Endolysins, Depolymerases, Holins, and VALs: Phage-Derived Proteins for Combating Antimicrobial Resistance.

The global rise of multidrug-resistant (MDR) bacteria has renewed interest in bacteriophage-derived enzymes as alternative antibacterials. Phage-encoded lytic proteins-including holins, endolysins, polysaccharide depolymerases, and virion-associated lysins (VALs)-act via distinct mechanisms to disrupt bacterial cells or their protective polysaccharide barriers. Holins accumulate in the bacterial membrane and suddenly form pores, enabling endolysins to access and cleave peptidoglycan bonds. Endolysins are peptidoglycan hydrolases (glycosidases, amidases, endopeptidases, etc.) that cause rapid osmotic lysis of Gram-positive pathogens and, if engineered (e.g., fused to membrane-penetrating peptides), can kill Gram-negative bacteria from without. Polysaccharide depolymerases degrade bacterial capsules, biofilm exopolysaccharides and LPS O-antigens, stripping away virulence factors and exposing underlying cells to phage or host defenses. VALs (or VAPGHs) are phage tail enzymes that locally cleave peptidoglycan at infection onset to permit viral DNA entry. Each class has been studied in natural and engineered forms (e.g., "artilysins" and "chimeolysins" with optimized activity or spectrum). These proteins exhibit rapid, specific bacteriolysis (often synergistic with antibiotics) and low resistance propensity, but challenges remain in delivery, stability, and immunogenicity. Here, we review the molecular actions, advantages and limitations, and the therapeutic applications of holins, endolysins, depolymerases, and VALs (natural and engineered) against MDR infections.