Synergistic/antagonistic antimicrobial effects of cosmetic ingredients in combination with 1,2-hexanediol

With the rise of the clean beauty trend in the cosmetics and personal care industry, consumers' interest in cosmetic ingredients, especially preservatives, continues to grow. Paraben, previously the most used preservative in cosmetics, has been excluded from many products owing to its potential risks. Therefore, a movement to lower the content of various preservatives is ongoing. One approach to achieve a suitable level of preservation is to use multifunctional ingredients as preservative boosters. In this study, we aimed to confirm the synergistic antimicrobial interactions between various cosmetic ingredients and 1,2-hexanediol, a preservative introduced as a substitute for paraben, using the checkerboard assay. We also measured the antagonistic effect by measuring the fold changes in the minimum inhibitory concentration of 1,2-hexanediol. Niacinamide, a form of vitamin B3, showed synergistic antifungal activity with 1,2-hexanediol, which lowered the content of 1,2-hexanediol in the oil-solubilized toner formulation. Among the substances, 50000 ppm of methyl methacrylate crosspolymer elevated the minimum inhibitory concentration of 1,2-hexanediol against bacteria and fungi by 2‒8 times. Through this study, we suggest applying the synergistic effects of various cosmetic ingredients in the formulation as a method to effectively reduce the content of preservatives.