Alphaproteobacteria (al-fa-PRO-te-o-bak-TE-re-uh) is derived from two Greek roots and a Greek letter meaning "alpha" (α) "changeable" (proteakos -πρωτεϊκός) "little stick" (bakterion -βακτήριον).  The name is in reference to Proteus, the name of a Greek sea god who could change his shape (Stackebrandt et al. 1988).



The Alphaproteobacteria, first defined as a subclass by Stackenbrandt et al. (1988), includes infectious agents, nitrogen-fixing symbionts, and pigmented autotrophic/heterotrophic taxa. The rickettsias are very small intracellular parasites of vertebrates, usually with a very complex lifecycle involving an invertebrate intermediate host.  Important rickettsial diseases include Typhus and Rocky Mountain Spotted Fever (together with othe spotted fever diseases).  They are transmitted by an arthropod vector (fleas transmit Typhus; ticks transmit Rocky Mountain Spotted fever).  In general, rickettsial diseases of economic importance attack haemopoetic tissue, cells of the immune system, and vascular endothelium of vertebrates.

The purple nonsulfur bacteria are anaerobic photosynthetic bacteria that use bacteriochlorophylls to capture light energy.  Mainly, they are photoheterotrophs, organisms that use light energy to make food using organic substrates rather that CO2.  However, these ecologically resourceful organisms also can be photosynthetic (photoautotrophs) when growing in the light, or chemotrophic when growing in the dark.  Hydrogen sulfide, which is a necessary substrate for the Purple Sulfur Bacteria (see the Betaproteobacteria) can be toxic to these organisms when it is in high concentrations.  The versatility of the purple nonsulfur bacteria allows them to be abundant in soil, aquatic environments (sediment and water column), activated sludge, and many marine environments.

The rhizobias are nitrogen-fixing bacteria, which allows them to use atmospheric nitrogen to make amino acids.  Rhizobium is a common genus that enters into symbiotic relationships with legumes and other nodule-forming plants, which may help to explain the great diversity of leguminous plants.  Rhizobia also exist as common members of the microbial flora of soil and and aquatic systems where they are important contributers to most nitrogen fixation.

Stackebrandt et al. (1988), using 16S rRNA sequences, defined a seemingly unrelated group of eubacteria as Proteobacteria, the purple bacteria, which they defined as a class called Proteobacteria.  Within that group, they defined five separate lines, each defined by a Greek letter: α, β, γ, δ, ε.  The second edition of Bergey's Manual of Systematic Bacteriology (Garrity et al. 2003) adopted Proteobacteria, but raised it to phylum level with each of the five groups becoming classes.  In order to bring the prokaryotes into line with kingdom-level divisions in the eukaryotes, I felt that it was necessary to raise the Proteobacteria to kingdom-level status with each of the five groups also raised to the level of phylum.

The Alphaproteobacteria has two major groups within it: the rickettesias and all other taxa (Williams et al. 2007).  We took this separation to be at the class-level (classes Rickettsiae and Rhodobacteriae).  The ordinal structure is from Garrity et al. (2003).






FIGURE 5.  Topology of the Proteobacteria with the relationships of the phyla and classes of the Alphaproteobacteria (in shaded box).







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By Jack R. Holt.  Last revised: 02/18/2013