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C. trachomatis is a human pathogen that causes trachoma , a disease of the eyes, often leading to blindness. C. trachomatis also causes the sexually transmitted disease lymphogranuloma venereum (LGV). This disease is often mildly symptomatic, manifesting as regional lymph node swelling, or it may be asymptomatic, but it is extremely contagious and is common on college campuses.

[link] summarizes the characteristics of important genera of Alphaproteobacteria.

A diagram showing the life cycle of Chlamydia. An epithelial cell is infected by small spheres labeldd elementary bodies. Within 12 hours, these form into reticulate bodies which divide  to form inclusions within 24 hours. Within the inclusions more elementary bodies are formed and within 72 hours these are released when the cell ruptures.
Chlamydia begins infection of a host when the metabolically inactive elementary bodies enter an epithelial cell. Once inside the host cell, the elementary bodies turn into active reticulate bodies. The reticulate bodies multiply and release more elementary bodies when the cell dies after the Chlamydia uses all of the host cell’s ATP. (credit: modification of work by Centers for Disease Control and Prevention)
Class Alphaproteobacteria
Genus Microscopic Morphology Unique Characteristics
Agrobacterium Gram-negative bacillus Plant pathogen; one species, A. tumefaciens , causes tumors in plants
Bartonella Gram-negative, pleomorphic, flagellated coccobacillus Facultative intracellular bacteria, transmitted by lice and fleas, cause trench fever and cat scratch disease in humans
Brucella Gram-negative, small, flagellated coccobacillus Facultative intracellular bacteria, transmitted by contaminated milk from infected cows, cause brucellosis in cattle and humans
Caulobacter 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)
Chlamydia Gram-negative, coccoid or ovoid bacterium Obligatory intracellular bacteria; some cause chlamydia, trachoma, and pneumonia
Coxiella Small, gram-negative bacillus Obligatory intracellular bacteria; cause Q fever; potential for use as biological weapon
Ehrlichia 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
Hyphomicrobium Gram-negative bacilli; grows from a stalk Similar to Caulobacter
Methylocystis Gram-negative, coccoid or short bacilli Nitrogen-fixing aerobic bacteria
Rhizobium 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)
Rickettsia 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
  • What characteristic do all Alphaproteobacteria share?


Unlike Alphaproteobacteria, which survive on a minimal amount of nutrients, the class Betaproteobacteria are eutroph s (or copiotrophs), meaning that they require a copious amount of organic nutrients. Betaproteobacteria often grow between aerobic and anaerobic areas (e.g., in mammalian intestines). Some genera include species that are human pathogens, able to cause severe, sometimes life-threatening disease. The genus Neisseria , for example, includes the bacteria N. gonorrhoeae, the causative agent of the STI gonorrhea , and N. meningitides , the causative agent of bacterial meningitis .

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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