Esculentin-1a Derived Peptides Potentiate Ceftazidime Activity against Multidrug-Resistant KPC-Producing Klebsiella pneumoniae.

Carbapenemase-producing Klebsiella pneumoniae represents a major clinical threat due to the multidrug-resistant phenotype of this Gram-negative bacterium and to the limited available therapeutic options. Currently, antimicrobial peptides are regarded as promising candidates for the development of novel anti-infective strategies. In this study, the antimicrobial activity of the frog skin-derived antimicrobial peptide Esc(1-21) and its three analogs, an Aib-containing peptide and two variants bearing d-amino acids (i.e., the diastereomer Esc(1-21)-1c and [Aib8]-Esc(1-21)-1c isoform), was evaluated against a panel of clinical carbapenemase-producing Klebsiella pneumoniae isolates. In addition, the ability of these d-amino acid-containing peptides to potentiate the effectiveness of ceftazidime, a widely used third-generation cephalosporin for treatment of severe Gram-negative bacterial infections, was investigated. Mechanistic analyses revealed that Esc(1-21)-1c promotes outer membrane permeabilization, thereby facilitating ceftazidime entry into the periplasmic space. Notably, the peptide/ceftazidime combination treatment resulted in enhanced outer membrane destabilization and increased cytoplasmic membrane perturbation compared to the peptide alone. These findings support a self-reinforcing mechanism in which ceftazidime-mediated cell wall damage further destabilizes the bacterial envelope, promoting additional peptide uptake and amplifying cytoplasmic membrane disruption. Time-kill assays confirmed the superior bactericidal activity of the peptide/ceftazidime combination compared to either agent used alone. Importantly, no outer membrane perturbation was observed in a strain lacking a synergistic response, suggesting that membrane susceptibility is a key determinant of this cooperative interaction. Overall, these results highlight the potential of Esc-derived peptides as adjuvants capable of restoring β-lactam efficacy against multidrug-resistant carbapenemase-producing Klebsiella pneumoniae, offering a promising strategy to combat antimicrobial resistance.