8 WORLD GASTROENTEROLOGY NEWS JULY 2014 Editorial | Expert Point of View | WDHD News | WGO & WGOF News | WGO Global Guidelines | Calendar of Events 300,000 microbial genes are common in the sense that they are present in at least 50% of individuals. Up to 98% of genes in the catalogue are bacterial, and the entire cohort of individuals harbors between 1,000 and 1,150 prevalent bacterial species, with at least 160 species per individual6. Interestingly, Bacteroides, Faecalibacte-rium and Bifidobacterium are the most abundant genera but their relative proportion is highly variable across individuals (Figure 1). Network analysis of species abun-dance across different individuals suggested that the overall structure of the human gut microbiota in each in-dividual conforms to discrete and dis-tinct patterns defined by interactions within community members. This hy-pothesis was investigated using a da-taset of metagenomic sequences from American, European and Japanese individuals. The phylogenetic analysis for taxonomic assignments was per-formed by mapping the metagenomic sequences to the reference genomes of fully sequenced bacteria. Multidimen-sional cluster analysis and principal component analysis revealed that all individual samples formed three robust clusters, which have been des-ignated as ‘enterotypes’7. Each of the three enterotypes is identifiable by the variation in the levels of one of three genera: Bacteroides (enterotype 1), Prevotella (enterotype 2) and Rumi-nococcus (enterotype 3). The basis for the enterotype clustering is unknown but appears independent of nation-ality, sex, age, or body mass index. The enterotype concept suggests that enteric microbiota variations across individuals are generally stratified, not continuous. This further indicates the existence of a limited number of well-balanced host-microbial symbi-otic states. Interestingly, it seems that the reported enterotype partitioning is related to long-term dietary patterns8. The Bacteroides enterotype was associ-ated with diets enriched in protein and fat. In contrast, the Prevotella entero-type was linked to diets with predomi-nance of carbohydrates and sugars. Sequencing analysis has allowed describing not only differences in mi-crobial communities between humans but also intra-individual variability (Figure 2). Factors such as diet, drug intake or travelling may have an impact on microbial composition over time in a unique host. A recent study9 collected samples from three different body sites (gut, mouth and skin) of two healthy subjects on a daily basis for a period of 15 and six months, respectively. Com-munity differentiation by body site is highly stable over time but, within the same body site, a low stability across time was noted. At species level, very few microbial members would con-stitute the so-called ‘core human gut microbiota’10, since only 5% of species Figure 2: Temporal variation in genus abundance in fecal samples from a single human individual, who was sampled daily for 15 months. Columns represent microbial composition of each sample at genus level, and colors indicate genera as follows: Bacteroides, red; Faecalibacterium, beige; Akkermansia, pale green; Roseburia, light blue; Parabacteroides, yellow; other bacteroides, black; Bifidobacterium, grey, etc. (Source: from Additional file 8 in: Caporaso JG, Lauber CL, Costello EK et al. (2011) Genome Biol 12(5):R50; with permission).
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