Brock Biology of Microorganisms Chapter 23 Microbial Symbioses Lecture Notes

Symbiotic Associations

  • Symbioses: Close, long-term relationships between organisms. - Characterized by effects of relationship on BOTH parties.

The Spectrum of Symbiotic Associations

  • Relationships: - Mutualism: Both species benefit (Effect on Species A: +++, Effect on Species B: +++)

    • Parasitism: One species benefits at the other's expense.

    • Commensalism: One species benefits while the other is unaffected. (Effect on Species A: 0, Effect on Species B: +)

    • Antagonism: One species harms another (Effect on Species A: +, Effect on Species B: -)

Microbe-Microbe Symbioses

Surfaces and Biofilms = Microenvironments

  • Biofilms: Assemblages of bacterial cells adhered to surfaces and enclosed in adhesive matrix excreted by cells. - Matrix Components: Typically a mixture of polysaccharides.

    • Extracellular Polymeric Substance (EPS): Long-chain sugar macromolecules form a gel-like layer.

    • Transparent Exopolymer Particles (TEP): Gel-like particles of acidic polysaccharides found in water.

    • Initiation: Begins with attachment of cell to surface, followed by expression of biofilm-specific genes.

Advantages of Biofilms

  • Allow cells to: - Remain in a favorable niche and maintain close association with one another.

    • Trap nutrients for growth.

    • Resist physical forces that would sweep away unattached cells.

    • Help resist phagocytosis by immune system cells.

    • Retard penetration of toxins (e.g., antibiotics).

    • Environmental Impact: Good for environmental films but make biofilms difficult to treat and resistant to standard disinfectants when a sterile surface is needed.

Medical and Industrial Implications of Biofilms

  • In Medical Conditions: Biofilms implicated in several conditions, e.g., periodontal disease, kidney stones, tuberculosis, Legionnaires' disease, Staphylococcus infections.

  • In Industrial Settings: - Biofilms can slow the flow of liquids through pipelines, including drinking water pipes.

    • They accelerate corrosion of inert surfaces, constituting a significant issue in food production.

  • Treatment Challenges: - Few effective antibiofilm agents available.

    • Biofilms exhibit recalcitrance (condition that is stubbornly resistant to treatment).

    • Scrubbing often isn’t 100% effective.

    • Phage “therapy” is being studied if proper host-phage combinations can be identified.

Lichens: True Mutualism

  • Structure: Leafy or encrusting microbial symbiosis involving fungi and algae found on bare rocks, tree trunks, roofs, and surfaces of bare soils.

  • Recent Evidence: 16s rRNA indicates a three-way mutualism involving fungi, yeast, and algae/cyanobacteria. - The yeast/fungus forms the structure with algae embedded within it.

  • Historical Context: This symbiosis has been known since 1867, but the role of yeast was a mystery until 2016.

  • Species Diversity: Estimated to have ~20,000 species and covers 6-8% of Earth's surface.

Microbe-Insect Symbioses

Symbionts of Insects

  • Acquisition Sources: - From environmental reservoirs (horizontal transmission) or parents (vertical or heritable transmission).

  • Heritable Symbionts: - Obligates: Required for reproduction and lack a free-living replicative stage.

    • Secondary: Not required for reproduction, often present in individual insects, can invade different cells, and must provide benefits (e.g., nutritional protection from pathogens).

Honey Bee Endosymbionts

  • Familiarity with plant-bee relationships as pollinators also includes a mutualism in the bee gut involving bacterial metabolic symbiosis. - Bee guts contain only ~5 species of essential bacteria, making them susceptible to environmental stresses and pesticides.

    • Gilliamella apicola: Degrades pectin from pollen grains essential for bee nutrition.

    • Snodgrassella alvi: Oxidizes fermentative products of G. apicola.

    • G. apicola forms volatile fatty acids (VFAs) crucial for honey bee health and metabolism.

Termite Endosymbionts

  • Cellulose Decomposition: Termites rely on symbionts to decompose cellulose.

  • Anaerobic Community: Certain termites harbor diverse anaerobic communities, including cellulolytic anaerobic bacteria and protists. - Bacteria break down cellulose into glucose and ferment it to acetate and VFAs, which termites utilize.

  • Example: Trichonympha makes up about 1/3 of termite gut contents.

Defensive Symbioses

  • A widespread strategy among insects to deter pathogens and predators through the production of toxic and antimicrobial chemicals often synthesized by microorganisms associated with insects.

  • Example: - Paederus Beetle (Rove Beetle): Uses chemical pederin synthesized by endosymbiotic Pseudomonas species to deter predators.

    • The cytotoxic chemical accumulates in the insect’s hemolymph and is deposited in its eggs, deterring predation on eggs.

  • Leafcutter Ants: Established obligate mutualism with a fungus, using small leaf fragments to feed it, demonstrating elaborate symbiotic associations between multiple microbial species and insects.

Other Microbe-Insect Endosymbiosis

  • Buchnera aphidicola: Endosymbiotic bacteria of pea aphids that synthesize essential amino acids not fully absorbable from plant sap. - W. glossinidia: Bacterial endosymbiont of tsetse flies, producing essential vitamins obtainable from blood meals.

    • Several endosymbionts educate an insect's immune system against other bacterial infections.

    • Wolbachia and Spiroplasma: Can influence hosts’ reproductive strategies.

Microbe-Plant Symbioses

Legume-Root Nodule Symbiosis

  • A critical mutualistic relationship between leguminous plants and diazotrophic (N2-fixing) bacteria, which is essential in agriculture. - N2 Fixation: The biological conversion of atmospheric N2 gas into ammonia (NH3).

    • Legumes: Plants with seeds in pods, e.g., soybeans, clover, alfalfa, beans, peanuts, and peas.

    • Rhizobia: N2-fixing Alphaproteobacteria or Betaproteobacteria that infect leguminous plants.

    • Specificity: Certain Rhizobia species can only infect specific legumes, indicating exquisite specificity.

Root Nodule Formation

  • The infection of legume roots by N2-fixing bacteria leads to the formation of root nodules where nitrogen fixation occurs. - This allows nodulated legumes to thrive in low-nitrogen soils and lessens the need for nitrogenous fertilizer.

Steps of Root Nodule Formation

  1. Recognition and Attachment: Mediated by rhicadhesin.

  2. Curling: Bacterium secretes Nod factors causing root hair to curl.

  3. Invasion: Rhizobia penetrate the root hair and multiply within an 'infection thread.'

  4. Growth towards Root Cell: Bacteria in the infection thread grow towards root cell.

  5. Bacteroid Formation: Bacteria inside root cells form a bacteroid state.

  6. Nodules Formation: Continued division of plant and bacterial cells leads to nodules, stimulating nearby cells to divide.

Other Non-Legume N2-fixing Symbiosis

  • Azolla Water Fern: Contains heterocystous nitrogen-fixing cyanobacteria (Anabaena) supplying nitrogen to rice crops.

Mycorrhizae: Mutualistic Association of Plant Roots and Fungi

  • Ectomycorrhizae: Found primarily in boreal and temperate forests; fungal cells form extensive sheaths around roots. - Plants gain significantly greater ability to absorb nutrients and water from soil.

  • Endomycorrhizae: Present in >80% of terrestrial plant species; fungi mine nitrogen and phosphorus from soil and convert them into arginine and polyphosphate. - Fungi also collect trace metals like zinc and copper, while obtaining sugars (energy) from plants.

  • Both ecto- and endomycorrhizae enhance plant diversity and strengthen terrestrial ecosystem health.

Microbe-(Aquatic) Invertebrate Symbioses

Aquatic Invertebrates: Hawaiian Bobtail Squid

  • A mutualistic relationship between the marine bacterium Aliivibrio fischeri and Hawaiian bobtail squid exemplifies animal-bacterial symbioses. - Synergy involves horizontal transmission whereby the light organ is colonized shortly after juvenile squid hatch.

    • Bacteria provide bioluminescence that serves as camouflage from predators.

    • The squid provides essential nutrients to A. fischeri.

Aquatic Invertebrates: Reef-Building Corals

  • The most ecologically significant symbiosis involves stony corals and dinoflagellates (typically Symbiodinium).

  • Algal symbionts are found in coral eggs, and developing corals can ingest these dinoflagellates to establish symbiosis.

Aquatic Invertebrates: Deep-Sea Hydrothermal Vents

  • Hydrothermal vents support diverse invertebrate communities fueled by chemolithotrophic microbes.

  • Chemolithotrophic prokaryotes utilize vent-emitted inorganic materials and form endosymbiotic relationships with invertebrate species like tube worms. - Tube worms harbor specialized structures (trophosome) that house symbionts and employ hemoglobin to facilitate growth while protecting these microbes.

Microbe-Animal Symbioses

Cellulose and Starch

  • Both cellulose and starch are long polymers of glucose but differ in molecular orientation. - Cellulose: Glucose molecules rotated 180° relative to one another, constituting 20-50% of plant matter.

    • Starch: Glucose monomers oriented similarly, serving as a stored energy source.

    • Digestion: Humans can digest starch but not cellulose due to the lack of necessary enzymes, making gut microbes critical for nutrient extraction from cellulose (plant fiber).

Fermentation in the Mammalian Gut

  • Animals with gut microbe associations can efficiently catabolize plant fiber. - Two Digestive Plans in Herbivores:

    1. Foregut Fermentation: Digestion occurs in a fermentation chamber before entering the small intestine.

    2. Hindgut Fermentation: Digestion occurs in the cecum and/or large intestine.

Differences Between Foregut and Hindgut Fermentation

  • Foregut: Fiber degraded before hitting stomach acid (e.g., ruminants like cows).

  • Hindgut: Fiber degraded after small intestine (e.g., horses and cecal animals).

  • Advantages: Foregut fermenters also gain protein from their fermenters.

Rumen (Foregut) and Ruminant Animals

  • Ruminants, such as cows, sheep, and goats, possess a specialized digestive organ known as the rumen where cellulose and other polysaccharides are digested with microbial help. - Rumen can hold up to 150 liters.

  • Rumen microbial community performs critical metabolic functions, including synthesizing amino acids and vitamins for their host and facilitating nutrient absorption.

Rumen Microbes

  • Contains 10^10 to 10^11 microbes per gram of ingested materials, consisting of approximately 300-400 different species, primarily anaerobes. - Fermentation: Conducted by cellulolytic microbes that hydrolyze cellulose into glucose, which is fermented, producing volatile fatty acids (VFAs) such as acetic, propionic, and butyric acid.

    • CO2 produced is utilized by Archaea to produce methane (CH4), while VFAs are absorbed by the bloodstream as the main energy source for the animal.

    • Rumen microbes provide protein through digestion and allow the animal to thrive on nutrient-poor diets.

Things to Know for Chapter 23

  • Key Concepts: - Definition of biofilm, formation processes, and advantages of living in biofilms.

    • Differences between mutualism and commensalism, parasitism and antagonism.

    • Familiarize with examples of each type of symbiotic relationship:

    • Microbe-Microbe

    • Microbe-Insect

    • Microbe-Plant

    • Microbe-Aquatic Invertebrate

    • Explain the benefits of these symbiotic relationships.