Mechanism for carbapenem resistance of clinical Enterobacteriaceae isolates.

Carbapenemase-producing 'super bacteria', particularly NDM-1 and its variants, have become a major public health concern worldwide. The present study aimed to explore the molecular mechanism for carbapenem resistance of clinical Enterobacteriaceae isolates. Seventy-eight non-repeated Enterobacteriaceae strains resistant to any carbapenem were screened at the First Affiliated Hospital of Zhengzhou University (Zhengzhou, China) between December 2011 and December 2015. Outer membrane porin (OMP) proteins were detected using SDS-PAGE. Carbapenemases, extended-spectrum β-lactamases (ESBLs) and plasmid AmpC enzyme genes were detected using polymerase chain reaction (PCR). PCR and SDS-PAGE demonstrated that 60.3% (47/78) of the strains produced carbapenemases, of which 33.3% (26/78) produced KPC-2 carbapenemase, suggesting that the strains resisted carbapenems primarily through carbapenemases. SDS-PAGE showed that the OMP proteins in the majority of carbapenem-resistant Enterobacteriaceae (CRE) strains were deleted or decreased compared with those in the sensitive strains. Of the 44 Klebsiella strains, 59.1% (26/44) did not express or expressed less OmpK35 or OmpK36. Among the 34 strains of other enterobacteria, 97.1% (33/34) did not express or expressed less OmpC or OmpF. Of all CRE strains, 35.9% (28/78) lost at least one OMP protein, indicating that the strains resisted carbapenems also by producing ESBLs and/or plasmid AmpC enzyme, as well as by losing OMP proteins. The resistance of clinically isolated CRE strains may primarily be attributed to the production of carbapenemases, and may also involve the deletion of OMP proteins or mutation of OMP genes.