A New Endolysin Lys59: A Broad-Spectrum Phage Endolysin Targeting Both Gram-Negative and Gram-Positive Bacteria

To address the emerging multidrug-resistance crisis caused by Klebsiella pneumoniae, we expressed the endolysin Lys59 derived from phage VB_KpP_HS106 and performed a comprehensive analysis of its antibacterial activity and structural features. Molecular modeling revealed that Lys59 carries a highly positively charged N-terminus and an amphipathic helix at the C-terminus. In vitro antibacterial assays showed that Lys59 exhibited significant bactericidal activity against K. pneumoniae with an approximately 4 log reduction at 50 µg/mL in 2 h. Meanwhile, Lys59 exhibited potent, broad-spectrum activity against both Gram-negative and Gram-positive bacteria. Stability analysis indicated that Lys59 retained high activity over a pH range of 3–9 and a temperature range of 4–55 °C. Notably, the antibacterial activity of Lys59 was found to be regulated by metal ions. Molecular docking indicated that K+ can enhance binding stability by interacting with ASN35 and VAL57. In contrast, Mg2+ and Ca2+ suppressed catalytic function by binding to the essential GLU17 residue. Furthermore, treatment with 200 µg/mL of Lys59 resulted in a 44.6% reduction in K. pneumoniae biofilm biomass. Overall, this study identified a phage-derived endolysin with broad-spectrum antimicrobial activity and demonstrated its potential as an antibacterial agent against multidrug-resistant K. pneumoniae.