Two receptor-targeting mechanisms of lambda-like siphophage Gifsy-1 of Salmonella Typhimurium

The receptor-targeting mechanisms by which λ-like siphophages establish infection in gram-negative bacteria remain poorly characterized. This study demonstrated that the λ-like phage Gifsy-1, which exhibits broad lytic activity in Salmonella enterica, employs two receptor-targeting mechanisms mediated by the side tail fiber Stf and central tail tip J dependent on O-polysaccharide (OPS) production. In rough (OPS-deficient LPS) Salmonella Typhimurium strains, Gifsy-1 employs multiple receptor-targeting: the J protein binds OmpC, OmpX, and BtuB, while the Stf protein targets galactose II (Gal II) of the lipopolysaccharide (LPS) core oligosaccharide. OmpC uniquely serves dual roles as the primary receptor (mediating initial adsorption) and secondary receptor (facilitating DNA ejection), whereas the other three receptors function exclusively as primary receptors to prompt high-efficiency phage adsorption. In contrast, the surface OPS in smooth Salmonella Typhimurium blocks J protein interactions with membrane proteins. Instead, Gifsy-1 utilizes core oligosaccharide Gal II, located between the OPS layer and outer membrane, as its necessary receptor for both adsorption and DNA ejection. This study intriguingly identified a shift in the receptor role of the core oligosaccharide in Gifsy-1 infection, which confers Gifsy-1 adaptation to OPS switching. The adaptability of the two targeting-mechanisms contributes to the understanding of the biological functions of Gifsy-1 and provides a blueprint for engineering phage therapy against multidrug-resistant Salmonella enterica.