The invention relates to a method for determining the suitability of a product for use in a cosmetic or
pharmaceutical formulation for topical use, the product being a chemical or biochemical substance of any type (S) or a composition of chemical or biochemical substances of any type (C), the method comprising a step of suspending, in a single culture medium suitable for
bacterial growth, the following species: C. xerosis, M. luteus, S. epidermidis, C. acnes and S. mitis, in order to form, in the co-culture medium, a co-culture of the consortium of bacterial strains of the five aforementioned species, in proportions close to those found on the
skin; followed by a step of bringing the co-culture, in the co-culture medium, into contact with the product for a period of between seven and nine hours, and a step of calculating a
score (Sc): Sc = Σ(RL species X), wherein RL species X represents the following logarithmic reduction for a given bacterial species, and a step of determining the suitability of the product for use in a cosmetic or
pharmaceutical formulation for topical use, wherein the determination step consists in selecting the substance (S) or the composition (C) when the
score (Sc) is less than or equal to 5 and greater than or equal to -5 and rejecting the substance (S) or the composition (C) when the
score (Sc) is greater than 5 or less than -5 and the individual RL species X for each species are between -3 and 3.