Effects of cosmetic ingredients on growth and virulence factor expression in Staphylococcus aureus: a comparison between culture medium and in vitro skin model medium.

The effect of cosmetic ingredients on growth and virulence factor expression in Staphylococcus aureus may vary between culture medium and skin. Researchers have used an in vitro skin model with human heel callus to assess bacterial survival and growth on the stratum corneum of the epidermis. Here, we reconstituted a skin model using keratin as a base (instead of callus) and compared it with brain heart infusion (BHI) medium. We investigated the effects of five cosmetic ingredients (macadamia nut oil, sodium myristoyl methyl taurate, methyl p-hydroxybenzoate, 2-phenoxyethanol, and zinc oxide) on growth and virulence factor expression in S. aureus. Interestingly, the survival pattern of S. aureus in our skin model was similar to that reported in models using callus. Upon the addition of cosmetic ingredients to BHI or skin model medium, the sensitivity of S. aureus to these cosmetic ingredients differed between the two media. Notably, after adding the two tested cosmetic ingredients, the expression level of staphylococcal enterotoxin A in S. aureus reduced significantly in skin model medium compared with that in the BHI medium. Additionally, the expression levels of other S. aureus virulence factors (RNAIII, icaA, and hlb) differed between the two media. These findings suggest that our skin model is a valuable tool for evaluating the effects of cosmetic ingredients on growth and virulence factor expression in S. aureus.