Epsilonproteobacteria
** Examples and Characteristics
Overview
Epsilonproteobacteria is a class of the Phylum Proteobacteria that consists of Gram-negative bacteria. Members of this class can be found in both terrestrial and aquatic habitats where they exist as free-living species or in association with various mammals and birds. Epsilonproteobacteria is largely divided into two main families namely, Helicobacteraceae and Campylobacteraceae.
Some representatives of the class Epsilonproteobacteria include:
Arcobacter
Sulfospirillum
Wolinella
Hydrogenimonas
* Though Nautiliaceae formerly belong to the class Epsilonproteobacteria, it was re-classified and placed within the class Nautiliia in 2020.
Characteristics and Examples of Epsilonproteobacteria
To get a better understanding of the Epsilonproteobacteria, it's important to take a closer look at the two families that make up the group.
The family Campylobacteraceae consists of three genera which include Arcobacter, Campylobacter, and Sulfuspirillum. The family Helicobacteraceae, on the other hand, contains about five genera namely, Helicobacter, Sulfuricurvum, Sulfurimonas, Thiovulum, Wolinella, and Sulfurovum.
Distribution and Habitat
Campylobacteraceae species are ubiquitous in nature and can be found in both terrestrial and aquatic habitats. Campylobacter species, for instance, can be found in soil, mud, untreated and environmental water, and beach sand. They can also be found in association with a number of animals and some unicellular organisms.
For instance, the bacterium Campylobacter jejuni has been isolated from soil amoebas (Acanthamoeba polyphaga). In this host, they are able to grow and multiply before spreading to new hosts.
As for the protist, they can also be found in a number of other hosts including human beings, wild and domesticated mammals (pigs, sheep, cattle, etc.), and birds. In these animals, they often colonize the mucosal surfaces of the gastrointestinal tract where some species exist as commensals.
For example, the bacterium Campylobacter jejuni lives as a commensal in the intestinal mucosa of various birds, rodents, cats, and dogs among other animals. However, these species can also cause diseases and bacteria in various hosts (E.g. meningoencephalitis, endocarditis, and cellulitis in human beings and Campylobacteriosis in animals).
* Some Campylobacter species can also live in and survive in bovine manure.
Arcobacter species have also been isolated from a variety of habitats in the environment. Some of the most common habitats they inhabit include wetland soils, groundwater and rivers, lakes, and seawater. If ingested from these sources, they can infect both human beings and animals.
Accordingly, they can be found in the feces of various animals as well as sewage and contaminated surface water (E.g. A. marinus, A. defluvii and A. halophilus). Given that they are associated with a number of animals, Campylobacteraceae species can also be found in various animal products and raw milk as well as mussels and clams (e.g. A. venerupis, and A. ellisii).
* The majority of Sulfurospirillum species are free-living bacteria. They are also metabolically diverse and can be found in aquatic habitats (e.g. deep-sea vents and marine sediments) as well as sulphidic terrestrial niches.
Whether Campylobacteraceae species can survive is dependent on a variety of factors including oxygen, temperature, available nutrition, and pH. While some of the species are microaerobic (Campylobacter species prefer O2 concentration of 5–10 %), others exist as anaerobes and/or aerobes. Some Arcobacters can grow anaerobically but others can tolerate aerobic conditions.
The majority of species in the family prefer temperatures of between 25 and 42 degrees C. However, a few of the species can be found in areas with very high or very low temperatures. For instance, the bacterium Arcobacter butzleri has been shown to be moderately psychrophilic and can be found in areas with a temperature range of between 15 and 37 degrees C. Others, like Campylobacter jejuni and Campylobacter coli are thermophilic and can survive at temperatures above 41 degrees C.
Members of the family Helicobacteraceae can also be found in several habitats including springs, sulfidic caves, freshwater reserves, hydrothermal vents, deep-sea sediments, and iron and sulfur-rich water sources. Individuals like Helicobacter pylori can be found in the mucosa of human beings (the species is a major cause of gastroduodenal diseases).
Other Helicobacter species can also be found in association with other animal hosts including mice (Helicobacter ganmani and Helicobacter hepaticus), chicken (Helicobacter pullorum), cats (Helicobacter baculiformis), and dogs Helicobacter cynogastricus. Some of the other popular hosts of Helicobacter species include swine, whales, cheetahs, pigs, and birds among others.
Like Campylobacteraceae species, different members of the family Helicobacteraceae can also be found in specific niches with certain conditions that support growth. The bacterium Helicobacter acinonychis, for instance, is microaerophilic and cannot grow aerobically or anaerobically. Moreover, it grows optimally at 37 degrees C.
Helicobacter aurati, on the other hand, is capable of growth (microaerobically) at 37 and 42 degrees C but not at 25 degrees C. Unlike some of the other species, most of which are microaerophiles, Helicobacter hepaticus can grow under microaerobic and anaerobic conditions at 37 degrees C. However, it's incapable of growing under aerobic conditions.
Morphological Characteristics of Epsilonproteobacteria
Like other Proteobacteria, members of the family Epsilonproteobacteria are Gram-negative species that exhibit diversity in morphology. These variations are also evident in size, structure, and G+C content.
The family Campylobacteraceae, which consists of five genera, largely consists of individuals with a curved, straight, or spiral morphology. They have a low G+C content of between 29 and 47 percent and range from 0.2 to 5.0 um in length.
The Genus Campylobacter consists of slender, curved, or helical rods but some of the species have been shown to become coccal or spherical in stressful conditions. Arcobacters, on the other hand, can be S-shaped, curved, or helical in shape and range from 0.4 to 2.2 um in length. However, some species might grow to be about 10 um long.
Helicobacteraceae species are also generally small in size with shapes ranging from straight and twisted rods to curved and spiral. Helicobacter pylori, one of the most popular members of the family, has a G+C content of between 38.3 and 39.3 percent and ranges between 0.5 and 5um in length. Like many other Helicobacter species, it has an S-shape and polar flagella. However, some species might be short rods (e.g. Helicobacter mustelae). Other are helical (Helicobacter suis and Helicobacter heilmannii), or fusiform (e.g. Helicobacter rappini).
The bacterium Helicobacter acinonychis is a short, spiral rod, measuring about 0.3 um in diameter and between 1.5 and 2.0 um in length while Helicobacter mesocricetorum is a spiral/curved rod measuring 2 to 3 um in length. Following prolonged incubation, Helicobacter species like Helicobacter pylori and Helicobacter typhlonius exhibit a coccoid morphology.
Motility
Many Campylobacteraceae species are highly motile by means of a single or multiple flagella. However, there are also many non-flagellated species in the group. In some of the species, the flagella are also sheathed.
Campylobacters species like Campylobacters showae have a single non-sheathed flagellum (polar flagellum) that causes a darting, corkscrew-like motion when the organism moves. Others, like Campylobacters gracilis and Campylobacters hominis, however, do not have this structure and are therefore immotile. In the Genus Arcobacter, some species move by means of a single polar flagellum while others have one at both ends of the cell body. For this species, a darting, corkscrew-like movement has also been reported.
* In Campylobacter, the flagellum comprises two homologous flagellins (FlaA and FlaB). FlaB plays an important role in the adhesion and colonization of gastrointestinal tract cells.
As compared to Campylobacteraceae, flagellated Helicobacteraceae species have one or multiple flagella. The species Helicobacter pylori, for instance, moves by means of 4 to 8 unipolar or bipolar sheathed flagella. Multiple flagella can also be found in a number of other Helicobacter species including Helicobacter pylori felis (14-20), Helicobacter bizzozeronii (10-20), Helicobacter salomonis (10-23), and Helicobacter cynogastricus (6-12)
* For bacteria with sheathed flagella, a sheath layer covers the entire length of the flagellum.
Nutrition and Metabolism
A good number of Epsilonproteobacteria species exist as chemoheterotrophs or chemolithoautotrophs. As such, they can use a variety of sources for energy generation. Campylobacter jejuni, a member of the Genus Campylobacter is an example of a chemoheterotrophic bacterium.
Chemoheterotrophs include organisms that use various organic chemical substances as a source of energy. Unlike many other chemoheterotrophs, Campylobacter jejuni does not have the common metabolic pathways that allow for the breakdown of glucose, galactose, and other carbohydrates. However, it's capable of using a range of organic sources including serine, proline, amino acids asparate, and glutamate.
Moreover, recent studies have shown that the bacterium is capable of using L-fucose which helped dismiss the notion that it's asaccharolytic.
Although the metabolic pathway of the organism lacks a number of important components to transport sugars like glucose (lacks the necessary transporters to take up glucose) as well as several crucial enzymes commonly found in the glycolytic pathway, it's suspected to catabolize molecules like glycerol-3-phosphate.
It possesses a number of important metabolic enzymes including transketolase, ribose-5-phosphate isomerase, transaldolase, and ribulose-3-phosphate epimerase.
* Though it lacks a number of important enzymes, Campylobacter jejuni is still dependent on the citric acid cycle for energy generation. Here, pathways that produce fumarate, pyruvate, and oxaloacetate end up in the citric acid cycle.
* Campylobacter jejuni is also capable of sulphite respiration.
Unlike chemoheterotroph, chemolithoautotroph are autotrophic organisms that generate energy by oxidizing inorganic compounds. Some members of the genus Sulfuricurvum like S. kujiense (as well as Sulfurimonas like S. autotrophica) are examples of chemolithoautotrophs.
Sulfuricurvum kujiense is a facultative anaerobe that uses sulfides or thiosulfate as the electron donor. Here, nitrate is used as the electron acceptor. Like Sulfuricurvum kujiense, Sulfurovum lithotrophicum is also a sulfur and thiosulfate oxidizing bacteria. To generate energy, sulfur or thiosulfate is used as electron donors with oxygen ad nitrate acting as the electron acceptors.
Other examples of chemolithoautotrophs are:
S. paralvinellae
S. denitrificans
S. gotlandica
Pathogenic Epsilonproteobacteria
There are several members of the class Epsilonproteobacteria that exist as obligate or opportunistic pathogens and are capable of causing diseases in human beings and various animals. In the family Campylobacteraceae, these species are particularly common in the genera Arcobacter and Campylobacter.
In the Genus Arcobacter, some of the most common pathogens include Arcobacter jejuni, Arcobacter fetus, Arcobacter concisus, and Arcobacter upsaliensis.
Like Arcobacter jejuni, Arcobacter coli cause disease in human beings by colonizing and releasing toxin in the gut mucosa. Arcobacter jejuni, in particular, can produce well over 30 proteins that allow for adhesion, colonization, and invasion of the mucosa. The two species are associated with a number of infections including colitis, extra-intestinal infections such as nephritis, endocarditis, and pancreatitis.
Arcobacter fetus, on the other hand, can be found in the gastrointestinal and genital tract of animals like sheep and cattle. Like Arcobacter rectus, the bacterium is characterized by an external microcapsule that consists of paracrystalline S-layer proteins. This structure provides protection to the bacterium by allowing it to evade antibody-mediated destruction.
In human beings (especially in compromised individuals such as those with AIDS, or liver cirrhosis), the infection is characterized by diarrhea, relapsing illness, and bacteremia. In cases where the bacterium spreads to the vascular system, it can cause cellulitis, thrombophlebitis, and mycotic aneurysms. In animals like cattle, the infection can result in infertility and abortion.
Like some of the other Campylobacter species, Campylobacter upsaliensis also invades the gastrointestinal tract where it binds to mucins. The infection can result in inflammatory and non-inflammatory diarrhea in human beings.
In the family Helicobacteraceae, Helicobacter pylori is one of the most popular pathogens. In human beings, the bacterium colonizes the gastric mucosa where it causes gastritis and gastric ulceration. If untreated, studies have shown that the bacterium can also cause gastric cancer. Moreover, it has been associated with a number of other neurological conditions including Alzheimer's disease, Parkinson's disease, as well as ischemic heart disease.
A number of other Helicobacter species can cause disease in human beings. For instance, H. heilmannii can be found in the antral gastric mucosa and is responsible for mild chronic gastritis and some cases of peptic ulcers. H. cinaedi and H. fennelliae are associated with proctitis and gastroenteritis while H. macacae have been shown to play a role in the development of colitis.
In animals, Helicobacter species are also responsible for a number of infections/diseases. A good number of species have been detected in the gastrointestinal mucosal surface of various domestic and feral animals. In animals like cats and dogs, for instance, H. heilmannii has been shown to cause various gastric infections (chronic gastritis).
Some of the other infections/diseases may include gastritis and associated ulceration of the pars oesophagea, lymphocytic gastritis, and gastric adenocarcinoma.
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References
Delong, E., Lory, S., Stackebrandt, E., and Thompson, F. (2014). The Prokaryotes: Deltaproteobacteria and Epsilonpoteobacteria
On, S., Miller, W., Houf, K., Fox, J., and Vandamme, P. (2017). Minimal standards for describing new species belonging to the
families Campylobacteraceae and Helicobacteraceae:
Campylobacter, Arcobacter, Helicobacter and Wolinella spp.
O'Rourke, J. and Bode, G. (2001). Helicobacter pylori: Morphology and Ultrastructure.
Stahl, M., Butcher, J., and Stintzi, A. (2012). Nutrient acquisition and metabolism by Campylobacter jejuni.
Links
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/campylobacteraceae
https://www.researchgate.net/publication/230239809_Nonpathogenic_Epsilonproteobacteria
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