A Pseudomonas aeruginosa endolytic muramidase targets cell-wall peptidoglycan in bacterial competition

Pseudomonas aeruginosa is an opportunistic pathogen that frequently resides in multispecies communities. During chronic infections, P. aeruginosa employs a diverse arsenal of antibacterial weapons to compete with other bacteria for resources and space. Using genetic and biochemical approaches, we identified a type VI secretion system-dependent antibacterial effector-immunity pair, PseM (P. aeruginosa secreted endolytic muramidase) and PA0990 in P. aeruginosa. Our findings demonstrate that PseM functions as an endolytic muramidase, targeting prey bacteria by hydrolytically cleaving cell-wall peptidoglycan, whereas its immunity partner PA0990 provides self-protection. The X-ray crystal structure of PseM reveals a homodimeric configuration, with its active site formed by segments from both monomers. Through structural analysis and macromolecular docking simulations, we further elucidate the substrate-binding residues critical for the activity of PseM. Importantly, we show that PseM contributes to P. aeruginosa growth among bacterial competition. Together, these results uncover a novel antibacterial mechanism mediated by PseM, highlighting the dynamic nature of interspecies and intraspecies competition within bacterial populations.