Ionic regulation of Gram-positive phage adsorption governs host range and improves phage isolation efficiency

Abstract Antimicrobial-resistant Staphylococcus aureus and S. epidermidis remain major causes of invasive infection, yet isolation of therapeutically robust lytic phages targeting Gram-positive pathogens is often constrained by adsorption barriers imposed by thick peptidoglycan cell walls. We demonstrate that these limitations are primarily methodological. By optimising the ionic microenvironment during isolation through divalent cation supplementation (10 mM CaCl 2 and MgSO 4 ), controlled enrichment, phage polyclonal mixtures, and limited lysozyme-assisted release, we recovered 28 genomically distinct lytic phages compared with six obtained using conventional protocols. Host-range profiling across 103 clinical isolates showed broad infectivity, with 51.7% of interactions supporting high-efficiency infection and multiple phages exhibiting cross-species activity. Ion-supplemented adaptive passaging restored and expanded lytic capacity against resistant strains within five rounds. These findings show that ionic regulation of adsorption reshapes Gram-positive phage-host dynamics and provide a scalable framework for precision targeting of resistant Staphylococcus infections.