Fusobacteria is a phylum - Fusobacteria is a phylum under the domain Bacteria. It consists of the families Fusobacteriaceae and Leptotrichiaceae and several genera including Cetobacterium, Propionigenium, Fusobacterium, and Ilyobacter (belonging to the family Fusobacteriaceae) while the genera Leptotrichia, Sebaldella, Streptobacillus and Sneathia belong to the family Leptotrichiaceae.
They can be found in different types of habitats - While the phylum only consists of two known families, it consists of diverse species that can be found in different types of habitats.
While members of the genus Fusobacterium are commonly found in the oral cavity of human beings as normal oral flora (some species can be found in mud and other animals), a few species classified under this phylum can be found in nature where they exist freely while others like Streptobacillus can cause zoonotic diseases.
Members of the phylum are either microaerophilic or obligate anaerobes - As such, they can only survive in environments with very little oxygen or no oxygen. This is because they can be easily killed by the normal concentration of oxygen in the atmosphere.
DNA G+C Content - The majority of species in this phylum consist of DNA G+C content of between 26 and 34 mol%. However, the species Fusobacterium has been shown to contain between 52 to 57mol % of DNA G+C which is higher compared to the other species.
Despite the diversity of species in the phylum Fusobacteria, the two main families in this phylum Fusobacteriaceae and Leptotrichiaceae are, for the most part, distinguished based on the branching in the 16s rRNA gene trees.
Some examples of bacterial that belong to the phylum Fusobacteria include:
· Fusobacterium nucleatum
· Fusobacterium somerae
· Fusobacterium equinum
· Fusobacterium canifelinum
· Streptobacillus species (e.g. S. felis, S. moniliformis, S. ratti, and S. hongkongensis)
· Sebaldella termitidis
· Sneathia amnii
· Sneathia sanguinegens
Morphological and Structural Characteristics
The word Fusobacteria was derived from the Latin word "fusus" which means spindle (or spindle-shaped). Virtually all members of the phylum are characterized by this morphology and are therefore described as having a fusiform appearance (having pointed or tapered ends). However, because they are elongated, they are classified as bacilli bacteria.
While the majority of species in this group have a characteristic slender spindle-shaped, they have been shown to vary in size with limited motility. When viewed under the microscope, some of the species like Fusobacterium nucleatum occur in pairs while others like Streptobacillus species form elongated chains (unbranched).
Structurally, all members of the Phylum Fusobacteria are classified as Gram-negative bacteria. As such, they are characterized by a thin cell wall consisting of a thin peptidoglycan layer. For this reason, they are unable to retain the primary stain used in gram staining which causes them to appear pink or pale red in color (color of the counter/secondary stain).
While members of this phylum have a thin cell wall, they, like many other Gram-negative bacteria, have an outer membrane covering the thin cell wall. This membrane has been associated with a number of functions ranging from acting as a protective barrier to regulating material that enters the cell.
Like many other bacteria, members of the phylum Fusobacteria are characterized by a simple cell structure lacking membrane-bound organelles.
Some of the organelles that can be found in these species include:
· Plasmids - e.g. the shuttle plasmid found in Fusobacterium nucleatum
· DNA - Not contained in a nucleus
· Ribosome - Involved in protein synthesis
· Inner cell membrane
* In the outer membrane of species like Fusobacterium nucleatum, researchers have been able to isolate lipopolysaccharides (LPS). These are large molecules that consist of lipid and polysaccharide. While other Fusobacteria also possess this endotoxin in their outer membrane, their capacity to stimulate the secretion of cytokines by the host varies between species.
For the most part, members of the phylum Fusobacteria are normal animal flora and are therefore mostly associated with different types of animals. For instance, while some members of the family Leptotrichiaceae are found in the respiratory tract of rodents like squirrels and gerbils (e.g. Streptobacillus moniliformis), others, like Leptotrichia species can be found in the intestinal tract, the oral cavity, and genitourinary tract in human beings.
Sneathia amnii, a species of the genus Sneathia, on the other hand, can be found in the amniotic fluid where it can cause inflammation under certain conditions.
Most members of the genus Fusobacterium are commonly found in the mouth of human beings where they exist as normal oral flora. With the exception of a few species, e.g. Fusobacterium necrophorum, the majority of these species rarely cause serious diseases. In marine and freshwater environments, some species have been found in different types of fish and crabs and a number of other aquatic organisms.
For instance, Cetobacterium somerae, a species of the genus Cetobacterium (also found in human beings) has been isolated from Arapaima gigas (also known as paiche) in the freshwaters of the Amazon River basin. As well, species like Psychrilyobacter atlanticus, which is a member of the genus Psychrilyobacter, have been isolated from the intestinal mucosa of some marine fish as well as marine sediment in the Atlantic Ocean.
* Marine and aquatic members of the phylum Fusobacteria include some of the species with the genera Llyobacter, Cetobacterium, Psychrilyobacter, and Propionigenium.
In both terrestrial and aquatic organisms, the majority of Fusobacteria exist as microaerophiles or as obligate anaerobes. This means that they can only survive and thrive in environments with very little to no oxygen.
Streptobacillus moniliformis belonging to the Genus Streptobacillus is one of the best examples of a microaerophile within the Phylum Fusobacteria. Found in small rodents like rats and mice among others, the bacterium has been shown to be a highly fastidious organism that requires microaerophilic conditions to thrive.
In culture, consisting of Trypticase soy agar, serum, 20 percent blood, and 8 and 10 percent carbon dioxide the bacterium grows slowly; taking an average of 2 to 3 days. While microaerophiles are incapable of growth at normal atmospheric oxygen concentration, they need oxygen at lower concentrations for respiration.
Obligate anaerobes make up the majority of Fusobacteria. Unlike microaerophiles such as Streptobacillus moniliformis, obligate anaerobic Fusobacteria cannot grow in the presence of oxygen given that they largely depend on anaerobic respiration. For this reason, they can be found in habitats characterized by absence or very little (less than 0.5 percent) oxygen.
Here, members of the genus Fusobacterium are some of the most common obligate anaerobes. In the mouth, some of these species can be found in the periodontal pockets and gingival crevicular fluid where they form biofilms.
Compared to microaerophiles which require some oxygen concentration for respiration, metabolism in obligate anaerobic Fusobacteria occurs in the absence of oxygen. In the process, this results in the development of plaque by species like Fusobacterium nucleatum.
Some of the main sources of energy for the bacterium include amino acids like Histidine, Glutamate, and Lysine as well as glucose.
In a study involving the bacterial cells during their resting phase, the energy derived from the fermentation of amino acids (lysine and glutamate) was found to help in the uptake of glucose. In conditions with high amounts of carbohydrates/sugars, the sugar is stored as polyglucose rather than undergoing glycolysis to produce energy. It's not until the amino acid sources were depleted that the bacterium starts to breakdown the polymer in anaerobic conditions to produce energy.
* Obligate anaerobes lack important enzymes like catalase and peroxidase that help in the detoxification process of oxygen free radicals and peroxidase. For this reason, they cannot carry out oxidative phosphorylation because they would otherwise be destroyed by oxygen.
Compared to obligate anaerobes, microaerophiles need oxygen for metabolism. While they have protective enzymes, these enzymes are not as effective as those found in aerobic bacteria or facultative anaerobes. For this reason, metabolism only occurs in conditions characterized by lower oxygen concentration.
* Compared to the other organisms within the Phylum Fusobacteria, members of the genus Fusobacterium are distinguished by their ability to produce butyric acid alone. In the other genera, different types of species produce n-butyric acid along with such metabolic end produces like iso-valeric acids and iso-butyric acid.
Free-living Fusobacteria: A number of species under the phylum Fusobacteria have been shown to live freely without associating with other organisms in their surroundings. Most of these include species found in marine environments such as members of the Genus Llyobacter and Psychrilyobacter.
In particular, one of the best examples of free-living Fusobacteria is the species Psychrilyobacter atlanticus which can be found in marine sediment of the Atlantic Ocean. In this and other aquatic environments, the bacterium has been shown to play an important role in the degradation of various materials originating from land as well as the open sea through fermentation.
Generally, the process starts with the hydrolysis of more complex material (polymers) by exoenzymes to produce monomers that undergo the fermentation process to produce such products as alcohols, carbon dioxide, hydrogen, and short fatty acid chains - In the case of Psychrilyobacter atlanticus, acetate and hydrogen are the primary products of fermentation.
Commensalism: Commensalism refers to the relationship between two organisms where one of the organisms benefits from the relationship while the other neither is harmed nor benefits. In human beings and other types of animals, both terrestrial and aquatic animals, various members of the phylum Fusobacteria have been shown to contribute to the normal flora in the oral cavity, intestinal tract, as well as the urogenital tract, etc.
Like free-living Fusobacteria residing in aquatic environments, these species (e.g. members of the genus Fusobacterium) depend on their hosts for conducive conditions that allow them to survive (e.g. 35 degrees C, 5 percent carbon dioxide, as well as below 0.5 carbon dioxide oxygen, etc.).
In addition, various energy sources are readily available to them (e.g. proteins and carbohydrates in the oral cavity and gastrointestinal tract consumed by the host).
With the optimal environmental conditions and available energy sources, they can exist in harmony with the host without causing serious diseases.
Parasitism: While the majority of Fusobacteria species can exist in a harmonious relationship with their respective hosts, others can become parasitic and cause disease.
While Streptobacillus moniliformis causes Rat Bite Fever (RBF) characterized by swelling or development of an ulcer at the wounded site, Fusobacterium nucleatum is considered to be an opportunistic parasite that can cause periodontal disease.
In the case of Fusobacterium nucleatum, one of the most common Fusobacterium species disease outcome has been shown to be the result of the bacterial activities and host response. The outer membrane of the bacterium consists of adhesin that contributes to the adhesion of salivary proteins as well as the adhesion of other microorganisms resulting in the formation of biofilms and plague.
In the process, the bacterium activates the host cells in this region to produce matrix metalloproteinases which in turn results in periodontal inflammation and ultimately to the development of periodontal disease.
Some of the other diseases/infections associated with various Fusobacteria species include:
· Peritonsillar abscess
· Ulcerative colitis
· Otitis media
· Oropharyngeal trauma
· Inflammatory bowel disease
* While given Fusobacteria species can cause given infections or diseases, they rarely cause serious diseases in human beings.
Some of the other characteristics of Fusobacteria include:
They are mostly non-motile - While some of the species were initially suggested to have flagella for movement, this is absent in most of the species that have already been identified. Some of the species are suspected to move through a process known as gliding (moving on a flat surface using polysaccharide slime). In this case, the slime is produced on the outer membrane for motility purposes.
Fusobacteria species do not form spores - Spores (also known as endospores) represent the dormant form of bacteria that allow the organisms to survive harsh environmental conditions.
Compared to bacteria like members of the genus Clostridium and Bacillus etc., Fusobacteria are non-spore formers and thus do not have a dormant phase – Cell division among Fusobacteria occur along one plane with some cells remaining attached to each other to form long chains
Bacteriology as a field of study
Caitlin A. Brennan and Wendy S. Garrett. (2019). Fusobacterium nucleatum — symbiont, opportunist and oncobacterium.
K. W. Bennett and A. Eley. (1993). K. Fusobacteria: New taxonomy and related diseases Free.
Kevin Afra, Kevin Laupland, Jenine Leal, Tracie Lloyd, and Daniel Gregson. (2013). Incidence, risk factors, and outcomes of Fusobacterium species bacteremia.
Radhey Gupta and Mohit Sethi. (2014). Phylogeny and Molecular Signatures for the Phylum Fusobacteria and its Distinct Subclades.
Thomas Thurnheer, Lamprini Karygianni, Manuela Flury, and Georgios N. Belibasakis. (2019). Fusobacterium Species and Subspecies Differentially Affect the Composition and Architecture of Supra- and Subgingival Biofilms Models.