Abstract

This study systematically elucidates how ST88-KL186 Klebsiella aerogenes evolved carbapenem resistance through a dual synergistic mechanism involving porin deficiency and inducible AmpC β-lactamase, providing a mechanistic explanation for carbapenem resistance independent of carbapenemase production. From seven homologous strains (single-nucleotide polymorphism [SNP] difference 3–30) consecutively isolated from a patient with post-pancreaticoduodenectomy infection, three subsequent ascites isolates (R5–R7) developed carbapenem resistance after imipenem treatment. All strains carried the OmpK36 G138S mutation, which reduces pore diameter by ~15%, and displayed an inducible AmpC enzyme. However, an additional Q172Ter truncating mutation in OmpK36 was acquired exclusively in the resistant strains (R5–R7). Molecular dynamics simulations confirmed that the truncated protein loses conformational stability, resulting in a complete loss of functional porin. Although the susceptible strains already had a narrowed pore and inducible AmpC enzyme, residual porin function allowed enough drug entry to maintain susceptibility. In contrast, in resistant strains, loss of OmpK36 served as the primary barrier restricting drug entry, while the small amount of imipenem that entered was sufficient to induce high-level AmpC enzyme expression. The synergistic interplay between these two mechanisms: a “permeability barrier” coupled with “enzymatic hydrolysis”—ultimately overcame the carbapenem resistance threshold. This study underscores the importance of phenotypic screening for inducible AmpC enzymes in clinical practice and provides key evidence for optimizing the diagnosis and management of carbapenem-resistant Gram-negative bacteria.