Bacteroidetes

The phylum Bacteroidetes is composed of three large classes of Gram-negative, nonsporeforming, anaerobic or aerobic, and rod-shaped bacteria that are widely distributed in the environment, including in soil, sediments, and sea water, as well as in the guts and on the skin of animals.
Although some "Bacteroides spp." can be opportunistic pathogens, many "Bacteroidetes" are symbiotic species highly adjusted to the gastrointestinal tract. "Bacteroides" are highly abundant in intestines, reaching up to 1011 cells g−1 of intestinal material. They perform metabolic conversions that are essential for the host, such as degradation of proteins or complex sugar polymers. "Bacteroidetes" colonize the gastrointestinal already in infants, as non-digestible oligosaccharides in mother milk support the growth of both "Bacteroides" and "Bifidobacterium spp." "Bacteroides spp.", is selectively recognized by the immune system of the host through specific interactions.
History.
"Bacteroides fragilis" was the first "Bacteroides" species isolated in 1898 as a human pathogen linked to appendicitis among other clinical cases. By far, the ones in the Bacteroidia class are the most well-studied, including the genus "Bacteroides" (an abundant organism in the feces of warm-blooded animals including humans), and "Porphyromonas", a group of organisms inhabiting the human oral cavity. The class Bacteroidia was formerly called Bacteroidetes; as it was until recently the only class in the phylum, the name was changed in the fourth volume of Bergey's "Manual of Systematic Bacteriology".
For a long time, it was thought that the majority of Gram-negative gastrointestinal tract bacteria belonged to the genus "Bacteroides", but in recent years many "Bacteroides spp." underwent reclassification. Based on current classification, the majority of the gastrointestinal "Bacteroidetes spp." belongs to "Bacteroidaceae", "Prevotellaceae", "Rikenellaceae", and "Porphyromonadaceae" families.  
This phylum is sometimes grouped with Chlorobi, Fibrobacteres, Gemmatimonadates, Caldithrix, and marine group A to form the FCB group or superphylum. In the alternative classification system proposed by Cavalier-Smith, this taxon is instead a class in the Sphingobacteria phylum.
Medical and ecological role.
In the gastrointestinal microbiota "Bacteroidetes" have a very broad metabolic potential and are regarded as one of the most stable part of gastrointestinal microflora. Reduced abundance of the "Bacteroidetes" in some cases is associated with obesity. This bacterial group appears to be enriched in patients suffering from irritable bowel syndrome and involved in type 1 and type 2 diabetes. "Bacteroides spp". in contrast to "Prevotella spp". were recently found to be enriched in the metagenomes of subjects with low gene richness that were associated with adiposity, insulin resistance and dyslipidaemia as well as an inflammatory phenotype. "Bacteroidetes" species that belong to classes "Flavobacteriales" and "Sphingobacteriales" are typical soil bacteria and can only occasionally detected in the gastrointestinal tract, except "Capnocytophaga spp." and "Sphingobacterium spp." that can be detected in the human oral cavity.
Bacteroidetes are not limited to gut microbiota, they colonize a variety of habitats on Earth. For example, "Bacteroidetes", together with "Proteobacteria, Firmicutes" and "Actinobacteria", are also among the most abundant bacterial groups in rhizosphere. They have been detected in soil samples from various locations, including cultivated fields, greenhouse soils and unexploited areas. "Bacteroidetes" also inhabit freshwater lakes, rivers, as well as oceans. They are increasingly recognized as an important compartment of the bacterioplankton in marine environments, especially in pelagic oceans. Halophilic "Bacteroidetes" genus "Salinibacter" inhabit hypersaline environments such as salt-saturated brines in hypersaline lakes. "Salinibacter"  shares many properties with halophilic "Archaea" such as "Halobacterium" and "Haloquadratum" that inhabit the same environments. Phenotypically, "Salinibacter" is remarkably similar to "Halobacterium" and therefore for a long time remained unidentified.
Metabolism.
Gastrointestinal "Bacteroidetes" species produce succinic acid, acetic acid, and in some cases propionic acid, as the major end-products. Species belonging to the genera "Alistipes", "Bacteroides", "Parabacteroides", "Prevotella", "Paraprevotella", "Alloprevotella", "Barnesiella", and "Tannerella" are saccharolytic, while species belonging to "Odoribacter" and "Porphyromonas" are predominantly asaccharolytic. Some "Bacteroides spp." and "Prevotella spp." can degrade complex plant polysaccharides such as starch, cellulose, xylans, and pectins. The "Bacteroidetes" species also play an important role in protein metabolism by proteolytic activity assigned to the proteases linked to the cell. Some "Bacteroides spp." have a potential to utilize urea as a nitrogen source. Other important functions of "Bacteroides spp." include the deconjugation of bile acids and growth on mucus. Many members of the "Bacteroidetes" genera ("Flexibacter", "Cytophaga", "Sporocytophaga" and relatives) are coloured yellow-orange to pink-red due to the presence of pigments of the flexirubin group. In some "Bacteroidetes" strains, flexirubins may be present together with carotenoid pigments. Carotenoid pigments are usually found in marine and halophilic members of the group, whereas flexirubin pigments are more frequent in clinical, freshwater or soil-colonizing representatives.
Genomics.
Comparative genomic analysis has led to the identification of 27 proteins which are present in most species of the phylum Bacteroidetes. Of these, one protein is found in all sequenced Bacteroidetes species, while two other proteins are found in all sequenced species with the exception of those from the genus "Bacteroides". The absence of these two proteins in this genus is likely due to selective gene loss. Additionally, four proteins have been identified which are present in all "Bacteroidetes" species except "Cytophaga hutchinsonii"; this is again likely due to selective gene loss. A further eight proteins have been identified which are present in all sequenced Bacteroidetes genomes except "Salinibacter ruber". The absence of these proteins may be due to selective gene loss, or because "S. ruber" branches very deeply, the genes for these proteins may have evolved after the divergence of "S. ruber". A conserved signature indel has also been identified; this three-amino-acid deletion in ClpB chaperone is present in all species of the Bacteroidetes phylum except "S. ruber". This deletion is also found in one Chlorobi species and one Archaeum species, which is likely due to horizontal gene transfer. These 27 proteins and the three-amino-acid deletion serve as molecular markers for the Bacteroidetes.
Relatedness of Bacteroidetes, Chlorobi and Fibrobacteres phyla.
Species from the Bacteroidetes and Chlorobi phyla branch very closely together in phylogenetic trees, indicating a close relationship. Through the use of comparative genomic analysis, three proteins have been identified which are uniquely shared by virtually all members of the Bacteroidetes and Chlorobi phyla. The sharing of these three proteins is significant because other than them, no proteins from either the Bacteroidetes or Chlorobi phyla are shared by any other groups of bacteria. Several conserved signature indels have also been identified which are uniquely shared by members of the phyla. The presence of these molecular signatures supports their close relationship. Additionally, the phylum Fibrobacteres is indicated to be specifically related to these two phyla. A clade consisting of these three phyla is strongly supported by phylogenetic analyses based upon a number of different proteins These phyla also branch in the same position based upon conserved signature indels in a number of important proteins. Lastly and most importantly, two conserved signature indels (in the RpoC protein and in serine hydroxymethyltransferase) and one signature protein PG00081 have been identified that are uniquely shared by all of the species from these three phyla. All of these results provide compelling evidence that the species from these three phyla shared a common ancestor exclusive of all other bacteria, and it has been proposed that they should all recognized as part of a single “FCB” superphylum.
Phylogeny.
The phylogeny is based on 16S rRNA-based LTP release 123 by 'The All-Species Living Tree' Project.
Taxonomy.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI).
Notes
♠ Strains found at the National Center for Biotechnology Information, but not listed in the LPSN
♪ Prokaryotes where no pure (axenic) cultures are isolated or available, i.e., not cultivated or cannot be sustained in culture for more than a few serial passages