DESCRIPTION OF THE PHYLUM ALPHAPROTEOBACTERIA (STACKENBRANDT ET AL. 1988)
| EUBACTERIA> PROTEOBACTERIAE> ALPHAPROTEOBACTERIA |
PHYLUM ALPHAPROTEOBACTERIA LINKS
| 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). |
| INTRODUCTION TO THE ALPHAPROTEOBACTERIA 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 (text with tooltip) Bacteriochlorophylls are prokaryotic photosynthetic pigments that are related to chlorophylls of eukaryotes. 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) (text with tooltip) Phototrophs utilize light energy to provide metabolic energy. Sometimes this energy is stored asreduced organic compounds (food). when growing in the light, or chemotrophic (text with tooltip) Chemoheterotrophs are those organisms that use compounds from the environment to provide metabolic energy. 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 (text with tooltip) Nitrogen fixation is the ability to use energy to take nitrogen gas and reduce it to ammonium, nitrite, or nitrate. The initial step requires a nitrogenase enzyme and the energy of about 38 ATP molecules to fix one nitrogen molecule. bacteria, which allows them to use atmospheric nitrogen to make amino acids. Rhizobium is a common genus that enters into symbiotic (text with tooltip) Symbiotic (adj.) literally means 'living together'. Its usual meaning is that two species have a necessary association. However, strictly applied symbiosis varies from parasitism, to commensalism, to mutalism. 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) |
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| FIGURE 1. TEM micrograph of Rickettsia inside a host cell. Image from the CDC and in the Public Domain | FIGURE 2. Rhodospirillum, a purple nonsulfur bacterium. Image from http://www.de.mpi-magdeburg.mpg.de/research/projects/1010/1014/1020 | FIGURE 3. SEM micrograph of Rhizobium attached to plant roots. Image from http://www.arc.agric.za/institutes/ppri/main/divisions/plantpathology/rhizobium.htm | FIGURE 4. Nodules on the root system of a legume caused by a symbiosis with Rhizobium. Image from http://biology.unm.edu/ccouncil/Biology_203/Images/Monera/rhizobium.jpg |
FIGURE 5. Topology of the Proteobacteria with the relationships of the phyla and classes of the Alphaproteobacteria (in shaded box).
| LITERATURE CITED Black, J. G. 2002. Microbiology, Principles and Explorations. 5th ed. John Wiley and Sons, Inc. New York. Brock, T. D., M.T. Madigan, J.M. Martinko, and J. Parker. 1994. Biology of Microorganisms. 7th ed. Prentice Hall. Englewood Cliffs, NJ. Emerson, D., J. A. Rentz, T. G. Lilburn, R. E. Davis, H. Aldrich, C. Chan, and C. L. Moyer. 2007. A novel lineage of Proteobacteria involved in formation of marine Fe-oxidizing microbial mat communities. PLoS ONE. 2007 (8): e667. http://www.plosone.org Gao, B. and R. S. Gupta. 2007. Phylogenetic analysis of proteins that are distinctive of Archaea and its main subgroups and the origin of methanogenesis. BMC Genomics. 8:86. http://www.biomedcentral.com/1471-2164/8/86. Garrity, G. M., M. Winters, and D. Searles. 2001. Bergey’s manual of systematic bacteriology. 2nd ed. Springer-Verlag. New York. Garrity, G. M., J. A. Bell, and T. G. Lilburn. 2003. Taxonomic Outline of the Prokaryotes. Bergey’s Manual of Systematic Bacteriology. 2nd edition. Release 4.0. Springer-Verlag. New York. pp. 1-397. Margulis, L. and K. Schwartz. 1988. Five kingdoms, an illustrated guide to the phyla of life on earth. 2nd Edition. W. H. Freeman and Co. New York. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Co. New York. Stackebrandt, E., R. G. E. Murray, and H. G. Trüper. 1988. Proteobacteria classis nov., a name for the phylogenetic taxon that includes the “Purple Bacteria and Their Relatives”. International Journal of Systematic Bacteriology. 38(3): 321-325. Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York. Williams, K. P., B. W. Sobral, and A. W. Dickerman. 2007. A robust species tree for the Alphaproteobacteria. Journal of Bacteriology. 189(13): 4578-4586. Woese, C. R. and G. E. Fox. 1977. Phylogenetic structure of the prokaryotic domain: the primary kingdoms. Proceedings of the National Academy of Sciences USA. 74:5088-5090. Woese, C. R., O. Kandler, and M. L. Wheelis. 1990. Towards a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya. Proc. Natl. Acad. Sci. USA. 87: 4576-4579. |
| By Jack R. Holt. Last revised: 02/18/2013 |
