In-Depth Notes on Microbial Symbioses

Microbial Symbioses

Types of Microbial Symbiotic Associations

• Mutualism: Both organisms benefit (e.g., lichens, which consist of fungi and algae).

• Commensalism: One organism benefits, and the other is unaffected.

• Amensalism: One organism is harmed while the other is unaffected.

• Parasitism: One organism benefits at the expense of the other. (parasite + host)

Legume-Rhizobia Symbiosis

• Nitrogen Fixation in Soybean Roots:

  • Infection of legume roots by Rhizobium bacteria.

  • Formation of root nodules by nitrogen-fixing bacteria (rhizobia) that fix nitrogen essential for plant growth.

  • Leghemoglobin in the nodules protects nitrogen-fixing enzymes (nitrogenases) from oxygen.

Mycorrhizal Associations

• Mycorrhizae: Mutualistic associations between plant roots and fungi.

  • Fungal Benefits: Fungus transfers inorganic nutrients (N, P, K) to plants.

  • Plant Benefits: Provides carbohydrates to fungi.

  • Two types:

  • Ectomycorrhizae → outside root

  • Endomycorrhizae → inside root.

Bioluminescent Symbiosis in Squids

• Aliivibrio fischeri and Hawaiian Bobtail Squid:

  • Squid hosts A. fischeri in a light organ that emit light resembling moonlight aiding in camouflage, attract mates

  • Bioluminescence due to quorum sensing

  • A. fischeri bacteria receive nutrients from the squid.

Ruminants and Microbial Symbiosis

• Rumen Structure: Digestive organ hosting a complex microbial community.

  • Main function: Fermentation of plant materials (cellulose) via cellulolytic microbes.

  • Production of volatile fatty acids as the primary energy source for ruminants.

  • Abrupt dietary changes can alter rumen flora, potentially causing acidosis (rumen acidification).

Human Microbiome Overview

• Microbial Habitats in the Human Body:

  • Humans host a microbiome with approximately the same number of microbes as human cells.

• Core Questions:

  • Is there a shared core microbiome across individuals?

  • How does microbiota composition relate to host genotype and health?

  • The role of microbiota variations in disease susceptibility.

Gut Microbiota Dynamics

• Gut Microbiota Development:

  • Changes in microbial communities occur from infancy to adulthood.

  • Inflammatory Bowel Disease (IBD): Associated with dysbiosis and reduced gut microbiome diversity.

Antibiotics and Microbiome Health

• Impact of Antibiotics:

  • Can decrease all gut microbes, leading to dysbiosis and increased risk of conditions like IBD.

  • Clostridium difficile Infections: Associated with antibiotic overuse, leading to antibiotic resistance.

• Fecal Transplants: Used to restore normal microbiota in patients suffering from conditions such as C. difficile infections.

  • Success demonstrated by improved resolution rates following transplants from healthy donors.