Biogenic synthesis, purification and functional elucidation of pheomelanin from <i>Streptomyces spinoverrucosus</i> : a promising platform for next-generation typhoid vaccine development

The present study delineates the biotechnological potential of marine-derived actinomycetes for the biosynthesis of pheomelanin, a sulfur-containing pigment with its prospective application in typhoid vaccine development. The marine isolate <i>Streptomyces spinoverrucosus</i> was cultured under optimized conditions, and pheomelanin was harvested from the post-eumelanin purification supernatant, a typically discarded by-product. In an innovative approach, the cell-free supernatant was repurposed to fabricate melanin-typhoid polysaccharide microparticles, envisioned as a next-generation vaccine platform. These microparticulate constructs were meticulously characterized via Fourier-Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Hestrin's assay, confirming their structural integrity and biochemical composition. Immunogenicity assessment in murine models demonstrated that the microparticles elicited a robust T-cell mediated immune response against the typhoid polysaccharide antigen. Notably, the antibody titers achieved were equivalent to or surpassing those elicited by conventional glycoconjugate vaccines, despite using identical antigen dosages. This underscores the capacity of melanin-based carriers to convert poorly immunogenic polysaccharides into highly immunostimulatory agents. Collectively, these findings advocate the melanin-polysaccharide microparticles as a novel, facile, and immunologically superior alternative to existing typhoid conjugate vaccines, simultaneously advancing the valorization of microbial melanin as a versatile biomedical material.