Phylum Bacteroids (Sphingobacteria, or Bacteroides/Flavobacterium/Cytophaga group)

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  • Phylum Bacteroids
    • Class Bacteroidetes
      • Order Bacteroidales
        • Family Bacteroideceae (e.g. Bacteroides, Pontibacter)
        • Family Rikenellaceae (e.g. Rikenella, Alistipes)
        • Family Porphyromonadaceae (e.g. Porphyromonas)
        • Family Prevotellaceae (e.g. Prevotella, Xylanibacter)
    • Class Flavobacteria
      • Order Flavobacteriales
        • Family Flavobacteriaceae (e.g. Flavobacterium, Polaribacter)
        • Family Blattabacteriaceae (Blattabacterium)
        • Family Cryomorphaceae (e.g. Algoriphagus, Cryomorpha)
    • Class Sphingobacteria
      • Order Sphingobacteriales
        • Family Sphingobacteriaceae (Sphingobacterium, Pedobacter)
        • Family Saprospira (e.g. Saprospira, Haliscomenobacter)
        • Family Flexibacteraceae (e.g. Flexibacter, Cytophaga)
        • Family Flammeovirgaceae (e.g. Flexithrix, Persicobacter)
        • Family Crenotrichaceae (e.g Chitinophaga, Rhodothermus)

About this phylum


This is a very large and diverse phylum, encompassing at least 140 genera. The phylum is composed of 3 major groups, the Bacteriods (Bacteroidetes), Flavobacteria (Flavobacteriales) and Sphingobacteria (Sphingobacteriales, sometimes known as the “Cytophaga and relatives”). Although very common in the environment, there are no spectacular human pathogens or industrial organisms in this phylum, and so they are generally unfamiliar and not well studied. The Spingobacteria are specifically related to the Chlorobi, and are sometimes grouped together into a single phylum.


These organisms are generally saccharolytic, either obtaining sugars directly from the environment or releasing them from long-chain polysaccharides such as cellulose and chitin. The Bacteroids are generally anaerobes, the Flavobacteria and Sphingobacteria are aerobes. Most are naturally resistant to aminoglycoside antibiotics. Members of this group synthesis and incorporate sphingolipids into their membranes. Sphingolipids are otherwise known from the cytoplasmic and vesicle membranes of eukaryotes, but especially those of the nervous system of animals.


The Sphingobacteria are rod-shaped cells, typically long and thin and often with slightly tapered ends. The most conspicuous exceptions to this are Bacteroides, which are typical rod-shaped cells, and Sporocytophaga, which are typical thin tapered rods during log-phase growth but form spherical resting spores in stationary phase. Cells are motile by gliding, or non-motile.


Sphingobacteria are common in a wide range of environments, especially in soils, sediments, and the gut contents of animals; all environments in which long-chain polysaccharides and their decomposition products are degraded. These organisms are found in both moderate temperatures (mesophiles) and cold environments (cryophiles); thermophiles are not known.


Bacteroids are obligately anaerobic, non-spore-forming rods. The familiar species in this Family are Bacteroides and Porphyromonas. Bacteroides and similar relatives are non-pigmented and saccharolytic or peptidolytic, preferring complex polysaccharides and producing primarily acetate and succinate as their fermentation products. Unlike most polysaccharide degraders,which secrete hydrolytic enzymes into the environment or onto the outer surface of their cells, Bacteroides transport insoluble particles of polysaccharide into the periplasm for degradation; the sugars released by hydrolysis are transported directly into the cell. Porphyromonas are similar morphologically, but are heavily pigmented with protoheme and protoporphyrin, producing very dark brown or black colonies. Their preferred growth substrate is peptides, which are converted into a wide range of organic acids. Both genera are abundant symbionts of anaerobic mucous membranes of humans and other animals. Although they are considered normal flora, they can also be opportunistically pathogenic. Porphyromonas in high numbers is associated with gum disease and gingivitis.

Example : Bacteroides thetaiotaomicron

Bacteroides thetaiotaomicron
: He and L. T. Angenent / Washington Univ. (St. Louis)

B. thetaiotaomicron and related species (e.g. B. fragilis) is a major component of the human colonic flora, comprizing about 1/3rd of Bacteria in feces. (Most of the rest are a variety of Clostridium species.) Gastrointestinal infections caused by Bacteroides are usually caused by a close relative, B. fragilis. B. thetaiotaomicron can degrade and grow on starch, but not other polysaccharides.


The Flavobacteria are obligately aerobic, yellow-pigmented rods, usually long rods with slightly tapered ends. They are common in marine and freshwater environments. Most are non-motile, but some can move rapidly by gliding. Some species can hydrolyze chitin, gelatin, or starch if allowed to attach directly to the surface of these polymers. Many are also proteolytic. They are rarely involved in human disease (usually meningitis in infants), but their innate high-level resistance to many antibiotics often results in a dangerous delay in effective treatment. However, they are very important pathogens of fish, especially in aquacultured trout and salmon.

Example : Flavobacterium johnsoniae

Flavobacterium johnsonii : Shawn S. Nelson, Sreelekha Bollampalli, and Mark J. McBride J. Bacteriol. 2008;190:2851-2857.

F. johnsoniae is a common aquatic (freshwater and marine) chitin-degrading organism. It is a rapid glider; up to 10μm per second, which is often mistaken for flagellar motility. However, this gliding motility requires contact with the substrate; they are incapable of swimming. It is a model system for the study of the unusual mechanism of gliding motility in Bacteroids. Interestingly, non-motile mutants of F. johnsoniae are all also incapable of chitin degradation, implying a link between these processes; the nature of this link is not understood.


Cytophaga and relatives are typically long thin flexible rods, but vary from short rods to filaments or open spirals. Motile by gliding these organisms are capable of degrading a wide range of biopolymers: chitin, agar, starch, cellulose, pectin, nucleic acids, and proteins. These organisms are abundant in nutrient-rich aquatic environments, sediments and soils. Some are pleomorphic, forming rod-shaped or filamentous cells during log phase growth, and spherical or short rod-shaped spores in stationary phase. Cultures of these pleomorphs are often thought to be contaminated because of the different distinct cell morphologies. Colonies are usually feathery as a result of cells streaming on the agar surface. Cells placed on glass slides (e.g. in a wet mount) are often arranged in side-by-side monolayers.

Example : Cytophaga hutchinsonii

Scanning electron micrograph of C. hutchinsonii cells digesting cellulose filter paper. Bar, 10 μm.
Appl Environ Microbiol. 2007 June; 73(11): 3536–3546.

C. hutchinsonii is a long flexible rod-shaped cellulose degrader from soil. Cellulose degradation requires the organisms to be in direct contact with the substrate; extracellular cellulases are not produced. Cells adhere to cellulose, and orient themselves and glide along the axis of the cellulose fibers. Only cellulose and its decomposition products cellobiose and glucose can sustain growth; other simple monoacchrides (except gluconate) cannot be used.