Lacticaseibacillus rhamnosus GG inhibits infection of human keratinocytes by Staphylococcus aureus through mechanisms involving cell surface molecules and pH reduction

Beneficial bacteria represent an emerging tool against topical diseases, including infection caused by Staphylococcus aureus. Here, we investigated several anti-pathogenic mechanisms of the model probiotic Lacticaseibacillus rhamnosus GG against a clinical S. aureus isolate by implementing various mutants lacking important cell surface molecules. We analysed adhesion of L. rhamnosus and competitive adhesion with S. aureus to primary human keratinocytes, L. rhamnosus and S. aureus auto- and co-aggregation, S. aureus growth inhibition, keratinocyte viability increase, and monocyte Toll-like receptor (TLR) activation by L. rhamnosus as such, or with S. aureus. L. rhamnosus mutated in SpaCBA pili exhibited reduced adhesion to keratinocytes, reduced ability to prevent S. aureus adhesion to keratinocytes and reduced co-aggregation with S. aureus. Mutants in cell wall exopolysaccharides showed enhanced adhesion to keratinocytes and TLR activation in monocytes, suggesting involvement of additional cell surface molecules masked by exopolysaccharides. All L. rhamnosus strains inhibited S. aureus growth, likely due to acidification of the medium. Live (but not UV-inactivated) L. rhamnosus significantly reduced inflammatory TLR activation in monocytes by S. aureus. These data suggest the key role of SpaCBA pili and additional contribution of other cell surface molecules as well as secreted components of L. rhamnosus GG in the multifactorial inhibition of S. aureus adhesion and toxicity in the skin niche.