Staphylococcal proliferation on skin models to investigate novel anti-infective treatments against dysbiosis.

Inflammatory skin conditions like Acne inversa are characterized by dysbiosis, an imbalance of commensal and pathogenic bacteria, posing challenges for specific treatments. Consequently, we investigated how biofilm formation, low-nutrition skin environments, and air interfaces influence susceptibility to anti-infective treatments in mixed bacterial cultures. To achieve this in a cost-effective and reproducible manner, we developed a simplified substrate made of gelatin, hyaluronic acid, chondroitin sulphate, and alginate (=Gel-Alg). This in vitro model simulates biofilm cultivation on skin surfaces for aerobic bacteria. We selected Staphylococcus aureus and Staphylococcus epidermidis as two clinically relevant strains, which are also abundant in Acne inversa. We tested single and mixed cultures under different conditions: (i) nutrient broth, (ii) Gel-Alg substrate, (iii) EpiDerm™ commercial skin model, and (iv) ex vivo human skin. Proliferation, measured by colony-forming units, was comparable across most conditions, except for human skin. Metabolic activity, assessed via Presto Blue staining, revealed significant differences. Dual-species cultivation and quantification by viability PMA qPCR indicated dominance of S. epidermidis over S. aureus in skin-like environments. Treatments with biofilm-dissolving rhamnolipids, the antibiotic vancomycin, and combinations thereof demonstrated varying efficacy in single and mixed cultures. While the drug combination could almost completely eradicate staphylococcal biofilms in broth, susceptibility varied in skin-like models and moreover strongly depended on temperature (37°C vs. 32°C). In conclusion, this study suggests that reductionistic models, while mimicking key features, could be valuable for early selective antimicrobial drug development for specific applications like Acne inversa therapy.