loss at both the genetic and protein levels, and efflux pump overactivity contribute to ertapenem resistance in non-carbapenemase-producing CRKP. Our results underscore the complexity of resistance mechanisms and highlight the importance of integrated molecular and phenotypic assessments to inform appropriate antimicrobial therapy. IMPORTANCE This study highlights the complex, multifactorial nature of carbapenem resistance in carbapenem-resistant Klebsiella pneumoniae (CRKP), involving enzyme-mediated resistance, reduced membrane porin expression, and overactive efflux pumps. These findings provide valuable insights into CRKP resistance mechanisms and can aid in controlling CRKP in China.