THE EUBACTERIA
| SYSTEMATIC BIOLOGY |
THE EUBACTERIA |
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| PHYLUM ALPHAPROTEOBACTERIA | |||||
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 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 versitility 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.
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A. TEM micrograph of Rickettsia inside a host cell. |
B. Rhodospirillum, a purple nonsulfur bacterium. |
C. SEM micrograph of Rhizobium. |
D. Nodules on the root system of a legume caused by a symbiosis with Rhizobium. |
| Images taken from: A: http://www.cdc.gov/ncidod/eid/vol7no1/images/raoult1b.jpg B: http://www.de.mpi-magdeburg.mpg.de/research/projects/1010/1014/1020 C: http://www.arc.agric.za/institutes/ppri/main/divisions/plantpathology/rhizobium.htm D: http://biology.unm.edu/ccouncil/Biology_203/Images/Monera/rhizobium.jpg |
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SYNOPTIC DESCRIPTION OF THE ALPHAPROTEOBACTERIA
| The following description comes mainly from Margulis and Schwartz (1998), Barnes (1984), Brock et al... (1994), and Tudge (2000). |
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I. SYNONYMS: α-proteobacteria, rickettsias, purple nonsulfur bacteria, nitrogen-fixing bacteria, and nodule-forming bacteria. II. PHYLUM CHARACTERISTICS A. Structure Cell Form: Unicellular rods, cocci, spirals; and filaments. Cell Wall: Gram-. Motility: Non-motile or motile with polar or peritrichous flagella. B. Physiology O2 Tolerance: Aerobes and facultative anaerobes. Substrates: Rickettsias have limited metabolic ability and rely on the host cell. The purple nonsulfur bacteria are very diverse chemoorganotrophs, chemolithotrophs, photoautotrophs, and some as methylotrophs. Rhizobias fix molecular nitrogen by oxidation of small organic compounds such as ethanol, succinate, malate, fumerate, pyruvate, etc. Products: Rickettsias: have limited metabolic ability and rely on the host cell. Purple nonsulfur bacteria: produce food (or reduced organic carbon). Rhizobias: Ammonium; symbiotic species release glutamine and other N-rich compounds to their host plant. C. Other: Very diverse group of bacteria. D. Ecology: Enter into parasitic (rickettsias), commensal, or mutualistic (many rhizobias) symbiotic relationships. Free-living taxa free-living in soil and aquatic systems. |
SYSTEMATICS OF THE ALPHAPROTEOBACTERIA
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 that they 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). I took this separation to be at the class-level (classes Rickettsiae and Rhodobacteriae). The ordinal structure is from Garrity et al. (2003).
HIERARCHICAL CLASSIFICATION OF THE ALPHAPROTEOBACTERIA
I generally follow the system of Garrity et al. (2003), but used Williams et al. (2003) as support for the large separation at the class-level. |
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CLASS RICKETTSIAE
CLASS RHODOBACTERIAE
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This page is maintained by Jack R. Holt. Last revised: 02/05/2008.
