An Innovative Model Based on Wild Type Bacteria Co-Culture to Identify Cosmetic Ingredients That Respect The Skin Microbiota

Human skin is the home of thousands of microorganisms that are well adapted to that hostile environment. Divesting from the hygienist theory, microorganisms are not considered as potential pathogens anymore, but as symbionts that must be preserved. With this consideration in mind, cosmetic formulators have sought ingredients able to preserve and not interfere with cutaneous bacteria. The aim of the present study is to design a relevant and representative in vitro model of bacteria coculture able to identify ingredients that do not harm skin microbiota. A protocol was designed to obtain a coculture of the five main bacterial representatives of the face’s microbiota: Cutibacterium acnes; Staphylococcus epidermidis; Streptococcus mitis; Micrococcus luteus, and Corynebacterium xerosis, from wild isolates, and in proportions representative of the cheek microbiota. After 8 hours of contact with the test ingredients, bacteria concentrations were measured. A score establishing the impact of ingredients was implemented, based on the logarithmic reduction of the five species. This methodology correctly discriminated between known substances with neutral or bactericidal effects. Finally, four substances used in cosmetic products were evaluated. The proposed model is adapted to screen for new microbiota-friendly ingredients as a first step to design formulations intended to preserve the skin microbiota.