The mechanism of high-level carbapenem resistance in Klebsiella pneumoniae: underlying Ompk36-deficient strains represent a threat of emerging high-level carbapenem-resistant K. pneumoniae with IMP-1 β-lactamase production in Japan.

The mechanisms of high-level carbapenem resistance in Klebsiella pneumoniae isolated in Japan were investigated. High-level carbapenem-resistant K. pneumoniae Mkp4437 and a less carbapenem-sensitive K. pneumoniae strain, Mkp4365, were recovered from the same patient. These two strains were found to be homologous by pulsed-field gel electrophoresis, and both strains contained blaIMP-1, blaDHA-1, blaCTXM-14, blaTEM-1, and blaSHV-1. Based on the sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, the lack of Ompk36 was observed in Mkp4437. Direct sequencing of the ompK36 gene demonstrated that a new insertional sequence in the open reading frame of the ompK36 gene was found in Mkp4437. Three clinical isolates (minimum inhibitory concentration [MIC] 2-4 mg/L to imipenem) were identified upon screening the strains of K. pneumoniae isolated in the University hospital with MICs of ≥ 1 mg/L to imipenem. Interestingly, these three isolates all lacked OmpK36. Conjugation of the plasmid harboring IMP-1 to these three OmpK36-deficient strains led to the isolation of high-level carbapenem-resistant transconjugants. In conclusion, the mechanisms of high-level carbapenem resistance in K. pneumoniae entail not only the production of IMP-1 β-lactamase but also the lack of OmpK36. It is vital to monitor for the presence of less carbapenem-sensitive K. pneumoniae strains, which lack OmpK36, because blaIMP-1 transmission to these strains may result in isolates with a high-level carbapenem-resistant phenotype.