Intranasal immunization in mice with spore-associated serine-aspartate repeat-containing protein D from...

Staphylococcus aureus is a major human pathogen causing diseases ranging from mild skin infections to life-threatening conditions due to its diverse virulence factors. The emergence of antibiotic-resistant strains underscores the urgent need for an effective vaccine, though none has yet achieved clinical success. The nasal mucosa serves as a natural reservoir in about 30% of individuals and represents a key source of subsequent infections, particularly in immunocompromised patients. Among its virulence factors, SdrD, a surface-anchored MSCRAMM adhesin associated with methicillin-resistant strains (MRSA), promotes adherence to squamous epithelial cells by binding to desmoglein 1, facilitating epithelial attachment, evasion of mucociliary clearance, and subsequent invasion. Bacillus subtilis, an endotoxin-free organism recognized for its safety, has been extensively developed as a platform for secreted recombinant protein production. This study aimed to utilize the host strain B. subtilis to express and secrete recombinant SdrD antigen, using the control of a strong IPTG-inducible Pgrac212 promoter in a pHT expression vector for B. subtilis. Following IPTG induction, extracellular SdrD was analyzed by SDS-PAGE and confirmed by Western blotting using anti-SdrD polyclonal antibodies. The intranasal administration of concentrated secretions, combined with B. subtilis spores as adjuvants, to mice elicited significant antigen-specific IgA and IgG responses, providing both mucosal and systemic immunity. The co-administration with spores enhanced both the magnitude and persistence of antibody responses compared to antigen alone. These findings demonstrate that B. subtilis can serve as an effective platform for recombinant protein secretion and mucosal antigen delivery, supporting its application in vaccine development against S. aureus infection via mucosal route.