An iPAS-optimized phage cocktail delivered by a skin-derived ECM hydrogel for antimicrobial-regenerative treatment of seawater immersion wounds infected with MDR E. coli

Seawater immersion (SWI) wounds are prone to multidrug-resistant (MDR) bacterial infections and impaired tissue repair, yet dual-function wound dressings that combine robust antibacterial activity with proregenerative capacity remain scarce. Through shotgun metagenomic sequencing of East China Sea seawater and validation in a rat SWI wound model, we identified Escherichia coli as the predominant cultivable opportunistic isolate in the rat wound model used in this study, with an MDR prevalence of approximately 79%. To overcome the limited efficacy of conventional antibiotics and rapid clearance of topically administered free phages, we isolated lytic phages targeting these seawater-derived MDR E. coli isolates and applied an iterative phage adaptive selection (iPAS) strategy to formulate an optimized cocktail (Cocktail II). This cocktail achieved approximately 93% lytic coverage against these isolates and effectively suppressed bacterial regrowth for up to 24 h in vitro. This phage cocktail was further encapsulated within a skin-derived thermosensitive extracellular matrix (ECM) hydrogel that underwent rapid in situ sol–gel transition within approximately 75 s at physiological temperature, conformed to irregular wound cavities, and enabled sustained phage release for up to 48 h. In a rat model of MDR E. coli -infected SWI wounds, the composite hydrogel reduced the bacterial burden by more than three orders of magnitude within 4 days, and was associated with an increased pro-reparative macrophage marker profile, increased angiogenic marker expression, ordered collagen deposition, and nearly complete wound closure by day 14. To our knowledge, this study is the first to integrate an iPAS phage resistance-mitigation strategy with a bioactive ECM hydrogel delivery vehicle, offering a proof-of-concept precision antibacterial-regenerative therapeutic modality for SWI wound treatment.