"Figure 2.24: Rickettsias require special staining methods to see them under a microscope. Here, R. rickettsii, which causes Rocky Mountain spotted fever, is shown infecting the cells of a tick. Figure description available at the end of the chapter.\n\n2.4 PROTEOBACTERIA\n\nIn 1987, the American microbiologist Carl Woese (1928–2012) suggested that a large and diverse group of bacteria that he called “purple bacteria and their relatives” should be defined as a separate phylum within the domain Bacteria based on the similarity of the nucleotide sequences in their genome. This phylum of gram-negative bacteria subsequently received the name Proteobacteria. It includes many bacteria that are part of the normal human microbiota as well as many pathogens. The Proteobacteria are further divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria.\n\nALPHAPROTEOBACTERIA\n\nThe first class of Proteobacteria is the Alphaproteobacteria, many of which are obligate or facultative intracellular bacteria. Some species are characterized as oligotrophs, organisms capable of living in low-nutrient environments such as deep oceanic sediments, glacial ice, or deep undersurface soil. Among the Alphaproteobacteria are rickettsias, obligate intracellular pathogens, that require part of their life cycle to occur inside other cells called host cells. When not growing inside a host cell, Rickettsia are metabolically inactive outside the host cell. They cannot synthesize their own adenosine triphosphate (ATP), and, therefore, rely on cells for their energy needs. Rickettsia spp. include a number of serious human pathogens. For example, R. rickettsii causes Rocky Mountain spotted fever, a life-threatening form of meningoencephalitis (inflammation of the membranes that wrap the brain). R. rickettsii infects ticks and can be transmitted to humans via a bite from an infected tick (figure 2.24). Another species of Rickettsia, R. prowazekii, is spread by lice. It causes epidemic typhus, a severe infectious disease common during warfare and mass migrations of people. R. prowazekii infects human endothelial cells, causing inflammation of the inner lining of blood vessels, high fever, abdominal pain, and sometimes delirium. A relative, R. typhi, causes a less severe disease known as murine or endemic typhus, which is still observed in the southwestern United States during warm seasons. Table 2.7 summarizes the characteristics of important genera of Alphaproteobacteria.\n\nGenus Microscopic Morphology Unique Characteristics\n\nAgrobacterium Gram-negative bacillus Plant pathogen; one species, A. tumefaciens, causes tumors in plants\n\nBartonella Gram-negative, pleomorphic, flagellated coccobacillus Facultative intracellular bacteria, transmitted by lice and fleas, cause trench fever and cat scratch disease in humans\n\nBrucella Gram-negative, small, flagellated coccobacillus Facultative intracellular bacteria, transmitted by contaminated milk from infected cows, cause brucellosis in cattle and humans\n\nCaulobacter Gram-negative bacillus Used in studies on cellular adaptation and differentiation because of its peculiar life cycle (during cell division, forms swarm cells and stalked cells)\n\nCoxiella Small, gram-negative bacillus Obligatory intracellular bacteria; cause Q fever; potential for use as biological weapon\n\nEhrlichia Very small, gram-negative, coccoid or ovoid bacteria Obligatory intracellular bacteria; can be transported from cell to cell; transmitted by ticks; cause ehrlichiosis (destruction of white blood cells and inflammation) in humans and dogs\n\nHyphomicrobium Gram-negative bacilli; grows from a stalk Similar to Caulobacter (above)\n\nMethylocystis Gram-negative, coccoid or short bacilli Nitrogen-fixing aerobic bacteria\n\nRhizobium Gram-negative, rectangular bacilli with rounded ends forming clusters Nitrogen-fixing bacteria that live in soil and form symbiotic relationship with roots of legumes (e.g., clover, alfalfa, and beans)\n\nRickettsia Gram-negative, highly pleomorphic bacteria (may be cocci, rods, or threads) Obligate intracellular bacteria; transmitted by ticks; may cause Rocky Mountain spotted fever and typhus\n\nTable 2.7: Class Alphaproteobacteria"