Reduced Susceptibility to Cefepime–Taniborbactam in Carbapenem-Resistant Klebsiella pneumoniae Attributed to Penicillin-Binding Protein 3 Modifications

Abstract Background The recent emergence of a carbapenem-resistant Klebsiella pneumoniae (CRKP) strain from a clinical specimen resistant to cefepime–taniboractam (FEP–TAN) raises serious concerns, necessitating further investigation of its resistance mechanisms. Methods Three CRKP clinical isolates were investigated: 1 FEP–TAN-resistant colonizing strain (CRKP068) from a throat swab of a patient with lung infection and 2 FEP–TAN-sensitive controls (CRKP288 and CRKP170141). Whole-genome sequencing and clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9-mediated mutagenesis of the ftsI gene, encoding penicillin-binding protein 3 (PBP3), in the FEP–TAN-sensitive CRKP288 were used to generate isogenic mutants and confirm that PBP3 alterations were responsible for the resistance. Results The concurrent Asp76Val/Ala498Val substitutions in PBP3 conferred FEP–TAN resistance in the mutants while inducing filamentous morphology and membrane permeability defects. These mutations also reduced bacterial fitness, evidenced by decreased growth rates (k) under nutrient limitation, suggesting a tradeoff between resistance and proliferative capacity. Conclusions Although PBP3-mediated FEP–TAN resistance entails measurable fitness costs, especially under nutrient-limited conditions, PBP3 polymorphism surveillance should be incorporated into resistance monitoring.