Antibacterial Activity of Bacteriophage Lytic Enzyme Ply900

<i>S. suis</i> is a prominent zoonotic pathogen responsible for diseases such as arthritis in piglets, swine septicemia, and meningitis. The emergence of multi-drug resistance (MDR) underscores the urgent need for the development of novel antibacterial strategies. In this context, a systematic evaluation of the antibacterial potential of the bacteriophage lytic enzyme Ply900 was conducted in this study, along with an analysis of its domain functions and an in vivo study of its therapeutic dynamics. Ply900 exhibits potent in vitro lytic activity against multiple bacteria, including <i>Streptococcus suis</i>, <i>Streptococcus agalactiae</i>, and <i>Staphylococcus aureus</i>. Notably, it possesses broad biochemical stability, with tolerance to diverse environmental conditions. In a mouse model of <i>S. suis</i> serotype 2 SC19 infection, both the direct Ply900 treatment group and the triple therapy group achieved effective eradication of <i>S. suis</i>, with markedly improved survival rates. The remaining bacteria remained susceptible to Ply900, with no evidence of induced resistance development. Mechanistic analysis revealed that the SH3B domain of Ply900 enhances targeted cleavage efficiency by binding synergistically to peptidoglycan with the CHAP domain, with CYS-34, HIS-59, and ASP-28 serving as key amino acid sites for Ply900's cleavage activity. Collectively, these findings lay the foundation for the potential dual applications of the lysin Ply900, both in the clinical treatment of <i>S. suis</i> infections and in the prevention and control of these pathogenic bacteria in livestock farming.