Characterization of Proline-Rich Antimicrobial Peptides with SbmA Transporter-Dependent and Independent Antimicrobial Activity toward Klebsiella pneumoniae

The rapid emergence of antibiotic resistance in Klebsiella pneumoniae is a pressing concern, largely attributed to its rapid development of antibiotic resistance. Our previous work demonstrated that Bac7 (1-35), a proline-rich antimicrobial peptide (PrAMP), displayed potent antimicrobial activity toward K. pneumoniae and aggregated with cell-associated polysaccharides produced by this species. Here, we investigated natural PrAMPs from diverse organisms to explore their antimicrobial activity and interactions with K. pneumoniae cell-associated polysaccharides. The PrAMPs apidaecin Cd3+, Tur1A, and PR-39 demonstrated activity against all tested strains, with minimum inhibitory concentrations (MICs) ≤ 1 μmol L⁻¹, while only Tur1A and PR-39 had biofilm disruption potential. These active PrAMPs shared common structural features, including a proline content above 36% and a net positive charge exceeding +5. Interestingly, both active and inactive PrAMPs aggregated with cell-associated polysaccharides, indicating that the antimicrobial activity and cell-associated polysaccharide aggregation potential are distinct features of PrAMPs. PrAMPs with SbmA transporter-independent activity did not lyse the membrane but caused membrane depolarization, where increased depolarization was observed with colistin-resistant K. pneumoniae. Bacterial mutants lacking LPS modifications conferring colistin resistance displayed rapid uptake of BODIPY-labeled PR-39 and Bac7 (1-35), whereas mutants lacking the SbmA transporter displayed less uptake of PR-39 than Bac7 (1-35). Overall, these findings highlight peptide charge as a critical determinant of membrane interaction and membrane-mediated uptake while also revealing mechanistic insight into how PrAMPs engage with oligosaccharides.