Probiotic-Mediated Inhibition of Skin Pathogens and Modulation of Innate Immune Responses in HaCaT Cells

Acne vulgaris is a highly prevalent inflammatory skin condition involving the interaction between skin microbes and host immunity, which lead with the changes in composition and activities of the skin microbiota disturbing its homeostasis. Several probiotic strains have been tested for their anti-pathogenic activity against the main acne-responsible bacteria, their capacity to contrast pathogenic adhesion to HaCaT cells and the in vitro down-regulation of innate immunity. To investigate whether probiotics have direct effects on the growth of Cutibacterium acnes, Staphyloccoccus aureus, Streptococcus pyogenes and Streptococcus epidermidis, the antimicrobial activity of six probiotics was analyzed by two different methods (modified cross-streak and agar well diffusion). The in vitro blockage of pathogens' adherence by the probiotic strains to HaCaT cells was also investigated, under three possible mechanisms: exclusion by adhered probiotics, displacement of adhered pathogens and competition for receptor sites. The inflammatory response was evaluated by the Luminex approach, targeting a selection of innate immune markers in the HaCaT cell culture media. All probiotic strains showed anti-pathogenic activity against the pathogens tested. The inhibition result on HaCaT cells highlights a significant (p < 0.05) competition of all probiotics against all pathogens. Each pathogenic strain alone led to an up-regulation of innate immune markers, while restoration of the microbiome diversity by probiotics presence may suppress inflammation via down-regulation of innate immunity. The results suggest that the tested probiotics could prevent colonization of the skin by relevant pathogens through barrier and interference mechanisms (mainly exclusion), suggesting a potential use in the future conventional therapies of skin disorders.