Effect of leave-on cosmetic antimicrobial preservatives on healthy skin resident Staphylococcus epidermidis

The effect of high doses preservatives in the leave-on cosmetic products to the skin microbiota is not clear. Studies have shown that the preservatives might alter the balance of the skin microbiota.

In this study, we aimed to evaluate antimicrobial effect of nine cosmetic chemical preservatives.

A total of 77 Staphylococcus epidermidis isolates from 46 healthy zygomatic skin samples were characterized by multilocus sequence typing (MLST). Nine preservatives used in leave-on cosmetics were analyzed by testing the minimal inhibitory concentrations (MICs) against S. epidermidis isolates. We also determined the mutant prevention concentration (MPC) and bactericidal kinetics on selected isolates.

More than 17 sequence types were recognized among 77 S. epidermidis isolates. Our data demonstrated that the maximum permitted doses of 2-bromo-2-nitro-1,3-propanediol, ethyl 4-hydroxybenzoate, hexadecyltrimethylammonium bromide, and imidazolidinyl urea were significantly higher than both their MICs and MPCs. We showed that, at the maximum permitted doses, two preservatives could completely kill 10⁷  CFU/mL S. epidermidis in less than 1 h in MH broth.

Our data demonstrated that certain preservatives of the leave-on cosmetics might inhibit or kill S. epidermidis cells and perturb the skin microbiota balance. The determination of the maximum permitted doses of the preservatives should not only be based on the toxicological data, but also antimicrobial susceptibility analysis. This comprehensive evaluation would ensure a balanced and healthy skin microbiota.