Cosmetics preservatives may affect the healthy skin microbiota?
Bacteria are infamous for their roles in causing skin problems including, but not limited to, acne formations. However, current studies have shown that most of the microbes that compose the skin microbiotas are harmless and play a commensal role under normal condition.  So, what is skin microbiota?
Skin microbiota is the ensemble of microorganisms such as bacteria, fungi, and even viruses that resides on our skin.  It may sound scary knowing we have microbes all over our skin even after thorough cleaning and washing, but the matter of fact is that they play a crucial role for skin homeostasis by educating the host immune cells and protecting the host against various pathogens. 
The imbalance of skin microbiota can lead to skin problems such as eczema, dandruff or acne.  One of the factors that could impact the delicate balance of the cutaneous skin microbiota’s bio-diversified ecosystem are the preservatives added in cosmetics products in the market.
Preservatives are fundamental ingredients included in various cosmetics products in order to keep the products safe, preventing contamination as well as prolonging its shelf life. Preservatives inhibit the growth of pathogenic bacteria that could cause infection and inflammation on skin, but these antimicrobial agents may also affect the growth of skin microbiota. 
Staphylococcus epidermidis (S. epidermidis) is one of the several so-called ‘good’ bacteria on the skin that are known to contribute to skin health. They can inhibit the biofilm formation in skin disorder and kill pathogenic bacteria by stimulating keratinocytes to produce antimicrobial peptides. 
However, S. epidermidis is considered to be an opportunistic pathogen related to nosocomial infections despite its known beneficial functions. One of the reasons that commensal bacteria such as S. epidermidis could become pathogenic is due to environmental stresses, such as the imbalance of the bio-diversified microbial ecosystem.  Having said that, maintaining a balance diversity of microbes on our skin is important to avoid skin problems from occurring.
As the skin microbiome topic in cosmetic market has become the growing interest, some companies have tried to include probiotics ingredients to prevent the inhibition and/or to promote the growth of these ‘good’ bacteria. However, these approaches still remain a challenge as far as regulations and safety of cosmetics products containing probiotics are concerned.  Preservatives, whether natural or synthetic, are fundamental in all cosmetic products. However, how to ensure product safety without affecting the skin's microbiota remains a question to be answered.
Being aware of the importance of maintaining the balance of our skin’s ecosystem biodiversity could provide us with crucial information on the future perspective innovation and formulation of cosmetic products. Although further in vivo studies (e.g. by means of metagenomics analysis) are needed to discover more about how preservatives in cosmetic products might influence our skin microbiota, for the time being, it’s important to figure the ideal concentration that inhibits pathogens growth and, meanwhile, allows maintenance of the microbiota Eubiosis. As for the consumer, we should pay attention to the ingredients that might affect the microbial balance on the skin before purchasing said cosmetic products.
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