Intestinal Microbiota Composition in Adults

New sequencing technologies together with the development of bioinformatics allow a description of the full spectrum of the microbial communities that inhabit the human intestinal tract, as well as their functional contributions to host health. Most community members belong to the domain Bacteria, but Archaea, eukaryotes (yeasts and protists), and viruses are also present. Only 7-9 of the 55 known divisions or phyla of the domain Bacteria are detected in fecal or mucosal samples from the human gut. Most taxa belong to just two divisions, Bacteroidetes and Firmicutes, but other divisions that have been consistently found are Proteobacteria, Actinobacteria, Fusobacteria, and Verrucomicrobia. Bacteroides,Faecalibacterium, and Bifidobacterium are the most abundant genera, but their relative proportion is highly variable across individuals. Full metagenomic analysis has identified more than 5 million nonredundant microbial genes encoding up to 20,000 biological functions related with life in the intestinal habitat. The overall structure of predominant genera in the human gut can be assigned to three robust clusters, which are known as ‘enterotypes'. Each of the three enterotypes is identifiable by the levels of one of three genera: Bacteroides (enterotype 1), Prevotella (enterotype 2), and Ruminococcus (enterotype 3). This suggests that microbiota variations across individuals are stratified, not continuous. The next steps include the identification of changes that may play a role in certain disease states. A better knowledge of the contributions of microbial symbionts to host health will help in the design of interventions to improve symbiosis and combat disease.