Bacteroidetes is a diverse group of bacteria common in human gut, in marine and freshwater systems. Their levels rise as body weight is decreased. The name of this bacterial phylum changed several times over the past years and is also known as the Cytophaga–Flexibacter–Bacteroides (CFB) group. It includes four classes: Bacteroidia (858 strains), Flavobacteria (3583 strains), Sphingobacteria (787 strains), and Cytophagia (765 strains), and many unclassified strains representing around 7000 different species (NCBI, October 2010). They are non-motile, flagellated, or move by gliding.

Bacteroidetes in the gut are responsible for degradation of high molecular weight organic matter, i.e., proteins and carbohydrates. Their genomes appear to be highly plastic and frequently reorganized through genetic rearrangements, helping them to adaptation to distinct ecological niches.