ABSTRACT
The gut is one of the most populated microbial ecosystems in nature; this microbiota reaches concentrations of 1012 cells/g of feces and comprises about 150 times the number of genes (microbiome) present in the human genome (Bäckhead et al., 2005). Very often the term gut microbiome is used to indiscriminately describe both the microbes and the pool of microbial genes present in the gut (Korecka and Arulampalam, 2012). Human intestinal microbiota contains microorganisms belonging to the three domains of life: Eukarya, Bacteria, and Archaea. Bacteroidetes and Firmicutes are the dominant bacterial phyla in adults, whereas the main archaea identied to date is the methanogenic Methanobrevibacter smithii. All these microorganisms coexist in a complex symbiosis after millennia of coevolution (Reyes et al., 2010). This dense and diverse microbial population displays an enormous metabolic potential; its ability to interact with host tissues and the new possibility of transplantation are reasons to consider the microbiota as a human organ. Metagenomics and next-generation sequencing techniques have allowed, and will allow, great advances in the knowledge of the entire genetic and metabolic potentials of the human microbiome (National Institutes of Health (n.d.); MetaHIT (n.d.)).
