7 WORLD GASTROENTEROLOGY NEWS JULY 2014 Editorial | Expert Point of View | WDHD News | WGO & WGOF News | WGO Global Guidelines | Calendar of Events Figure 1: Genus abundance variation box plot for the 30 most abundant genera of the human gut microbiota as determined by metagenomic sequencing of human fecal samples. Genera are colored by their respective phylum (see inset for color key). Inset shows phylum abundance box plot. Genus and phylum level abundances were measured using reference-genome-based-mapping (Source: from Figure 1b in: Arumugam M et al, Enterotypes of the human gut microbiome. Nature 2011; 473:174-180; with permission). also telling about biological functions present in the community. The Gut Microbiota Estimates suggest that the colon, the largest ecological niche for microbial communities in the human body, harbors over 1014 microbial cells, i.e. several hundred grams of microbes, most of them belonging to the domain Bacteria. Molecular studies based on 16S rRNA gene sequenc-ing have highlighted that only seven to nine of the 55 known divisions or phyla of the domain Bacteria are detected in fecal or mucosal samples from the human gut4-7. Moreover, such studies also revealed that more than 90% of all the phylotypes belong to just two divisions: Bacteroidetes and Firmicutes (Figure 1). The other divisions that have been consistently found in samples from the human dis-tal gut are Proteobacteria, Actinobac-teria, Fusobacteria, and Verrucomicro-bia. Of the 13 divisions of the domain Archea, only one or two species seem to be represented in the human distal gut microbiota. Thus, at the division level, the human intestinal ecosystem is less diverse than other ecosystems on earth, like soils and ocean waters which may contain 20 or more divi-sions5. However, at a lower taxonomic level (species or strain), there is a considerable variation in the composi-tion of the fecal microbiota among human individuals. Strain diversity between individuals is highly remark-able so that studies have found that a large proportion of the identified strain-level phylotypes are unique to each person5. Each individual harbors his or her own distinctive pattern of bacterial composition. In a cohort of 124 European adult subjects, a total of 3.3 million microbial genes were characterized by full metagenomic analysis of fecal samples6. This effort has provided the first gene catalogue of the human gut microbiome. Each individual carries an average of 600,000 non-redundant microbial genes in the gastrointestinal tract. This figure suggests that most of the 3.3 million genes in the catalogue are shared. It was found that around
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