From Natural Product to Topical Antimicrobial Candidate: Evaluating Nigella sativa Seed Oil as a Broad-Spectrum Topical Antimicrobial in Multi-Tiered Preclinical Models

Background: Polymicrobial skin and soft tissue infections (SSTIs) are frequently complicated by methicillin-resistant Staphylococcus aureus (MRSA) and co-colonizing Gram-negative pathogens like Pseudomonas aeruginosa (P. aeruginosa). Mupirocin, the clinical gold standard, is limited by rising resistance and an intrinsic “mupirocin gap” against P. aeruginosa. This study evaluates a novel Nigella sativa (NS) seed oil topical formulation as an alternative. Methods: A 4-tier preclinical platform assessed the NS formulation against MRSA, methicillin-sensitive S. aureus (MSSA), Streptococcus pyogenes, and P. aeruginosa. The pipeline included: (1) in vitro agar diffusion, (2) a gauze biofilm prevention model, (3) an ex vivo porcine ear skin model challenging epidermal lipid barriers, and (4) an in vivo Galleria mellonella model evaluating trans-cuticular systemic protection. Results: The NS formulation produced extensive diffusion zones, completely inhibiting S. pyogenes and outperforming controls against MSSA and P. aeruginosa. In the gauze model, NS achieved complete eradication of MSSA and S. pyogenes, while significantly suppressing MRSA and P. aeruginosa biofilms (p < 0.001). In the ex vivo porcine model, NS yielded >1.5 to >2.5 log reductions across all pathogens at 24 h (p < 0.001). Furthermore, in the in vivo G. mellonella model, topical NS significantly reduced the systemic bioburden of MSSA, S. pyogenes, and P. aeruginosa (p < 0.001), though MRSA reduction lacked statistical significance. Conclusions: The novel NS formulation demonstrates potent broad-spectrum antimicrobial activity. By effectively bridging the “mupirocin gap” against P. aeruginosa and demonstrating significant efficacy against MRSA in in vitro and ex vivo environments, it represents a promising plant-based pre-clinical candidate that strongly warrants future evaluation in live mammalian wound healing models.