Human skin is populated by various microorganisms, the so-called microbiota, such as bacteria, viruses, yeasts, fungi, and archaea. The skin microbiota is in constant contact with the surrounding environment which can alter its eubiotic state. Recently it has been also observed that the application of cosmetic products can alter the balance of the skin microbiota. This effect may be attributed to many factors including the residual activity of the preservatives on the skin. In the present work, we studied the effect of eleven preservatives commonly found in cosmetic products on Propionibacterium acnes, Staphylococcus epidermidis, and Staphylococcus aureus in vitro using 3D skin models and culture-dependent methods. Also, the effect on Histone deacetylase 3 (HDAC3) has been investigated. Among tested combinations, three resulted as the best suitable for restoring a pre-existing dysbiosis since they act moderately inhibiting C. acnes and strongly S. aureus without simultaneously inhibiting the growth of S. epidermidis. The other four combinations resulted as the best suitable for use in topical products for skin and scalp in which it is necessary to preserve the eubiosis of the microbiota. Some of the tested were also able to increase HDAC3 expression. Taking together these data highlight the role of preservatives of skin resident microflora dynamics and could provide a reference for correctly choice preservatives and dosage in cosmetic formulations to preserve or restore homeostasis of skin microbiota.

The skin microbiome is in a very close mutualistic relationship with skin cells, influencing their physiology and immunology and participating in many dermatological conditions. Today, there is much interest in cosmetic ingredients that may promote a healthy microbiome, especially postbiotics, mainly derived from fermented products. In the present work, we studied the effects on skin microbiota of new patented natural oils obtained by unique fermentation technology in vivo. Three fermented oils were evaluated: F-Shiunko (FS), F-Artemisia® (FA) and F-Glycyrrhiza® (FG). The active components were included as single active component or in combination (FSAG) in an emulsion system. A total of 20 healthy women were recruited, and skin microbiota from cheek were analyzed by mean of swab sampling at T0 and T1 (after 4 weeks of a one-day treatment). 16S sequencing revealed that the treatment with fermented oils improved microbiome composition and alpha-diversity. It was shown that higher biodiversity reflects in a healthier microbial ecosystem since microbial diversity decreases in the presence of a disease or due to aging. The treatment also resulted in a more “beneficial” and “younger” microbial community since a significant decrease in Proteobacteria and the increase in Staphylococcus were reported after the treatment with fermented oils.