From The Microbiology Society.
Humans coexist with more than 100 trillion micro-organisms, including bacteria, fungi, archaea, viruses and protozoa that reside in multiple sites of the body, including the skin, lungs and reproductive tract. Those with the highest density and diversity are found in the gastrointestinal (GI) tract (the intestinal microbiota) and make up the microbiome.
The collective genome of all the micro-organisms that reside in and on the human body encodes approximately 3.3 million genes, which is 100-fold greater than the human genome. Whereas approximately 99.9% of genes within the human genomes of two unrelated humans are identical, only 10–20% of genes within the microbiome are shared by two unrelated humans. This emphasises the importance of the microbiome in defining humans as individuals.
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