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Bacteroidota comprises four orders: Bacteroidales, Cytophagales, Flavobacteriales, and Sphingobacteriales. Bacteroidales contains obligately anaerobic, fermentative bacteria including the genus Bacteroides, which dominates the human large intestine. Cytophagales consists mainly of obligate aerobes that degrade complex polysaccharides. Flavobacteriales and Sphingobacteriales include aerobic and facultatively aerobic chemoorganotrophs found in diverse aquatic and terrestrial environments.

Bacteroides constitutes a significant portion of the microbial biomass in the human large intestine. These obligately anaerobic bacteria metabolize sugars and proteins, producing acetate and succinate. Bacteroides thetaiotaomicron specializes in polysaccharide degradation, encoding numerous carbohydrate metabolism genes absent in the human genome. Additionally, Bacteroides uniquely synthesizes sphingolipids, a lipid class rarely found in bacteria.

Cytophaga species are obligate aerobes capable of degrading complex polysaccharides including cellulose, agar, and chitin. These long, slender rods exhibit gliding motility and contribute to cellulose digestion in soil and freshwater environments. Cytophaga hutchinsonii lacks processive cellulases, relying on direct contact with cellulose fibers for degradation. Some species, like Cytophaga columnaris and Cytophaga psychrophila, are fish pathogens causing tissue destruction in aquaculture.

Flavobacterium species inhabit aquatic and food-processing environments, with some like Flavobacterium meningosepticum being pathogenic. Cold-adapted genera such as Polaribacter and Psychroflexus thrive in polar waters and sea ice, representing specialized members of the order Flavobacteriales adapted to extreme environmental conditions.

Sphingobacterium species are gram-negative, nonmotile rods without flagella, commonly found in soil, freshwater, and clinical environments. Many species produce distinctive yellow to orange carotenoid pigments in their cell walls, contributing to their characteristic coloration. These bacteria degrade complex organic compounds and play important roles in bioremediation and nutrient cycling.

Bacteroidota play crucial roles in nutrient cycling and organic matter degradation across diverse environments. While most are beneficial, some species like Bacteroides and Cytophaga are opportunistic pathogens. Their metabolic diversity makes them important in human health, microbiology, and industrial applications, with applications of molecular taxonomy helping identify and classify these organisms.

All members of phylum Bacteroidota are gram-negative, non-sporulating rods exhibiting saccharolytic capabilities. They can be aerobic or fermentative, encompassing obligate aerobes, facultative aerobes, and obligate anaerobes. Many species display gliding motility, though some are nonmotile or use flagella for movement, representing diverse phenotypes within this single phylum.