Like Alpha and Zetaproteobacteria, Betaproteobacteria is a class of the phylum Proteobacteria. With over 11 families, 75 genera, and 400 species, Betaproteobacteria is a heterogeneous group whose members can be found in a range of habitats from wastewater and hot springs to the Antarctic.
While the majority of species are free-living individuals in soil and various aquatic habitats, the group also consists of important symbionts (found in association with plants) as well as pathogenic species that cause disease in human beings and animals.
Some examples of species within the class Betaproteobacteria include:
Betaproteobacteria species are widely distributed and can be found in a number of habitats throughout the world.
Species with phototrophic capabilities (e.g. some Rhodoferax species, Limnohabitans, and Rhodocyclus purpureus) are particularly common in stagnant aquatic systems, saline ponds, and various freshwater habitats, as well as wastewaters.
Rhodoferax fermentans and Algoriphagus antarcticus, members of the genus Rhodoferax are moderate psychrophilic species that have been isolated from microbial mats in the Antarctic (in regions with a temperature range of between 15-18 degrees C).
The two species are facultative anaerobes and thus capable of growth in the presence or absence of oxygen in their habitats. Although they are capable of anoxygenic photosynthesis, they have been shown to prefer photoheterotrophic growth under anoxic conditions. For this reason, they are more likely to be found in habitats with organic compounds (as carbon sources).
Phototrophic species are also less likely to be found in sulfide-rich water because of their inability to use sulfide as the electron donor (they have a very low tolerance for sulfide).
In stagnant freshwater bodies, they largely inhabit regions where they can not only access light but also a range of organic compounds.
Rhodocyclus purpureus, which is a rare member of phototropic Betaproteobacteria, has been isolated from swine waste lagoon which is evidence that some of the phototrophs can also survive in wastewater.
Pathogenic Betaproteobacteria (facultative and obligates) can be found in association with plants, human beings, and animals. However, they can also be found in some terrestrial and aquatic habitats.
Some of the most common pathogens in the group can be found in the Genera Neisseria and Bordetella. In human hosts and animal hosts, Neisseria species tend to colonize mucosal surfaces where they are part of the normal flora (especially in the oropharynx and nasopharynx).
Neisseria gonorrhoeae, which is always pathogenic, can be found in various regions of the genitourinary system as well as the upper respiratory system in human hosts. These include the urethra, cervix, and pharynx. However, it can also be found in the rectum and eyes.
In animals like dogs, monkeys, and pigs, Neisseria species can be found in the throat among other mucosal membranes (particularly in the respiratory system). Like Neisseria species, some members of the Genus Bordetella can also be found in the respiratory system.
B. pertussis, the most common pathogen in the group, can be found along the respiratory tract in human beings where it is responsible for Pertussis or whooping cough. Other species, like Bordetella bronchiseptica and Bordetella avium can be found in the respiratory of animals and also cause diseases.
Pathogens of plants can be found in the Genus Burkholderia and some members of the family Comamonadaceae. The Genus Burkholderia is a heterogeneous group whose members can be found in soil and in association with plants and human beings.
Although some of the species in the group promote plant growth (Rhizobacteria), others like Burkholderia gladioli and Burkholderia glumae are plant pathogens that cause disease in plants like corn and rice. In rice, for instance, the two pathogens are responsible for blight disease (characterized by grain rot and leaf browning). In the roots of leguminous plants, however, a number of species (e.g. Burkholderia vietnamiensis) play an important role in nitrogen fixation.
As mentioned, there are many Betaproteobacteria that can be found in terrestrial and aquatic habitats where they exist as free-living bacteria. Aside from phototrophs, which can also be found in soil and some aquatic habitats, other free-living species exist as chemolithotrophs, methylotrophs, and chemoorganotrophs.
Some of the species capable of chemolithotrophic growth include Hydrogenophaga flava, Variovorax paradoxus, and Acidovorax facilis. They can be found in several habitats including glacier mud, farm soil, and water, plant roots, as well as some habitats with heavy metals. Many other soil and aquatic species can be found in the genera Achromobacter, Spirillum, and Alcaligenes.
* Thiocacillus species are thermophilic acidophiles and therefore can be found in habitats with high temperatures and low pH. They are also strict aerobes and survive by oxidizing Iron and sulfur. They can be found in a number of habits including sewage lagoons, digestion tanks, marine sediments, and abandoned mines, etc.
Betaproteobacteria also exhibits significant Morphological Heterogeneity between species.
As members of the Phylum Proteobacteria, Betaproteobacteria are Gram-negative bacteria and thus have a thin peptidoglycan wall and outer membrane.
With respect to general morphology, they may be rod-shaped, circular, curved rods, coccid, spiral, and even sheathed. Some of the other differences can also be identified in the G+C content, arrangement, size, and external structures.
A good example of Betaproteobacteria with a circular shape/ring-like (or semi-circular) is the species Rhodocyclus tenuis. However, some of the other members of the Genus Rhodocyclus (e.g. Rhodocyclus tenuis) are generally slender or only slightly curved. Cocci species can be found in the genera Neisseria and Azoarcus.
Neisseria species are diplococci (which means that they exist in pairs). However, some individuals may have pear-like or dumbbell morphology with a few occurring singly. In the Genus Azoarcus, Azoarcus buckelii is the sole spherical species. Other members in the group may have an S-shaped morphology or appear slightly curved (slightly-curved rods).
Rod-shaped species can be found in a number of genera with variations in arrangements. For instance, in the Genus Rubrivivax, Rubrivivax gelatinosus are straight rods some of which may be attached. The Genus Alcaligenes, on the other hand, consists of rod-like species or coccibacilli (very short rods or ovoid). Rod-like species can also be found in the genera Achromobacter, Oligella, and Comamonas among others.
Spiral-shaped Betaproteobacteria may be gently curved or have a corkscrew spiral shape. They are particularly common in the Genus Aquaspirillum. Sheathed Betaproteobacteria are characterized by long filaments that form an external sheath layer. For instance, in the Genus Leptothrix, the species Leptothrix discophora has an external, tube-like sheath that consists of fibrils and an underlying capsular layer.
* With respect to size, Betaproteobacteria vary significantly from as small as 0.3 µm × 1 µm among Bartonella species to a helical length of between 14 and 60 um in the case of Spirillum volutans.
With respect to morphology, it is worth noting that some Betaproteobacteria form stalks or prosthecae and colonies. Essentially, stalks are thin extensions that originate from the cell body of the cell wall of certain bacteria.
Gallionella ferruginea, a member of the genus Leptothrix (sheathed bacteria), is a good example of stalked bacteria in the Class Betaproteobacteria. The twisted stalk consists of numerous organic fibers with the accumulation of iron oxides over time.
Although studies are ongoing, the stalk is suspected to play an important role in protecting the cell against toxic oxygen radicals that are produced during the oxidation of Iron. As such, it is an important adaptation feature that allows the organism to survive in areas with relatively high Iron concentrations.
Several members of the Class Betaproteobacteria also form colonies under various conditions. In culture, Bordetella avium has been shown to produce unpigmented colonies measuring about 1mm in diameter. These colonies are characterized by a glistering appearance and a smooth convex outline.
Bordetella hinzii produces rounded and raised grayish colonies that measure about 2 mm in diameter. Lautropia species produce several types of colonies that can be differentiated based on their general morphologies. For instance, the colonies might be flat, dry, and circular. As they age, they become larger in size, wrinkled, and crisp. They might also be raised, rounded, and mucoid and vary in size from a pinpoint size to about 5mm in diameter.
Some Betaproteobacteria also possesses and move by means of flagella. The structure and number of this structure also vary between different species. The bacterium Robbsia andropogonis has a single polar flagellum that is sheathed while members of the Genus Achromobacter are characterized by peritrichous flagella that are also sheathed.
Although studies are yet to conclude whether the peritrichous arrangement of most Achromobacter are membranous, there is evidence to suggest that the sheath of flagella found in the species Achromobacter xylosoxidans is. Many others are non-motile, but some are capable of moving without flagella.
The majority of Betaproteobacteria exist as chemoorganotrophs or as chemolithotrophs. While some species have been shown to be aerobes, others are facultative anaerobes and can therefore grow in the presence or absence of oxygen.
The bacterium Thiobacillus ferrooxidans is a good example of a chemolithotrophic species. It is commonly found in acid mines, coal waste, and certain caves. As the name suggests, this Betaproteobacteria is dependent on Iron for energy. For this reason, it is likely to be found in mineral-rich habitats (habitats with a relatively high concentration of ferrous iron).
* All Thiobacillus bacteria can oxidize reduced sulfur for energy.
* Thiobacillus ferrooxidans is the only member of the genus that actively uses ferrous Iron as an electron donor.
During metabolism, the energy obtained from the oxidation of inorganic compounds is coupled for the production of NADPH and the consequent synthesis of ATP. Here, carbon dioxide (source of carbon) is fixed through the Calvin-Benson Cycle.
This process involves the activity of two major enzymes namely, Ribulose biphosphate carboxylase and phosphoribulokinase. Aside from the Calvin-Benson Cycle, some carbon dioxide might also be fixed through the action of the enzyme phosphoenolpyruvate carboxylase. This results in the production of a few amino acids.
Other Betaproteobacteria capable of chemolithotrophy includes Thiobacillus species, Ralstonia species, Nitrosomonas species, Spirillum volutans, and some Paucimonas like Paucimonas lemoignei among others.
Many Betaproteobacteria rely on various organic sources as a source of energy. For this reason, they are referred to as chemoorganotrophs.
Some are free-living bacteria while others form a beneficial relationship with various plants and animals. However, some are pathogenic and end up causing diseases in their hosts.
Members of the Genus Comamonas are aerobic chemoorganotrophs that are ubiquitous in the environment. They can be found in mud, water, and soil as well as activated sludge. Although they have also been isolated from a number of clinical samples in hospital environments, they are not generally regarded pathogenic to human beings.
Comamonas species can degrade a number of aromatic compounds including nitrobenzene and polycyclic aromatic compounds (e.g. phenanthrene, anthracene, and naphthalene). Using the enzyme 1, 2-dioxygenase, the bacteria break down nitrobenzene to produce nitrohydrodiol. This is then decomposed into catechol and nitrite. Catechol then enters the meta-cleavage pathway where it is further degraded. These species can also use specific enzyme to degrade a number of other organic compounds including 4-toluene sulfonate, Terephthalate, and recalcitrant components among others.
* Comamonas species (C. terrigena and C. testosteroni) can reduce nitrate to nitrite. However, they are incapable of reducing nitrites.
Unlike Comamonas, members of the genus Chromobacterium mostly use carbohydrates through fermentation. Moreover, they are capable of reducing both nitrate and nitrite resulting in the production of gas.
As they are capable of carrying out oxidation and reduction reactions, they can survive in both aerobic and anaerobic conditions. In aerobic conditions, for instance, the species Chromobacterium violaceum can survive with diminished simple sugars like fructose and galactose. Here, they use the tricarboxylic acid cycle, glyoxylate cycle, and the Embden-Meyerhoff pathway to produce as much energy as possible.
In anaerobic conditions, the metabolism of glucose results in the production of acetic acid and formic acid. Aside from these sugars, the bacterium has also been found to use lipids and amino acids for energy generation.
Some other chemoorganotrophs in this group include Janthinobacterium species, Ideonella and Sphaerotilus.
In their association with various plants and animals, Betaproteobacteria also relies on a variety of organic compounds for growth. Some members of the Genus Azoarcus are capable of nitrogen fixation. They can be found in association with a few plants including some species of rice and grasses.
Although nitrogen is one of the most important elements for plants (it is a key component and building block of amino acids), it cannot be used directly by plants.
Biological fixation by bacteria like Azoarcus species makes the element available for plants. In the soil and some aquatic habitats, these bacteria can anaerobically degrade various aromatic compounds to generate energy. Through their association with plants like rice and grass, however, they have access to a variety of organic compounds that can serve as energy sources (this involves aerobic respiration).
* Azoarcus species fix nitrogen under microaerobic conditions. Aside from oxygen, they can use nitrate as the terminal electron acceptor.
* Unlike the symbionts, pathogenic Betaproteobacteria cause harm to the plant while benefiting from organic compounds.
In human beings and some animals, Betaproteobacteria like Neisseria exist as commensals in the mucous membrane. However, several species (N. gonorrhoeae and N. meningitidis) are opportunistic pathogens capable of causing disease, especially among immunocompromised individuals.
As commensals, Neisseria species have access to nutrients along the mucous membrane but do not cause harm to the host. As the bacteria switch from commensalism to a pathogenic life, they have also been shown to exhibit metabolic adaptation which not only allows them to use new energy sources but also survive in the new environments of the body.
* In studies aimed at investigating the metabolic process in Neisseria meningitidis, the bacterium was shown to break down carbohydrates using Entner-Doudoroff pathway or the pentose phosphate pathway.
As mentioned, some Betaproteobacteria are capable of photosynthesis. Also known as phototrophic purple β-Proteobacteria, these species have two main photosynthetic pigments, bacteriochlorophylls and carotenoids.
Phototrophic species in this group belong to the genera Rhodoferax (e.g. Rhodoferax fermentans), Rubrivivax (e.g. Rubrivivax gelatinosus), and Rhodocyclus (e.g. Rhodocyclus tenuis).
During photoautotrophic growth, molecular hydrogen is used as the photosynthetic electron donor. Photoheterotrophically (under anoxic conditions), on the other hand, is dependent on carbon sources from organic sources (not carbon dioxide).
* Some Betaproteobacteria have been shown to play an important role in arsenic cycling and accumulation of the element in groundwater. Arsenic is a toxic element that naturally occurs in combination with various minerals such as metals and sulfur. If an individual is exposed to the elements for a long period of time, they can develop skin problems and even cancer. Similarly, ingestion of the element from untreated groundwater can result in health problems. Based on studies conducted in parts of India and Bangladesh, several members (especially members of the families (Burkholderiaceae, Comamonadaceae, Gallionellaceae and Rhodocyclaceae) of the group were shown to contribute to the release and immobilization of arsenic into shallow aquifers by altering redox states and solubility of various compounds.
Chakraborty, A., DasGupta, C., and Bhadury, P. (2020). Diversity of Betaproteobacteria revealed by novel primers suggests their role in arsenic cycling.
DeLong, E., Lory, S., Stackebrandt, F., and Thompson F. (2014). The Prokaryotes
Alphaproteobacteria and Betaproteobacteria
Derkaoui et al. (2016). Transport and Catabolism of Carbohydrates by Neisseria meningitidis.
Dworkin, M., Falkow, S., Rosenberg, E., Schleifer, K., and Stackebrandt, E. (2006). A Handbook on the Biology of Bacteria. Volume 5: Proteobacteria: Alpha and Beta Subclasses