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Symbiosis
The living together of two different organisms, a specific symbiotic bacterium and a specific animal or plant host
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Commensal Enteric Sybiont
Living in the intestine of an animal, using nutrients in the intestine, beneficial or neutral
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Parasite
Living in or on an animal or plant, usually in a long-term association, and causing disease
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Saprophyte
Colonizes the surface of an animal or plant, uses nutrients excreted to the surface, beneficial or neutral
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Modes of Symbiont Transmission (from one host generation to the next)
- Veritcal- from parent to offspring, symbionts colonize the egg (e.g., intracellular bacterial symbionts of insects like Buchnera in aphids)
- Horizontal (environmental)- each new generation of the host picks up the symbiont from the environment (Rhizobium in legumes)
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Recognition/Attachment
Contact between the surface of the bacterium and the plant root cell, mediated by chemicals released by the root hair
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Excretion
Plant releases specific chemical factors to which the bacterial cells respond by growing and in turn producing nodulation (Nod) factors, that stimulate formation by the plant of an infection thread
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Invasion/Colonization
The bacterial cells travel through the infection thread as it forms, travel to the main root, stimulate the growth and division of cortical cells, which the bacteria colonize as bacteroids (swollen cells capable of nitrogen fixation) in a "symbiosome" (plant cell membrane enclosing many bacteroids)
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