L. reuteri can inhibit S. aureus adhesion to keratinocytes in vitro.

Probiotics are defined as “Live microorganisms which when administered in adequate amounts confer a health benefit on the host”. Studies in the gut have demonstrated the ability of probiotic bacteria to inhibit colonisation by pathogens through mechanisms including exclusion, competition and displacement of pathogen attachment to host tissues. Recent reports have indicated the potential of topical application of probiotic bacteria for the treatment or prevention of skin infections. We have investigated the protective effect of a Lactobacillus reuteri probiotic on the keratinocyte cytotoxicity of Staphylococcus aureus, a common cause of invasive infections. Normal human epidermal keratinocytes (NHEK) were pre-exposed to L.reuteri (108), simultaneously exposed or exposed 2 hours after S.aureus (106) addition and cell viability was assessed. Simultaneous and pre-exposure to L.reuteri for 1 hour resulted in a significantly lower decrease in NHEK viability compared to cells exposed to S.aureus alone (P=0.0016 and P=0.0003 respectively). Addition of L.reuteri 2 hours after exposure to S.aureus did not significantly affect cell viability (P=0.99). As adhesion of S.aureus to the cell is commonly the first step in infection, we examined whether L.reuteri was able to inhibit S.aureus adhesion. Pre-exposure to L.reuteri resulted in significantly less staphylococcal adhesion to keratinocytes (P=0.026), demonstrating that the probiotic excluded S.aureus from binding. Co-infection with L.reuteri also resulted in significantly lower staphylococcal adhesion (P=0.0078) suggesting that the probiotic competed with S.aureus for adhesion to the cell. In conclusion, we have shown that L.reuteri inhibits staphylococcal-induced cytotoxicity through bacterial exclusion and competition for adhesion.