Microbial Symbiosis Notes

Dissimilar organisms living together.
Symbiotic relationships are common in both terrestrial and aquatic (freshwater and marine) environments.
Allivibrio fischeri serves as an example for quorum sensing due to its light production.

The squid communicates with Allivibrio fischeri and facilitates selective colonization.
Squid secretes mucous, creating an environment that selects for A. fischeri.
Antimicrobials and nitric oxide secreted by the squid help create a monoculture.
A. fischeri sends signals to its host to develop its light organ.

Host (Squid):
- Development of the light organ.
- Production of light for camouflage at night, reducing predation.
Microbe (A. fischeri):
- Access to an environment (carbon, nutrients, and temperature) that supports growth.
- Reduced predation.
- Daily release back into the environment, maintaining higher population levels than would be expected without the squid.

The mutualistic relationship between legumes (plants with seeds in pods) and nitrogen-fixing bacteria is a crucial symbiosis.
Examples of legumes: soybeans, clover, alfalfa, beans, and peas.
Rhizobia are well-known nitrogen-fixing bacteria involved in these symbioses.
Rhizobia: A group of Alphaproteobacteria or Betaproteobacteria species that can grow freely in the soil or infect leguminous plants.

Infection of legume roots by nitrogen-fixing bacteria leads to the formation of root nodules that fix nitrogen.
This process significantly increases the amount of combined nitrogen in the soil.
Nodulated legumes thrive in areas where other plants cannot.
Nitrogen-fixing bacteria need $O<em>2$ to generate energy for $N</em>2$ fixation, but nitrogenase enzymes (responsible for nitrogen fixation) are inactivated by $O_2$ .
Leghemoglobin, an $O_2$ -binding protein within the nodule, acts as an "oxygen buffer" to protect nitrogenases from free oxygen.

Different rhizobia generally infect different species of legumes (e.g., rhizobia that infect peas differ from those infecting clover).
Cross-inoculation group: A group of related legumes that can be infected by a particular species of rhizobia.

Step 1: Recognition and attachment of bacterium to root hairs.
Step 2: Excretion of Nod factors by the bacterium.
Step 3: Bacterial invasion of the root hair.
Step 4: Travel to the main root via the infection thread.
Step 5: Formation of bacteroid state within plant cells.
Step 6: Continued plant and bacterial division, leading to the formation of a mature root nodule.

Bacteroid formation is essential for nitrogen fixation.
The nodulation process, from infection to effective nodule formation, takes about 1 month for soybeans.

Bacteroids rely on the plant for fuel (in the form of pyruvate) for nitrogen fixation.
The legume-bacteria symbiosis involves oxygen sequestration, various metabolic reactions, and nutrient exchange.

Host (Legume): Access to a usable form of nitrogen.
Microbes (Rhizobia): An environment providing a continuous supply of an electron donor and acceptor, with conditions optimized for nitrogen fixation.

Different types of mammals exist:
- Herbivores: Animals that consume plants.
- Carnivores: Animals that consume meat.
- Omnivores: Animals that consume both plants and meat.
Phylogenetic evidence suggests that different lineages evolved an herbivorous lifestyle.

Some herbivores are foregut fermenters, while others are hindgut fermenters.
Some mammalian carnivores consume only insects (insectivores, like bats) or fish (piscivores, like river otters).

Herbivorous mammals (e.g., cows, sheep, goats) possess a special digestive organ called the rumen.
Cellulose and other plant polysaccharides are digested with the help of microbes.
The rumen is well-studied due to the presence of an implanted sampling port.

The rumen contains $10^{10}$ - $10^{11}$ microbes per gram of rumen constituents.
Fermentation in the rumen is mediated by cellulolytic microbes that hydrolyze cellulose to free glucose, which is then fermented, producing volatile fatty acids (e.g., acetic, propionic, butyric) and $CH<em>4$ and $CO</em>2$ .
Fatty acids pass through the rumen wall into the bloodstream and are utilized by the animal as its main energy source.
Rumen microbes also synthesize amino acids and vitamins for the animal host.
Rumen microbes can serve as a protein source for the host when they are directly digested.
Anaerobic bacteria dominate in the rumen.
The rumen contains 300 to 400 bacterial "species."

Host (Ruminant): Breakdown of cellulose and conversion into volatile fatty acids that are directly used in catabolic and anabolic pathways. A consistent source of vitamins and amino acids.
Microbes: An environment that provides a warm, anoxic environment with a continuous input of carbon.

In symbiotic relationships, one, both, or neither species may benefit.
Mutualism: A relationship where both the host and associated microbes benefit.
Symbiosis: A close, long-term interaction between organisms of at least two different species. Involves at least two different species closely interacting together over a period. For the host and/or microbes, one, both, or neither species may benefit from the relationship. When both host and associated microbes benefit, the relationship is called mutualism. A close, long-term interaction between organisms of at least two different species.