Although Elie Metchnikoff, the discoverer of the phagocytosis, had formulated at the beginning of the 20th century the concept that some bacteria present in food, known today as probiotics, could have a positive effect on human health, bacteria present in humans were considered to be either causes of diseases or non-pathogens tolerated by the organism but devoid of any physiological activities of interest.
This paradigm has been present in the mind of most scientists for decades but recent advances have increased our understanding of the human microbiota, i.e. the set of micro-organisms (bacteria, archea, fungi, parasites) that our body hosts, and there has been a shift from thinking about germs that must be eliminated to thinking about achieving a healthy microbiotic environment. The gut microbiome was the first and most thoroughly investigated system since it was known for a long time that fermented food brought health benefits. However interest in the skin microbiota or skin microbiome (genes from the microbial cells) has been on the rise for a few years with several major players of the cosmetic industry trying to bet on probiotic skin care products that are using skin’s natural bacteria to improve the cutaneous condition. When one considers that bacteria hosted by the body outnumber human cells by a factor of 10 and that the microbiome has 150 times more genes than the human body does (Fig 1), it seems indeed reasonable to develop skin treatments that work with the skin’s natural bacteria rather than trying to fight or remove them.
During the past decade scientists have made major progress in understanding the microbiome’s influence upon human health and diseases. It is now widely accepted that the role of the microbiome, thanks to the bioactive metabolites produced by microorganisms, is huge. Microbiome exerts an influence on the central nervous system, on the immune function, etc. Several important new concepts have emerged: i) microbes influence numerous physiological mechanisms in different human organs and systems; ii) the microbiome is essential because of the metabolites it provides to its host; iii) its composition and genetic diversity are good indicators of health (a depleted microbiome is often the cause or is at least associated to diseases); iv) the microbiome is unique to each person although we all share some microbes; and v) the microbiome can be altered with prebiotic and probiotic products or the local application of bacteria for instance.
Microbiota of healthy and diseased skin
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