Bacteroidetes is a phylum of Gram-negative bacteria found in all ecosystems. They are particularly dominant in soils and the gut of animals and human beings where they exist as degraders of carbohydrates, carbohydrate-based substances, and proteins.
In the gut of animals and human beings, they play a role in the breakdown and release of energy from various organic molecules. They can also act as pathogens or release toxic products during metabolism thus causing harm to the host.
Currently, members of the phylum Bacteroidetes are divided into four main classes that include:
Some species of the class Bacteroidia include:
Like other members of the phylum Bacteroidetes, Bacteroidia species are characterized by a rod-shaped morphology with rounded ends. They are also classified as Gram-negative bacteria and thus have a thin peptidoglycan layer and outer membrane composed of protein, polysaccharides, and lipids.
In culture, they form colonies of between 1 to 3 mm in diameter. These colonies may appear white or gray in color with a smooth, convex surface.
Naturally, they occur singly or in pairs (diplobacilli). Depending on the species, a single cell may measure between 0.5 and 2.0 um in diameter and 1.6 to 12 um in length. They lack structures like flagella and thus are generally non-motile.
* B. fragilis, which is the most common Bacteroides in the colon, has a complex cell envelope composed of multiple layers. This complex structure contributes to their survival in the host colon.
* B. fragilis has also been found to possess a number of extracellular structures like fimbriae (peritrichous fimbriae), pili, and adhesins. Moreover, outer membrane vesicles have been identified with the help of transmission microscopy.
These vesicles may appear as surface blebs or as detached extracellular vesicles and are involved in hemaglutination.
The majority of Bacteroidia species, particularly members of the genus Bacteroides can be found in the human colon where they are part of the normal flora. However, a small population of these bacteria can also be found in the female genitalia.
As a result, they can be passed from the mother to the child during vaginal birth. In the colon, they exist as anaerobes and thus do not require oxygen for growth.
* Some species like Porphyromonas gingivalis and Tannerella forsythensis can be found in the oral cavity.
In the colonic region, these bacteria normally exist as commensals. As such, they compete with some of the other microorganisms for survival without causing harm to the host. Based on a number of studies, some of these bacteria (e.g. B. fragilis) have been shown to be resistant to bile and antibacterials like vancomycin which gives them a competitive advantage over the other microbiota.
While these bacteria are found in the colon under normal circumstances, they can spread to other parts of the body in the event of ruptured GI tract. In such cases, they can cause serious infections (e.g. bacteremia).
Generally, carbohydrates are the primary source of nutrition for Bacteroidia bacteria. As anaerobic bacteria, they break down these polymers through a process known as fermentation.
This process is also beneficial for the host given that some of the energy produced through carbohydrate fermentation is used by the host's cells. On the other hand, some of the bacteria can utilize carbohydrate-based compounds (e.g. some protein that contains carbohydrates) as a source of energy.
* In culture, some of the conditions required for growth include carbon dioxide, succinic acid, vitamin K, and Hemin.
Some species in the class Flavobacteriia include:
Compared to other Bacteroidetes, Flavobacteriia species have been shown to exhibit variations in morphology. While the majority of species are short rods with tapered or rounded ends (they can be rigid or flexible), others, filamentous species, are relatively longer in length.
Depending on the species, Flavobacteriia range between 2 and 5 um in length and 0.3 to 0.5 um in diameter. In culture, researchers have also identified coiled, helical, and coccoid cells. Though they lack flagella, some of the species are capable of motility through a process known as gliding.
* While the species Polaribacter irgensii has been shown to possess flagella, it was not used for motility.
* The DNA G+C content varies between 27 and 56% depending on the species.
* Reproduction is mostly through binary fission. However, budding has also been reported in a few species (E.g. Formosa agariphila).
When grown in agar, they also form colonies that significantly vary in appearance. For instance, while some are round in shape, others may appear flat, convex, or sunken. The color also varies from light and bright yellow to yellow and orange.
Compared to Bacteroidia, members of the class Flavobacteriia are widely distributed in marine and freshwater environments, and soil. However, they can be found in some hosts (human beings, fish, and some insects) where they exist as saprophytes or parasites.
In marine environments, where they occur abundantly, they play an important role in the degradation of organic matter thus contributing to nutrient turnover. Here, they can be found in association with a variety of organisms (both plants and animals) including diatoms and phytoplankton.
* In human beings, some plants, and various animals, Flavobacterria like Flavobacteriaceae are pathogenic and thus cause diseases. For instance, some of the species that infect and cause diseases to fish include F. columnare, F. branchiophilum, and F. johnsoniae.
While these bacteria are widely distributed in the environment, the majority are aerobic and thus need oxygen for growth. A few of the species are microaerophiles while the rest are anaerobes.
While the majority of Flavobacteria need oxygen to break down various organic matter for oxygen (aerobes), some only grow in habitats with very little oxygen (microanaerophiles) while others can produce energy in the absence of oxygen (anaerobes).
The environment in which a given bacteria is found and the mechanism of metabolism are largely influenced by the concentration of oxygen. However, given that the number of anaerobic species is small, fermentation is rare. Some of the main sources of energy include proteins, lipids, and carbohydrates.
Some species of the class Sphingobacteriia include:
Members of this class are characterized by a rod-shaped morphology. Compared to some of the other Bacteroidetes that exhibit structural variations, the majority of these species are straight rods measuring between 0.3 and 0.6 um in diameter and 0.5 to 0.6 um in length.
They are also classified as Gram-negative bacteria and thus consist of a thin peptidoglycan layer and outer membrane. Some species are capable of movement through a mechanism referred to as sliding motility.
* Members of the class Sphingobacteriia have a DNA G+C content of between 36 and 49%.
* They do not have flagella.
* They do not form spores.
Like many other bacteria, Sphingobacteriia can form colonies that may appear yellowish in color as a result of yellow pigmentation. In some of the species (E.g. Flavobacterium species), studies have identified large amounts of sphingophospholipid compounds within the cells that made it possible to distinguish them from members of the genus Flavobacteria.
Sphingobacteriia species can be found living freely in soil and compost manure. Here, free-living species exist as saprophytes and derive energy from dead and decaying matter (e.g. S. thalpophilum and S. multivorum).
Some of the species are opportunistic pathogens capable of causing disease in human hosts (e.g. S. spiritivorum). Opportunistic pathogens have been isolated from blood, feces, synovial and ventricular fluids, urine, and various tissues meaning that they can infect different parts of the body.
The majority of free-living Sphingobacteriia are aerobic mesophiles. As such, they can be found in aerated soils or manure at moderate temperatures. Here, they have been shown to degrade a variety of compounds for energy.
For instance, Sphingobacterium multivorum, which can be found in sludge has been shown to act on and degrade mefenacet which is a herbicide used to protect roots from pests. However, they can also use complex carbohydrates and some amino acids as an energy source.
Cytophagia is the newest class in the phylum Bacteroidetes and consists of many families that had been previously misclassified.
Some of the species found in this class include:
The majority of species in this group are characterized by a rod-shaped morphology (short or long). However, some of the species have been shown to form elongated filaments.
Depending on the species, the rods may measure between 0.3 and 0.5 um in width and 2 to 10 um in length. Some cells may grow to be 50 um in length. Like all the other Bacteroidetes, these Cytophagia are Gram-negative species and do not form spores. Some of the species are capable of motility through gliding.
* While many of the species are rod-shaped, some of the genera exhibit significant variation in shape, ranging from coiled and S-shaped to vibrioids and ring-like structures.
* Some members of the genus Balneola have been shown to possess flagella-like structures.
When grown in culture, these cells form rounded colonies that may be bright yellow, pink, red, or orange in color. The colonies are generally thin with characteristic shallow blobs on the surface.
Cytophagia species can be aerophilic, microaerophilic, or anaerophilic. As such, they are widely distributed in nature and can be found in terrestrial and aquatic environments.
While a good number of species are free-living mesophiles, some species like C. marina can be pathogenic to fish. A few species can be found in habitats characterized by very low temperatures. These are known as psychrophilic.
Cytophagia can use a variety of organic compounds as a source of energy. These include proteins, starch, lipids, chitin, and cellulose among others.
Bacteroidetes are widely distributed in nature and can be found in different environments across the globe. As mentioned, some of the species in this group are pathogenic and capable of causing diseases in human beings, animals, and plants.
In human beings, members of the genus Bacteroides are a normal part of the gastrointestinal microbiota. However, they can cause opportunistic infections if they enter circulation.
As well, Prevotella and Tannerella can cause diseases in the oral cavity while Flavobacterium species affect fish and vegetables. However, there are many species that play an important function in the environment with main benefits to animals.
Aside from members of the phylum Firmicutes, Bacteroidetes are some of the most abundant microbes in the human gastrointestinal tract. In particular, they can be found in large numbers in the colon.
They are also normal microbiota in a variety of other animals including mice, pigs, geese, and ostriches among others.
One of the biggest benefits of Bacteroidetes in the gut involves their role in the breakdown of polysaccharides. For human beings and many other animals, carbohydrates are a significant part of the diet. In the gut, simple sugars are broken down by normal enzymes to produce energy. However, these enzymes may be unable to degrade more complex polysaccharides.
Here, these bacteria are particularly important because of their ability to break down these complex polymers (e.g. xylan and cellulose, etc.). In doing so, they provide extra energy for the host from sources that would otherwise not be degraded.
In a study involving rats, findings showed that the absence of these bacteria results in the excretion of 87% more calories compared to rats with the bacteria.
In other studies, the presence of Bacteroidetes has been shown to promote normal development of the gastrointestinal tract.
* Bacteroidetes are particularly beneficial for herbivorous animals. Compared to many other animals, the diet of herbivores is largely composed of plants. The presence of these bacteria makes it easier to break down plant cell walls and produce more energy.
* In aquatic environments, Bacteroidetes are also found in association with sponges and where they are suggested to play a role in larval development.
Bacteroidetes also play a role in the activation of T-cell mediated responses through their interaction with the immune system thus contributing to the overall health of the host.
Based on a number of studies, the bacterium B. fragilis was found to activate the CD4+ T cells through the production of polysaccharides known as Zwitterionic polysaccharides.
The production of CD4+ Cells stimulates the release of interleukin-10 which in turn serves to prevent the given inflammatory responses and abscess formation.
Compared to some of the other bacteria found in the gastrointestinal tract, studies have shown some members of the phylum Bacteroidetes to be resistant to various antibiotics and bile among other substances. This gives them a competitive advantage.
In addition to this competitive advantage, these bacteria have been shown to play an important role in regulating the number of other normal flora and pathogens in the gastrointestinal tract.
Based on more recent studies, members of the genus Bacteroides were found to help limit the bacterium Clostridium difficile and thus prevent potential infections. This is not only achieved through the activation of the immune system but also through the production of paneth cell proteins that produce antibacterial peptides.
The majority of Bacteroidetes found in the environment belong to the classes, Cytophagia, Sphingobacteriia, and Flavobacteriia. As is the case with Bacteroidetes found in the gut, these bacteria play an important role in the degradation of complex organic matter (proteins and carbohydrates) into constituents that can be used by plants and other organisms in nature.
In the marine environment, this process has also significantly supported the life cycle of various plants and animals. Based on a number of studies, sponges are some of the organisms that have been shown to benefit through their association with these bacteria.
Bacteriology as a field of study
Elizabeth L. Johnson & Stacey L. Heaver & William A. Walters2 & Ruth E. Ley. (2016). Microbiome and metabolic disease: revisiting the bacterial phylum Bacteroidetes.
François Thomas et al. (2011). Environmental and gut Bacteroidetes: the food connection.
Hannah M. Wexler. (2014). Bacteroides: the Good, the Bad, and the Nitty-Gritty.
Noel R. Krieg et al. (2010). Bergey’s Manual of Systematic Bacteriology: The Bacteroidetes, Spirochaetes, Tenericutes (Mollicutes), Acidobacteria, Fibrobacteres, Fusobacteria, Dictyoglomi, Gemmatimonadetes, Lentisphaerae, Verrucomicrobia, Chlamydiae, and Planctomycetes.