As soon as multicellular beings appeared on earth, they were colonized by microbes, the first inhabitants of the biosphere. The relevance and robustness of the human-microbe symbiosis is therefore the fruit of a billion years of microbe-eukaryote multicellular coevolution. The term "symbiosis" was introduced by Anton de Bary in the 19thcentury as the rule of life shared by several species, "the living association of different species". The symbiotic association is often built around a large partner, the host, and smaller partners, the symbionts. This view is highly subjective, as in the human-microbe symbiosis, microbes far outnumber humans in terms of cells (× 10) and genes (× 150).
In this symbiotic, largely mutualistic relationship, bacteria benefit from a stable environment (temperature, oxygen, pH, nutrients). The host gains a broad spectrum of digestive, metabolic and nutritional capacities, protection against the intrusion of allogenic/pathogenic microorganisms (barrier effect), controlled stimulation of mucosal and systemic immunity, and even other functions whose diversity is beginning to be recognized.
Each body site corresponds to a specialized ecological niche, characterized by its own microbial consortia, different community dynamics and close interactions with tissues. The most studied of human microbial consortia is the intestinal microbiota. It is extremely diverse in terms of taxonomic, genetic and functional biodiversity.
