The phylum Bacteroidota (synonym 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.

Bacteroidota colonize the gastrointestinal tract already in infants, as non-digestible oligosaccharides in mother milk support the growth of both Bacteroides and Bifidobacterium spp.

Based on current classification, the majority of the gastrointestinal Bacteroidota species belong to the families Bacteroidaceae, Prevotellaceae, Rikenellaceae, and Porphyromonadaceae.

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.

Bacteroidota species that belong to classes Flavobacteriales and Sphingobacteriales are typical soil bacteria and are only occasionally detected in the gastrointestinal tract, except Capnocytophaga spp.

The Bacteroidota species also play an important role in protein metabolism by proteolytic activity assigned to the proteases linked to the cell.

[4] Many members of the Bacteroidota genera (Flexibacter, Cytophaga, Sporocytophaga and relatives) are coloured yellow-orange to pink-red due to the presence of pigments of the flexirubin group.

[6] Additionally, four proteins have been identified which are present in all Bacteroidota species except Cytophaga hutchinsonii; this is again likely due to selective gene loss.

A conserved signature indel has also been identified; this three-amino-acid deletion in ClpB chaperone is present in all species of the Bacteroidota phylum except S. ruber.

Through the use of comparative genomic analysis, three proteins have been identified which are uniquely shared by virtually all members of the Bacteroidota and Chlorobiota 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.

[6][12] The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature[2] Chlorobiota Balneolota Rhodothermota Saprospiria Chitinophagia Sphingobacteriia Cytophagia Bacteroidia Flavobacteriia Ignavibacteria Chlorobiia Rhodothermia Balneolia Raineyaceae Microscillaceae Cytophagales Saprospirales Chitinophagales Sphingobacteriales Bacteroidales Flavobacteriales "Kapabacteriales" "Kryptoniales" Ignavibacteriales Chlorobiales Balneolales Rhodothermales Cytophagales Chitinophagales Sphingobacteriales Bacteroidales Flavobacteriales