Lecture 23

  1. Symbiosis
    The living together of two different organisms, a specific symbiotic bacterium and a specific animal or plant host
  2. Commensal Enteric Sybiont
    Living in the intestine of an animal, using nutrients in the intestine, beneficial or neutral
  3. Parasite
    Living in or on an animal or plant, usually in a long-term association, and causing disease
  4. Saprophyte
    Colonizes the surface of an animal or plant, uses nutrients excreted to the surface, beneficial or neutral
  5. 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)
  6. Recognition/Attachment
    Contact between the surface of the bacterium and the plant root cell, mediated by chemicals released by the root hair
  7. 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
  8. 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)
Card Set
Lecture 23
Bacterial Symbiosis