Bacteroidota

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. Although some Bacteroides spp. can be opportunistic pathogens, many Bacteroidota 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. Bacteroidota colonize the gastrointestinal tract already in infants, as non-digestible oligosaccharides in mother milk support the growth of both Bacteroides and Bifidobacterium spp. Bacteroides spp. are selectively recognized by the immune system of the host through specific interactions. Bacteroides fragilis was the first Bacteroides species isolated in 1898 as a human pathogen linked to appendicitis among other clinical cases. By far, the species in the class Bacteroidia 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 species of Bacteroides have undergone reclassification. Based on current classification, the majority of the gastrointestinal Bacteroidota species belong to the families Bacteroidaceae, Prevotellaceae, Rikenellaceae, and Porphyromonadaceae.