Class β-proteobacteria

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  • Class β-proteobacteria
    • Order Burkholderiales
      • Family Alcaligenaceae (e.g. Alcaligenes, Bordetella, Achromobacter)
      • Family Burkholderiaceae (e.g. Burkholderia, Ralstonia)
      • Family Comamonadaceae (e.g. Comamonas, Acidovorax)
      • Family Oxalobacteraceae (e.g. Oxalobacter, Janthinobacter)
    • Order Hydrogenophilaceae
      • Family Hydrogenophilaceae (e.g. Hydrogenophilus, Thiobacillus)
    • Order Methylophilales
      • Family Methylophilaceae (e.g. Methylophilus, Methyobacillus)
    • Order Neisseriales
      • Family Neisseriaceae (e.g. Neisseria, Aquaspirillum, Vitreoscilla)
    • Order Nitrosomonadales
      • Family Gallionella (Gallionella)
      • Family Nitrosomonadaceae (e.g. Nitrosomonas, Nitrosospira)
      • Family Spirillaceae (Spirillum)
    • Order Procabacterales
      • Famly Procabacteraceae (Procabacter)
    • Order Rhodocyclales
      • Family Rhodocylaceae (e.g. Rhodocyclus, Azoarcus, Zoogloea)

About this Class

Diversity

Although less diverse phylognetically, and perhaps less abundant in the environment, than either the α- or γ-proteobacteria, the β-proteobacteria nevertheless are a diverse and abundant Class of Bacteria, consisting of at least 125 characterized genera.

Included in this Class are a number of organisms that were previously considered to be members of the genus Pseudomonas. This genus is now reserved for organisms specifically related to P. aeruginosa and P. fluorescens; other phylogenetic groups that had been lumped into this genus on the basis of generic phenotypic criteria have been divided into a number of new genera and reclassified on the basis of their phylogenetic relationships.

Metabolism

Like the other classes of proteobacteria, the β-protobacteria are very diverse metabolically and phenotypically. Most of these organisms are either heterotrophs (including some important pathogens, although mostly opportunists) or chemolithoautotrophs. They are generally aerobic or facultatively anaerobic.

Heterotrophic members of this Class are able to utilize a wide range of substrates for growth, including compounds important in waste management, such as phenol and lignin. Lithotrophic members are key players in the nitrogen cycle, primarily in nitrification. Many are also sulfur, ferrous iron, or manganese oxidizers, and those that grow autotrophically use the Calvin cycle to fix CO2. Also in this Class are the methylotrophs, methane oxidizers.

Although the bulk of the purple non-sulfur phototrophs are members of the α-proteobacteria, one main genus (Rhodocyclus) and a few less well-known genera (Rubrivivax, Roseateles) are members of the β-proteobacteria. Morphologically, Rhodocyclus are very tightly wound short helices, resembling old-fashioned lockwashers.

Morphology

Although typically rods and cocci, there are a number of conspicuous spirilla in this Class, and the non-cyanobacterial filamentous sheathed Bacteria. Filamentous iron- and manganese-oxidizing organisms typically become coated with granules of insoluble metal salts. Simonsiella and its relatives are gliding flattened multicellular filaments that inhabit the oral epithethium of mammals, often in great numbers.

Habitat

These organisms are abundant in most environments, particularly in organic-rich soils, sediments, wastewater and eutrophic aquatic systems. Also common are those found in association with plant and animal surfaces.

Heterotrophs and pathogens

The β-proteobacteria contain numerous heterotrophic species that are common in the environment, and are capable of utilizing a wide range of organic compounds. Most of these heterotrophs are aerobic rod-shaped organisms, and many are at least opportunistic pathogens of plants and animals. The most important animal pathogens include Bordetella, Burkholderia, and Neisseria.

Example: Ralstonia solanacearum

Ralsontia

R. solanacearum is an important plant pathogen, causing “Southern bacterial wilt” in a wide range of crop plants worldwide, including tobacco, potato (Brown rot), tomato, pepper and bananas (Moko disease). Species now in the genus Ralstonia were previously members of the genus Pseudomonas, and are obligately aerobic motile rods. The pathogen enters the plant through the root hairs, and grows and is transported throughout the plant in the xylem. The organism grows to such high numbers in the plant that one of the standard diagnostic tests is to touch the cut end of an infected stem to a container of water; if the infection is R. solanacearum, cells and exopolysaccharide can been easily seen as a milky stream flowing out of the xylem. R. solanacearum can overwinter in the soil or water, or persist for even longer periods of time. The genome of R. solanacearum is in two chromosomes, a 3.6Mbp circle containing most of the essential genes, and a 2.1Mbp “megaplasmid” that also contains essential genes and genes required for pathogenicity.

Chemolithoautotrophs

Most of the chemolithoautotrophs amongst the β-proteobacteria fall into four categories; sulfur oxidizers (that can sometimes oxidize metal ions as well), methylotrophs, ammonia oxidizers, and hydrogen oxidizers. None of these phenotypes is unique to the β-proteobacteria; all are also found at least the α- and β-proteobacteria. Many of these organisms are obligate autotrophs. The Calvin cycle is used for CO2 fixation.

The most prevalent group of chemoautotrophs in the β-proteobacteria are the sulfur oxidizers. These organisms use reduced sulfur compounds (sulfide, thiosulfate, thiocyanate, elemental sulfur) as the reductant for electron transport; oxygen is the common terminal electron acceptor, although many can use nitrate or nitrite as alternative terminal electron acceptors. Sulfuric acid is the product of sulfur oxidation, although some accumulate elemental sulfur as long a more reduced sulfur compounds are available. These organisms are ubiquitous in environments at the interface between aerobic and anaerobic zones, where sulfides and oxygen coexist. Many are capable of iron oxidation (ferrous to ferric); this, combined with their production of sulfuric acid from sulfur oxidation, makes them important contributors to the corrosion of plumbing. Sulfur oxidizers are used routinely in the mining process to extract metals from ores by leaching.

Example: Thiobacillus thioparus

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T. thioparus is an obligate chemolithotroph, oxidizing reduced sulfur compounds using oxygen or nitrate as the terminal electron acceptor. Nitrate is reduced to nitrite. This rod-shaped motile organism is an neutrophile, in contrast to the acidophilic γ-proteobacteria Athiobacillus species that were previously in this genus. Carboxysomes are present when grown under conditions of CO2 limitation; these are small proteinaceous organelles that concentrate CO2 and contain the enzymes of the Calvin cycle. Many organisms that use the Calvin cycle contain carboxysomes.

Sheathed filaments

The sheathed filamentous β-proteobacteria are commonly seen in polluted streams and wastewater. They are distinguished from other filamentous Bacteria, which are common in these environments, by their sheath, which is a tubular structure surrounding the cells of the filament. Sheathed β-proteobacteria usually “strengthen” their sheaths with a precipitate of iron hydroxide of manganese dioxide, which he organisms produce by oxidation of soluble reduced metal ions. These organisms are obligately aerobic heterotrophs; it is unlikely that energy from the oxidation of metals contributes to the energy needs of the cells. Sheaths often are surounded by a slime layer, and attached to the substrate by a terminal holdfast. In addition to attachment, the sheath protects the filaments from predation.

Example: Sphaerotilus natans

Sphaerotilus

S. natans is an iron-accumulating organism, perhaps the most commonly seen member of this group. Filaments contain false branches, breaks in the sheath from which filaments can protrude. Sheaths are thin and smooth, and are brown in color because it their impregnation with iron hydroxide. Filaments can sometimes leave their sheath and create now ones, or individual cells can leave to create new filaments. Empty space in sheaths is common. Filaments are 1.2-2.5μm in diameter, and individual cells are 2-10μm in length. S. natans is commonly found in aerated wastewater, here it can be a nuisance, contributing to “bulking”, poor settling of sludge.