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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.
| 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 |
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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