1. History

Bacteroides fragilis was the first Bacteroides species isolated in 1898 as a human pathogen linked to appendicitis among other clinical cases.^[1] 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.^[2]

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.  ^[1] This phylum is sometimes grouped with "Chlorobi", "Fibrobacteres", "Gemmatimonadates", "Caldithrix", and marine group A to form the FCB group or superphylum.^[3] In the alternative classification system proposed by Cavalier-Smith, this taxon is instead a class in the Sphingobacteria phylum.

2. 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^[4] and involved in type 1 and type 2 diabetes.^[1] 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 are only occasionally detected in the gastrointestinal tract, except Capnocytophaga spp. and Sphingobacterium spp. that can be detected in the human oral cavity.^[1]

"Bacteroidetes" are not limited to gut microbiota, they colonize a variety of habitats on Earth.^[5] For example, "Bacteroidetes", together with "Proteobacteria", "Firmicutes", and "Actinobacteria", are also among the most abundant bacterial groups in rhizosphere.^[6] They have been detected in soil samples from various locations, including cultivated fields, greenhouse soils and unexploited areas.^[5] "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.^[5] 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.^[7]

3. 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.^[1] 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.^[8]

4. 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.^[3] 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.^[3]

4.1. 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.^[3] 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.^[3][9] 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^[9] These phyla also branch in the same position based upon conserved signature indels in a number of important proteins.^[10] 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.^[3][9]

5. Phylogeny

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature^[11] and the phylogeny is based on whole-genome sequences.^[12]