1. symbiosis: refers to any interaction between 2 or more different organisms. Examples;

• commensalism- where one member benefits from the association, and the other(s) is/are unaffected, i.e., one organism or group of organisms takes advantage of the capabilities of others.
  
• parasitism- one partner, the parasite, benefits at the expense of one or more members.
  
• mutualism- two or more members benefit from association. If the result of this association is merely enhanced growth or product formation, this is referred to as synergism.

2. Examples of:

• Commensalism- all are microbe/microbe interactions (S. mutans with other microorganisms).
  
• Parasitism- Many examples
  • viruses with their host cell.
    
  • bacteria as human pathogens (discussed later in course).
    
  • Bacteria/bacteria- Bdellovibrio interaction with Gram-negative bacteria (E. coli)
    • Bdellovibrio "bores" into periplasmic space of E. coli.
    • kills E. coli (mechanism unknown), but E. coli remains intact.
    • macromolecules from E. coli leek into periplasmic space where they are degraded and used by Bdellovibrio for growth.
    • Bdellovibrio grows into a long filament, which fragments into individual cells (~20 cells).
    • Bdellovibrio lysis E. coli , and Bdellovibrio cells released.
      
• Mutualism- Many examples
  • microorganisms that comprise the normal human flora with human host.
    
  • bacteria (genus Rhizobium) with plants (legumes) symbiosis-
    • Nitrogen-fixation- The ability to utilize N₂ as a nitrogen source is confined to the procaryotic world. Some bacteria can carryout nitrogen fixation on their own, while other do so only in association with a plant. Bacteria of the genus Rhizobium are the most important nitrogen-fixing bacteria in terms of total amount of nitrogen fixed globally. Different species of Rhizobium enter into a mutalistic relationship with specific leguminous plants (legumes are plants that form pods that split in two when mature such as alfalfa, clover, peas, beans, lupin, and vetch). Steps:
      • invasion of root hairs, and formation of infectious thread.
      • some root cells become infected and begin to grow, giving rise to a root nodule. Bacteria dividing intracellularly.
      • within root nodule nitrogen fixation occurs (converts N₂ to a more utilizable form of nitrogen NH₃ or NH₄⁺ also known as fixed nitrogen). Process requires nitrogenase enzyme which is encoded by bacterial genes.
      • fixed nitrogen then used by bacteria and plants to grow. What does bacteria get out of this association?
      • nitrogen fixation can only occur in an environment devoid of oxygen, yet plant is producing oxygen and bacteria need oxygen to carry out aerobic respiration! The way this problem is resolved is that the plant and bacteria encode for a protein, called leghemoglobin which is similar to hemoglobin, which binds oxygen, and prevents from inactivating nitrogenase.
        
  • Tube worm/bacteria (chemoautotrophic) symbiosis-
    • 2 meter long tube worms lack mouth, digestive tract, and anus; tube worm made of spongy tissue called the trophosome (50% of weight), which is loaded with sulfur granules and bacteria that can oxidize sulfur (S_o, H₂S, and S₂0₃^-2). Chemolithotrophic, or chemoautotrophic, bacteria.
    • Sulfur is oxidized to produce energy (oxygen and nitrate serving as terminal electron acceptors).
    • Energy used to carryout CO₂ fixation to produce glycogen, and bacterial cell growth occurs.
    • bacteria are then lysed within tube worm cells, called trophosomes, and bacterial nutrients used by tube worm for growth.
    • red plume of tube worm contains unusually soluble hemoglobins which is required to bring sulfur compounds and oxygen to bacteria!
      
  • microbe/herbivore (grass eating animals such as sheep, cattle, goats, deer, moose, caribou, and giraffes) mutualism:
    • these animals do not have genetic capability of synthesizing enzyme, cellulase, needed to break down cellulose, the main chemical component of grasses.
    • The digestive tract of these animals consist of 4 stomachs. The first 2 are collectively called the rumen and this organ is essentially an enormous culture vessel (100 liters in a cow, and 6 liters in a sheep) teeming with bacteria and protozoa. The microbial population in the rumen can reach numbers around 1 X 10¹⁰ per ml. These microorganisms synthesize cellulase.
    • rumen is anaerobic and vast majority of metabolism is through fermentation.
    • fermentation products absorbed by herbivores and used for growth.
      
  • termites/bacteria symbiosis- similar situation as above, except that bacteria that make cellulase are within a protozoa which resides within termite!

Learning Objectives:

• How do the different forms of symbiosis differ from one another?
• What is the importance of the Rhrizobium/legume interaction. How is nitrogenase protected from oxygen?
• What is each partner get out the relationship in the herbivore/microbe interaction, and the termite/protozoan/bacterial interaction.
• Describe how the tube worm is able to grow in an environment completely devoid of organic materials?