OmpK36 Alterations Are Associated with Reduced Imipenem/Relebactam Susceptibility in KPC-2-Producing Klebsiella pneumoniae, with Dose-Dependent Restoration at Higher Relebactam Concentrations.

INTRODUCTION: Resistance to novel β-lactam/β-lactamase inhibitor (BL/BLI) combinations among KPC-2-producing Klebsiella pneumoniae poses a therapeutic challenge. Although resistance driven by KPC-2 mutations following drug exposure has been described, increased MICs are frequently observed in the absence of KPC mutations.

METHODS: We performed whole-genome sequencing and quantitative transcript analysis of 13 KPC-2-producing K. pneumoniae isolates that were non-susceptible to novel BL/BLI combinations, primarily imipenem/relebactam, despite being collected prior to their clinical introduction.

RESULTS: The 13 isolates were non-susceptible to ceftazidime/avibactam (7.7%), imipenem/relebactam (92.3%), or meropenem/vaborbactam (15.4%), and demonstrated dose-dependent reductions in BL MICs with increasing BLI concentrations. Whole-genome sequencing identified truncating ompK36 mutations in 8 isolates (61.5%) and non-truncating variants, including missense substitutions and/or in-frame indels, in 3 isolates (23.1%). Transcript analysis showed ≥2-fold reduced ompK36 expression in 6 isolates with ompK36 mutations, including four with marked (≥3-fold) and one with profound (≥10-fold) suppression. The only isolate that remained resistant to imipenem/relebactam despite a four-fold increase in relebactam concentration exhibited profound suppression of ompK35 and ompK36 and harbored concurrent truncating mutations in both genes. In contrast, blaKPC-2 expression varied by <2-fold across all isolates.

CONCLUSIONS: Loss or reduced expression of ompK36 was associated with reduced imipenem/relebactam susceptibility and dose-dependent responses to increasing relebactam concentrations, without evidence of β-lactamase overexpression or KPC-2 mutation.