Bacteroidetes general and systematic characteristics

Charles McCarthy

Bacteroidetes it is one of the phyla within which bacteria are classified. This edge comprises four classes (BacteroidiaFlavobacteriaSphingobacteria Y Cytophagia ) and more than 7,000 different species that have colonized all types of habitats on Earth.

They are present in soils, activated sludge, decomposing plant material, compost, oceans, fresh water, algae, dairy products, and sick animals. Distributed in temperate, tropical and polar ecosystems. The Bacteroidetes isolated in open habitats belong mainly to the classes Flavobacteria, Cytophagia and Sphingobacteria.

Bacteroides termitidis. Authors: Janice Haney Carr, CDC Brian J. Beck, PhD, American Type Culture Collection, USCDCP

Bacteroidetes are an important part of the human intestinal tract and that of other mammals and birds. In humans, they intervene in the activation of the immune system and in nutrition, through the degradation of polysaccharides and carbohydrates that generate by-products, reabsorbed by the host, which constitute an important source of energy.

Species of the phylum Bacteroidetes are mostly non-pathogenic, with the exception of the genus Bacteroides, made up of opportunistic pathogens, and of some Flavobacteriaceae, pathogenic to humans, other mammals, freshwater fish, or marine fish.

Article index

  • 1 General characteristics
  • 2 Systematic
    • 2.1 Class I. Bacteroidia
    • 2.2 Class II. Flavobacteria
    • 2.3 Class III. Sphingobacteria
    • 2.4 Class IV. Cytophagia
  • 3 Gut microbiota
    • 3.1 Mutualism
    • 3.2 Bile acid metabolism
    • 3.3 Energy harvesting
  • 4 References

Characteristics general

The bacteria classified in this phylum have a common evolutionary history and a wide morphological, physiological and ecological diversity. They can be short or long, straight, spindle-shaped or thin filaments. They are gram negative and do not form endospores.

They can be facultative anaerobic or strictly aerobic. They can be non-mobile, flagellated, or can move by sliding.

They are chemoorganotrophic, aerobic or facultatively anaerobic with respiratory metabolism, although there are some species with fermentative metabolism.


The edge Bacteroidetes, also known as the group Cytophaga-Flexibacter-Bacteroides, comprises four classes: BacteroidiaFlavobacteriaSphingobacteria Y Cytophagia, that group more than 7000 different species.

In previous classifications, the phylum Bacteroidetes comprised three classes (Bacteroidia, Flavobacteriia, and Sphingobacteriia). However, more recent studies, based on the sequence analysis of the 16S rRNA gene, justify the formation of a fourth class within this phylum, Cytophagia..

Included in this new class are many genera previously classified within the families Flexibacteraceae, Flammeovirgaceae, and Crenotrichaceae. Therefore, the phylum Bacteroidetes comprises at least four phylogenetic groups that are well delineated..

Class I. Bacteroidia

This class includes a single order called Bacteroidales. The order currently includes five families: Bacteroidaceae, Marinilabiliaceae, Porphyromonadaceae, Prevotellaceae, and Rikenellaceae..

It is represented in more than 850 species. Cells of this class are straight, spindle-shaped, or thin rods or coccobacilli with gram-negative staining. They do not form spores.

They are primarily anaerobic, although some are facultatively anaerobic. They ferment simple carbohydrates generating butyrate as a fermentation product, although they can degrade proteins and other substrates. They are non-mobile or mobile by sliding.

Class II. Flavobacteria

The class Flavobacteria includes a single order called Flavobacteriales. The order currently includes three families: Flavobacteriaceae, Blattabacteriaceae, and Cryomorphaceae. This constitutes the largest class of the phylum bacteroidetes, grouping more than 3,500 species.

The cells are rods or filaments that do not form spores, are gram-negative, without gas vesicles and intracellular granules. They are generally multiplied by binary fission.

Members of the family Blattabacteriaceae are intracellular symbionts of insects. The Flavobacteriaceae and Cryomorphaceae families are formed by aerobic or facultatively anaerobic chemoorganotrophic bacteria with respiratory metabolism, although there are some species with fermentative metabolism..

They are not mobile. Many members of these families require NaCl or seawater salts for growth..

Members of the Flavobacteriaceae family are widespread in soil or in fresh, brackish or marine water in temperate, tropical or polar areas, while members of the Cryomorphaceae family are so far restricted to low-temperature marine habitats..

Some members of the Flavobacteriaceae family are pathogens for humans, fish, or amphibians..

Class III. Sphingobacteria

This class includes only the order Sphingobacteriales, which groups three families (Sphingobacteriaceae, Chitinophagaceae and Saprospiraceae), 29 genera and 787 species..

Bacteria of this class are rod-shaped. They are nonmotile, non-spore-forming, with gram-negative staining. Growth aerobic or optionally anaerobic.

With limited fermentation capacities in some members. Some genres, mainly Sphingobacterium, contain high concentrations of sphingophospholipids as cellular lipid components.

Class IV. Cytophagia

This class includes only the order Cytophagales and some other phylogenetic groups that are treated as orders incertae sedis, named thus to indicate the inability to place them exactly within this classification.

Cells of this class can be short rods, long rods, or filaments. Some genera form S-shaped rings, coils or cells. They do not produce spores except for the genus Sporocytophaga. They are mobile by sliding or not mobile. The only genus with flagella is Balneola. Gram-negative staining.

Growth is usually strictly aerobic, but microaerobic and anaerobic growth occur in some limbs. They are chemoorganotrophic. They are widely distributed in nature.

Some genera are marine organisms that require salts of seawater for their growth. Most species are mesophilic, but there are psychrophilic and thermophilic members..

Gut microbiota

Bacteroidetes have colonized the different parts of the gastrointestinal tract in humans. They are also found in the microbiota of other mammals, such as echinoderms, mice, dogs, pigs, and ruminants; from domestic and wild birds, such as chickens, turkeys, goose, and ostriches; and in invertebrates such as millipedes and termites.


Most Bacteroidetes have a mutualistic relationship with their hosts. In humans, they interact with the immune system producing the activation of T cell-mediated responses and control the colonization of potential pathogenic bacteria.

These bacteria generally produce butyrate as the end product of fermentation, which has antineoplastic properties and therefore plays an important role in maintaining intestinal health..

Bile acid metabolism

They also participate in the metabolism of bile acids and the transformation of toxic and / or mutagenic compounds. They contribute to the degradation of polysaccharides in the large intestine, molecules difficult to degrade by mammals, resistant to the action of digestive enzymes.

Bacterial-mediated fermentation of these polysaccharides leads to the release of volatile short-chain fatty acids (mainly acetate, propionate and butyrate) that are reabsorbed by the host. Therefore, these gut bacteria help the host obtain energy from refractory carbohydrate sources..

Energy harvesting

In omnivorous mammals, especially humans, this additional energy source represents between 7% and 10% of the daily allowance. 

In rats, germ-free animals have been shown to excrete 87% more calories in feces than their normal counterparts, and they need to eat 30% more food to maintain their body weight.

 Therefore, the presence of the gut microbiota is necessary for optimal uptake of energy from the diet.


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