Deltaproteobacteria
** Examples and Characteristics
Overview
Deltaproteobacteria is a large class of Gram-negative bacteria within the Phylum Proteobacteria. It consists of ecologically and metabolically diverse members that exist as sulfate/sulfur-reducing bacteria or as intracellular pathogens. Some of the species, especially those of the families Geobacteraceae and Desulfuromonadaceae, are also capable of reducing elemental metals like iron to complete respiration.
Though there are many aerobic species in this class, it also consists of strictly anaerobic genera most of which possess dissimilatory sulfite reductases.
Some examples of Deltaproteobacteria include:
- Desulfobacter species
- Desulfococcus species
- Desulfuromonas species
- Desulfovibrio species
- Myxobacteria species
Characteristics and Examples of Deltaproteobacteria
* Before looking at the characteristics of Deltaproteobacteria, it's worth noting that some of the members have been re-classified (e.g. the order Bdellovibrionales now belongs to the class Oligoflexia). Moreover, there have been proposals to disassemble and re-classify most of the families within this class.
The majority of Deltaproteobacteria are known for their ability to reduce sulfur or sulfates. They include members of the families Desulfobulbaceae, Desulfarculaceae, Desulfohalobiaceae, and Desulfobacteraceae among others.
As mentioned, others exist as obligate or facultative pathogens while a few groups are capable of reducing elemental metals. These characteristics allow the class to inhabit a wide variety of habitats across the globe where they can derive energy from a wide range of sources.
Ecology/Habitat
Sulfur/sulfate-reducing Deltaproteobacteria can be found in a variety of habitats in both aquatic and terrestrial environments. Members of the family Desulfarculaceae, for instance, can be found in marine, brackish, and freshwater systems around the globe while Desulfobulbaceae species are common in sediment and various anoxic habitats.
Desulfohalobiaceae have been isolated from such habitats as hydrothermal vents of hyper-saline and haloalkaline lakes and marine whereas Desulfovibrionaceae occur in soil, sewage sludge, fresh and marine waters as well as animal and human hosts.
Members of the family Myxococcaceae, which exhibit predatory behavior, are particularly common in soil but can also be found in dung and barks of tree as well as decaying material. Given that some of the species in this family can tolerate relatively high salt concentration, they can also be found in the sandy shore sediments and some saline-alkaline soils.
Whereas some of the species are anaerobic in nature and thrive under anoxic conditions, others are aerobic and need oxygen for respiration. Therefore, the presence or absence of oxygen is an important factor with respect to the habitat in which certain Deltaproteobacteria inhabit. However, some are facultative anaerobes and can thrive in the presence or absence of oxygen.
Members of the genus Anaeromyxobacter within the family Cystobacteraceae are facultative anaerobes and can use oxygen as the terminal electron acceptor in low concentrations. In the absence of oxygen, they rely on several alternatives including fumarate, nitrate, and chlorophenolic compound.
Stigmatella species are aerobic in nature and rely on oxygen as the terminal electron acceptor. They can be found in a number of habitats ranging from Antarctic soil to deserts in parts of Africa and the United States. The genus Desulfarculus consists of several strictly anaerobic species which are known sulfate-reducing bacteria which can be found in marine systems.
Other Factors Influencing Habitation
Aside from oxygen, a number of additional factors also determine the habitat in which Deltaproteobacteria are found. These include pH, temperature, and acidity among others. The majority of species are mesophilic which means that they are commonly found in habitats with moderate temperatures (between 20 and 45 degrees C). These include most of the Myxobacteria, Desulfarculaceae, and Desulfobulbaceae among others. They are also likely to be found in terrestrial and various aquatic habitats.
A few groups consist of psychrophiles (psychrophilic species) which grow at temperatures of between 0 and 15 degrees C. These include some members of the genus Geobacter (E.g. G. psychrophilus), Archangium gephyra, and D. svalbardensis. Desulfuromusa ferrireducens and all members of the genus Desulfotalea which have been shown to be obligate psychrophilic. They can be found in various cold environments including the Swedish arctic, parts of Northern Canada, deeper in the ocean, and parts of Alaska and Iceland.
Thermophilic (60 -120 degrees C) and moderate thermophiles (50-60 degrees C) have also been identified. These include Desulfothermus species, some Desulfomicrobium species, and Aquificales among others. They can be found in various hot habitats including solfataric fields, hydrothermal vents, and hot springs. They have also been isolated from poultry feces, thermophilic deep-sea vents, and several marine habitats.
Whereas most of the species are neutrophilic and grow well under neutral conditions, others are alkaliphilic or acidophilic (or moderate acidophiles). For instance, H. jasoniae, which is considered a moderate acidophile, grows at a pH range of between 4.5 and 5.0 while H. alviniae can grow at a pH of 4.5. Some myxobacteria are alkaliphilic and can be found in alkaline lakes and soil with a pH of 9.5.
* Deltaproteobacteria species that are found in extreme environmental conditions (acidophiles, thermophiles, and psychrophiles among others) are collectively referred to as Extremophiles.
Morphological Characteristics of Deltaproteobacteria
The class Deltaproteobacteria consists of Gram-negative bacteria. As such, they are characterized by a thin peptidoglycan cell wall and an outer membrane. Due to the large size of the group, they exhibit significant variation in size and shape.
While some of the species have a spherical or rod-like morphology, others have a vibrio or filamentous appearance. However, some of the species exhibit changes in morphology as they develop with others having different shapes within the same family or genus. Under certain conditions (e.g. in stressful environments), some of the species may produce structures like fruiting bodies and myxospores that allow them to survive until conditions improve.
The vegetative cells of many Deltaproteobacteria have a slender morphology. When viewed under the microscope, they are likely to be elongated with a needle-like shape and tapered ends. These include members of the families Cystobacteraceae, Desulfobacteraceae, and Haliangiaceae among others (Haliangium tepidum and many other species have blunted/rounded ends).
Members of the order Syntrophales may be rounded or coccoid in shape while Desulfobacteraceae species vary from vibrio and coccoid to rod-shaped and filamentous. As mentioned, there is also significant variation in size not only between families and also within genera.
In the genus Desulfobacter, the bacterium Desulfobacter postgatei ranges between 1.7 and 2.5 um in length while Desulfobacter halotolerans is between 3.0 and 5.0 um in length. Desulfobacter latus and Desulfobacter vibrioformis, on the other hand, are relatively larger, ranging between 4.5 and 8.0 in length.
* Some of the species like Desulfobacter postgatei, Desulfobacter latus, and Desulfobacula toluolica have an oval shape.
Fruiting Bodies
Some Deltaproteobacteria, particularly Myxobacteria, are also capable of producing fruiting bodies and myxospores. Under stressful conditions (e.g. depleting nutritional sources), these cells aggregate through an inward movement pattern in order to form fruiting bodies.
Depending on the species, these multicellular biofilms vary from simple mounds/heaps to convoluted structures. Based on several studies, a number of important events have been shown to occur in order to ensure the survival of these organisms. The first event involves the accumulation and dispersion of secondary metabolites within these bodies. These metabolites include antifungal and antibacterial agents that protect the cells during this period.
Secondly, and most importantly, some of the cells gradually transform into non-reproductive cells while others differentiate into resistant spores capable of enduring harsh conditions.
The formation of fruiting bodies in Myxobacteria has proved useful in studying the social behavior of various bacteria. Here, motility is especially crucial in that it allows the cells to aggregate. When the cells come in contact at a given point, they stall and produce signaling molecules (A-signal and C-signal) which not only promote increased aggregation but also influence the production of fibrils which bundle adjacent cells together and maintain cellular cohesion. Some of the other substances produced during this process include lipopolysaccharide and retractile type IV pili.
* Fruiting bodies vary in shape, size, and texture. Whereas some have a slime-like cartilaginous consistency, most consist of pale or deep brown sporangioles of varying arrangements. They may also contain slime stalks with short, spheroid, or rod-like myxospores.
* Some Myxobacteria also form colonies known as swarms. These colonies can move in a coordinated manner which makes it easier to prey on other bacteria.
Motility
Some Deltaproteobacteria can move by gliding or by means of flagella. Many members of the family Desulfobulbaceae, for instance, have one or two polar flagella used for motility. These structures can be found in many other individuals including some members of the families Desulfovibrionaceae, Desulfuromonadaceae, Desulfarculaceae, and the Genus Bdellovibrio.
Using flagella, members of the genus Bdellovibrionales among other species are able to effectively target and capture prey. Moreover, this type of motility also allows them to easily penetrate the cell.
In some Myxococcaceae species and Chondromyces crocatus, microscopic studies have detected the presence of pilu and fimbria. Like flagella, they are used for motility and contact. More specifically, fimbria in Chondromyces crocatus was suggested to play an important role in swarming motility while the pili in Myxococcus xanthus are involved in contact-mediated cell to cell interaction.
In Chondromyces crocatus, pili are involved in the gliding movement as well as adhesion. Though not all Bdellovibrio species are flagellated, Type IVa pili have been identified in some of the species. Here, they are particularly essential for prey attachment as well as penetration into the periplasm.
The third type of movement among some of these cells is known as gliding motility or A-motility. Some of the Deltaproteobacteria that exhibit gliding include some members of the families Desulfobacteraceae, Cystobacteraceae, and Deltaproteobacteria.
As mentioned, gliding plays a crucial role in the formation of fruiting bodies as well as predation. In the case of Myxococcales, for instance, the cells swarm around the prey by gliding. Lytic enzymes are then secreted to destroy the prey.
Metabolism
Metabolically, Deltaproteobacteria can be divided into three major categories namely, chemoorganotrophs/chemoorganoheterotroph, chemolithoheterotrophs, and chemolithoautotrophs. Moreover, as mentioned, they can be divided based on whether they use oxygen for respiration or not and whether they are capable of fermentation.
Chemoorganoheterotrophs are species that require organic substrates as a source of carbon for growth. Energy is therefore derived from the oxidation of various organic compounds. As compared to chemoorganoheterotrophs, chemolithoautotroph are, for the most part, autotrophs and derive energy from the oxidation of inorganic compounds (e.g. ferrous iron, ammonia, and hydrogen sulfide). Chemolithoheterotrophs rely on inorganic compounds for energy and reduced organic compounds as carbon sources. For this reason, they are sometimes referred to as mixotrophs.
Chemoorganoheterotrophs can be found in just about every family within the group chemoorganoheterotroph. For instance, they make up the majority of species in the families Desulfobacteraceae, Desulfobulbaceae, Desulfohalobiaceae, Desulfomicrobiaceae, Desulfonatronaceae, and Desulfovibrionaceae. Many of the families also have some chemolithoheterotrophs with chemolithoautotrophs being the least common.
In the genus Desulfarculus within the family Desulfarculaceae, the sole species can exist as a chemoorganoheterotroph and chemolithoheterotroph. With sulfate serving as the electron acceptor, it oxidizes organic substrates to produce carbon dioxide. It's also anaerobic in nature and incapable of fermentation.
In the family Desulfobacteraceae, the majority of species also oxidize organic substrates to produce carbon dioxide. However, in some of the species, the process is incomplete and results in the production of acetate. Chemolithoautotrophs like D. baarsii, in the family Desulfarculaceae can survive in the presence of formate. Here, some of the most common electron acceptors include sulfite, thiosulfate, and sulfate. As well, the Genus Nitrospina, in the family Nitrospinaceae is an obligate chemolithoautotroph and uses nitrite as an energy source while carbon dioxide serves as the carbon source.
Chemolithoheterotroph can also be found in most of the families. They are particularly common in the family Desulfomicrobiaceae where they are as many as chemoorganoheterotrophs. They can use sulfate and thiosulfate as electron acceptors and can also carry out fermentation using malate and fumarate.
Some Deltaproteobacteria species are also capable of fermentation. However, this is not the most popular method of energy production. In the family Desulfomicrobiaceae, a good number of species are strict anaerobes that carry out a fermentative type of metabolism. A variety of simple organic molecules can be used as electron donors and the process (incomplete oxidation of organic substrates) results in the production of acetate.
Some of the other species capable of fermentation include some species of the families Desulfomicrobiaceae and Desulfonatronaceae.
Pathogenic Deltaproteobacteria
Some of the most common pathogens in the class Deltaproteobacteria can be found in the families Desulfomicrobiaceae (Desulfomicrobium orale) and Desulfovibrionaceae (Lawsonia and Bilophila species). Like many other members of the class, Desulfomicrobium orale is a rod-shaped, Gram-negative bacterium. It measures between 2.0 and 3.0 um in length and moves by means of a polar flagellum.
In recent years, studies have revealed that the bacterium is associated with periodontitis, a severe gum infection that can result in tooth loss and complications like preeclampsia. As the disease progresses, with more bacteria building in periodontal pockets, a patient is likely to experience tender and bleeding gums, teeth sensitivity, a gradual separation of the gum from the teeth, and teeth loss. This is further aggravated by the fact that as the pathogen proliferates, they form biofilms that not only protect them but also allow them to spread.
Lawsonia intracellularis, a member of the genus Lawsonia, is an obligate intracellular bacterium that causes Lawsonia intracellularis and proliferative enteropathy in a number of animals (pigs, rabbits, horses).
In horses, an infection occurs when the bacterium is ingested and enters the small intestine. Here, it penetrates the cells of the intestinal lining causing it to swell. Some of the symptoms associated with the infections include diarrhea, colic, fever, and hyperplasia (increased number of cells in the tissue). Like Lawsonia intracellularis, Bilophila wadsworthia is an anaerobic organism that can be found in the gut.
Although it's less abundant, especially when compared to some of the other gastrointestinal microbiota, it's one of the most common pathogens among patients with gangrenous appendicitis. The bacterium can also spread into the bloodstream causing bacteremia. Some of the symptoms associated with the infection include the inability to pass gas, abdominal swelling, and loss of appetite, constipation/diarrhea, and abdominal pain.
* Application: A number of Deltaproteobacteria species produce a variety of products that can be useful in pharmaceutical and biotechnology industries. Some of these metabolites include cytotoxic, antiviral, insecticidal, and anti-tumor products.
Members of the family Myxococacceae, in particular, are important sources of antibacterial and antifungal productions. Some of the other important organisms in this regard include members of the families Polyangiaceae, Haliangiaceae, and Cystobacteraceae.
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References
Dorado, M., Torres, M., Bravo, E., Munoz, M., and Perez, J. (2016). Myxobacteria: Moving, Killing, Feeding, and Surviving Together.
Gaspari, F., Paitan, Y., Mainini, M., and Marinelli, F. (2004). Myxobacteria isolated in Israel as potential source of new anti-infectives.
Rosenberg, E. (2014). The Prokaryotes: Deltaproteobacteria and Epsilonpoteobacteria.
Waite, D. et al. (2020). Proposal to reclassify the proteobacterial classes Deltaproteobacteria and Oligoflexia, and the phylum Thermodesulfobacteria into four phyla reflecting major functional capabilities Open Access.
Links
https://www.researchgate.net/publication/278667796_The_Family_Myxococcaceae
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/deltaproteobacteria
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