Class α-proteobacteria

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  • Class α-proteobacteria
    • Order Caulobacterales
      • Family Caulobacteraceae (e.g. Caulobacter)
    • Order Parvularculales
      • Family Parvularculaceae (Parvularcula )
    • Order Rhodobacterales
      • Family Rhodobacteraceae (e.g. Rhodobacter, Paracoccus, Roseobacter)
    • Order Rhodospirillales
      • Family Acetobacteraceae (e.g. Acetobacter, Gluconobacter, Stella)
      • Family Rhodospirillaceae (e.g. Rhodospirillum, Magnetospirillum)
    • Order Rhizobiales
      • Family Aurantimonadaceae (e.g. Aurantimonas)
      • Family Bartonellaceae (e.g. Bartonella, Rochalimaea)
      • Family Beijerinckiaceae (e.g. Beijerinckia, Methylocella)
      • Family Bradyrhizobiaceae (e.g. Bradyrhizobium, Rhodopseudomonas)
      • Family Brucellaceae (e.g. Brucella, Ochrobactrum)
      • Family Hyphomicrobiaceae (e.g. Hyphomicrobium, Ancylobacter)
      • Family Methylobacteriaceae (e.g. Methylobacterium, Microvirga, Meganema)
      • Family Methylocystaceae (e.g. Methyloyctis, Pleomorphomonas)
      • Family Phyllobacteriaceae (e.g. Phyllobacterium, Mesorhizobium)
      • Family Rhizobiaceae (e.g. Rhizobium, Agrobacterium)
      • Family Rhodobiaceae (e.g. Rhodobium, Roseospirillum)
  • Order Rickettsiales
    • Family Anaplasmataceae (e.g. Anaplasma, Ehrlichia, Wolbachia)
    • Family Holosporaceae (Holospora)
    • Family Rickettsiaceae (e.g. Rickettsia, Orientia)
  • Order Sphingomonadales
    • Family Sphingomonadaceae (e.g. Sphingomonas, Erythrobacter)

General characteristics of the α-proteobacteria


The α-proteobacteria are a large group of organism encompassing 7 Orders, 20 Families and more than 200 genera. Also in this group are the mitochondria of eukaryotes, probably as relatives of the Rickettsiales although this is obscured by the very high evolutionary rates in the gene sequences of the mitochondria.


Most of the purple non-sulfur phototrophic Bacteria are members of the α-proteobacteria, but there are also a wide range of heterotrophic species as well, including some important pathogens. Also in this Class are autotrophic methane oxidizers (methylotrophs) and a many organisms that are capable of nitrogen fixation, usually as symbionts of plant hosts. Autotrophic members of this Class fix CO2 via the Calvin cycle. The Sphingomonads lack lipopolysaccharide (LPS) in their cell envelopes, having instead glycosphingolipids similar to those found in the central nervous systems of animals.


Although most of the members of this Class are generic rods and cocci, and a few spirilla, many of the appendaged Bacteria are members of this group. These appendages are extensions of the cell, surrounded by the cells envelop, unlike the stalks of the Planctomycetes in which these are fibrous extracellular structures. Appendages can be single stalks such as in Caulobacter, or multiple “arms” as in Ancalomicrobium. In the Hyphomicrobia, including the purple phototroph Rhodomicrobium, reproduction is by budding from the ends of appendages, often resulting in chains and networks of cells connected by thin bridging appendages.


Members of this class are abundant in nearly all habitats, and are predominant members of both aerobic and anaerobic aquatic environments.

Purple non-sulfur phototrophs

With the exception of the genus Rhodocyclus, and a couple of close relatives, all purple non-sulfur Bacteria are members of the α-proteobacteria. These organisms carry out anoxygenic photosynthesis using a single photosystem, i.e. cyclic photophosphorylation (see Chapter 9 for a discussion of photosynthesis). Although all can grow photoheterotrophically, using light for energy and organic compounds for carbon growth requirements, most are also capable of photoautotrophy using hydrogen or reduced sulfur compounds as a source of reducing power for carbon fixation; this is captured in the form of NADH as the product of reverse electron flow, and is used to fix carbon via the Calvin cycle. A few can grow fermentatively producing organic acids, CO2 and hydrogen, or by anaerobic respiration using nitrate or nitrite as terminal electron acceptors. Most are capable of nitrogen fixation. Photosynthesis takes place in internal membranes, which can take the form of vesicles lining the cytoplasmic membrane, flattened stacks resembling thylakoids, or concentric lamellae lining the cytoplasmic membrane.

Example: Rhodomicrobium vannielii

Rhodomicrobium vannielii
: from Imhoff & Truper, The Prokaryotes

Rhodomicrobium is a commonly-isolated purple phototroph, producing deep red growth in liquid culture or colonies. It is an appendaged ovoid or rod-shaped cell that reproduces by budding from the ends of appendages. It prefers to grow photoheterotrophically, and is capable of utilizing a wide range of organics, but in the absence of these organic compounds it grows photoautotrophically using sulfide or hydrogen as the source of reducing power for carbon fixation. Thiosulfate, not elemental sulfur nor sulfate, is the product of sulfide oxidation. Unlike some other purple non-sulfur Bacteria, R. vannielii is not capable of nitrogen fixation. It is also unique amongst these organisms in having peritrichous flagella and in forming moderately heat-resistant resting cysts (exospores). Photosynthetic membranes are lamellar.

Appendaged Bacteria

The appendaged, or “prosthecate” α-proteobacteria contain cytoplasmic extensions. Most are aerobic heterotrophs, and live in oligotrophic environments. If only a single appendage is present, it is referred to as a stalk. These stalks are used to attach to surfaces, with terminal holdfasts often containing powerful adhesives. In those with multiple appendages, these apparently serve to increase the surface area of the cells, and they may also make them resistant to grazing by some protists. Most divide by binary fission, but a few divide by budding from the ends of appendages (e.g. Hyphomicrobium).

Some stalked α-proteobacteria are dimorphic; reproducing by binary fission in which one offspring resembles the original non-motile stalked “mother” cell, but the other offspring is flagellated and lacks a stalk. These motile “swarmer” cells disperse in the environment, attach to a surface, and develop into sessile stalked cells. This dispersal prevents the accumulation of non-motile cells in one place in which they would compete with each other for resources in their usual oligotrophic environment; this is the same reason most sessile reef animals have planktonic larvae.

Example: Caulobacter crescentus

Caulobacter crescentus division : from Yves Brun, Indiana University

C. crescentus is a well-studied dimorphic appendaged bacterium. Cells are vibriod or fusiform. Mature cells have a single thin terminal stalk. Division is by binary fission; before division is complete, a single flagellum is created at the pole opposite the stalk. Although the mother and swarmer cells produced are approximately the same size, the DNA of the swarmer cell is condensed and transcriptionally inactive, and metabolism in the swarmer is much reduced compared to the mother cell. When the swarmer cell comes in contact with a solid substrate, it adheres, loses its flagellum, and develops a stalk (with a terminal holdfast) from the same end of the cell that previously had the flagellum. Caulobacter is ubiquitous in aquatic samples, and is most readily observed attached to diatoms and eukaryotic algae. Caulobacter, and other appendaged Bacteria, are readily isolated from the surfaces inside laboratory distilled water containers.

Nitrogen-fixing plant symbionts

These organisms, known as the Rhizobia, form intimate symbioses with leguminous plants. The Bacteria enter the plant via the root hairs, enter the body of the root through an infection thread, and induce the formation of root nodules. In these nodules, the bacterial cells reproduce, then develop into the symbiotic form (“bacteroids”), that fix atmospheric nitrogen both for the bacterial symbionts and for the host plant. These bacteroids are terminally differentiated; they cannot revert to vegitative bacteria, nor can they reproduce. The host provides the bacteroids with nutrients and vitamins for grow and metabolism in return. The growth of these leguminous plants, with their nitrogen-fixing rhizobia, is commonly used to replenish impoverished soil.

As previously in the discussion of nitrogen fixation by cyanobacteria, nitrogenases (the enzymes that reduce N2 to NH4+) are strongly inhibited by oxygen. Rhizobia, however, are obligate aerobes. In culture, Rhizobia can be grown and fix nitrogen microaerophilically. In nodules, the bacteroids are provided with plenty of oxygen bound to a legume-hemoglobin, leghemoglobin, that is readily available for respiration but maintains a very low concentration of free oxygen that would inhibit nitrogenase.

The production of effective (nitrogen-fixing) nodules is a complex process requiring a matching pair of compatible host and bacterial symbiont. Mis-matched pairs of host:symbionts often can produce ineffective nodules that fix little or no nitrogen.

When the host plant dies, the small numbers of vegetative (non-bacteroid) rhizobia escape from the decaying nodule and persist in the soil, capable of infecting emerging host plants.

Example: Rhizobium etli

root nodules
Rhizobium etli root nodules : source unknown

This species, previously known as Rhizobium leguminosarum biovar phaseoli type I, is specific for the host Phaseolus vulgaris, the Latin American common bean. The genome of R. etli is comprised of a single large chromosome (4.38Mbp) and 6 smaller chromosomes ranging from 0.18 to 0.64Mbp. Because the smaller chromosomes contain mostly non-essential genes, they would usually be considered plasmids, but they contain a wide range of important genes, including genes required for normal cell cycling and the nod genes required for their normal symbiotic life cycle. The largest of these so-called plasmids are as big as the entire genomes of some obligately parasitic Bacteria!

Obligate intracellular parasites

These include a wide range of obligately intracellular symbionts or parasites of eukaryotes, especially animals. Best known are the insect symbionts (e.g. Wolbachia) and insect-borne human and mammal pathogens (e.g. Rickettsia). Also a member of this group phylogenetically, Bartonella (previously Rochalimaea) is an obligate parasite, but lives on the outside surface of the host cell rather than intracellularly. These organisms are often compared to viruses, but although small and obligately intracellular parasites, they are otherwise typically bacterial.

Also members of this group are the mitochondria of eukaryotes. These seem to be specifically related to Rickettsia, the causative agents of spotted fevers and typhus. Both groups lack enzymes for glycolysis (this being carried out in the cytoplasm of eukaryotes), but contain the enzymes of the complete TCA cycle and electron transport. Amino acids and nucleotides precursors are provided by the host in either case. The sequences encoding these proteins in Rickettsia resemble those of mitochondria, even in cases where these genes now reside in the nuclear genome rather than in the mitochondrial genome.

Example: Wolbachia pipientis

Wolbachia on insect egg
In a stained egg of the wasp, Trichogramma kaykai, are brightly staining Wolbachia. The bacteria accumulate at the end of the egg that is destined to develop into the reproductive organs. Photo Credit: Merijn Salverda and Richard Stouthamer

W. pipientis is the only formally described species of this genus. Wolbachia is a very common intracellular symbiont of arthropods and nematodes. A majority of insect species seem to be susceptible to infection, and perhaps 15-20% of individuals are infected. Although infection with Wolbachia is not associated with outright disease, it does cause a range of phenotypes in the reproductive biology of the host. This is because infected females transmit the symbiont directly to their eggs, but infected males do not transmit the symbionts to their offspring via infected sperm. As a result, in an effort to maximize the number of infected offspring their host generates, the symbiont manipulates the host in favor of producing females at the expense of males. Often this means inducing parthenogenesis, feminization (causing genetically male eggs to develop into females), or embryonic lethality in males (son-killing). In insects in which infected males are viable, they can usually only successfully fertilize the eggs from females infected with the same strain of Wolbachia, resulting in the phenomenon of “cytoplasmic incompatibility”; this seems to be mediated by imprinting of the hosts chromosomes in both sperm and eggs. This can create a reproductive barrier favoring host speciation in the absence of geographic isolation.