Microbial Population and Community Dynamics

Microbial Population vs Community

• Microbial population: One species living in association with others
• Microbial community: Consists of multiple microbial populations

Key Factors Influencing Microbial Species Richness and Abundance

• Species richness: Total number of different species present
• Species Abundance: Proportion of each species in the community
• Habitat conditions: Factors like available nutrients and prevailing conditions directly impact richness and abundance

Microbial Habitats

• Microorganisms thrive in diverse environments including common habitats like soil and water, extreme environments, and on/within other organisms.
• Each environment has defined physicochemical conditions shaped by the microbial community’s metabolic activities

Microenvironments

• Microbes experience a limited local environment called their microenvironment.
• Microenvironments can change rapidly within a single habitat.

Microbial Nutrient Levels & Growth Rates

• Microorganisms often face a “feast-or-famine” existence.
• Extended periods of exponential growth are rare in nature.
• Growth rates are usually lower than laboratory maximum rates.

Microbial Competition & Cooperation

• Competition for resources is intense with factors like nutrient uptake rate, metabolic rates, and growth rates impacting the outcome.
• Syntrophy: Microbes working together in metabolic transformations that are not possible individually.
• Examples of complementary metabolisms: Nitrifying bacteria and archaea.

Microbial Surfaces

• Surfaces provide important habitats offering:
  • Greater nutrient access
  • Protection from predation and environmental disturbances
  • A way for cells to remain in favorable habitats, modify them, and avoid being washed away.
• Flow across a surface increases nutrient transport.
• Surfaces can be provided by other organisms or by nutrients like organic matter particles.

Surface Colonization

• Can be sparse, with microcolonies invisible to the naked eye.
• Can be so dense, that microbial accumulation is visible.
• Thickness can reach centimeters in extreme environments without grazing animals.

Mutualism (Positive Interaction)

• Mutualism is an obligatory relationship where both organisms benefit metabolically.
• Mutualists are specifically dependent on each other, and cannot be replaced by other species.
• Physical contact between organisms is required.
• Allows organisms to thrive in habitats they could not occupy alone.
• They act as a single organism.

Mutualism Examples

• Rhizobium-legume association: Nitrogen fixation.
• Mycorrhizae: Symbiosis between plant roots and fungal hyphae.
  • Ectomycorrhizae, Endoomycorrhizae, and Ectendomycorrhizae.
• Lichens: Symbiosis between a fungus and alga.
  • Fungus (mycobiont) is the dominant partner.
  • Alga (phycobiont) can be cyanobacteria.
• Herbivore-microbial interactions: Intestinal bacteria in ruminants.
  • Ruminococcus flavefaciens, Ruminococcus albus, and Fibrobacter succinogenes.

Commensalism (Positive Interaction)

• Commensalism is a unidirectional relationship where one organism benefits, and the other is unaffected.
• Examples:
  • Disease-causing microbes opening lesions for other microbes.
  • Staphylococcus: Some species are commensal skin flora, while others cause infections.
  • Aspergillus: Lives in the gastrointestinal tract and can cause aspergillosis in immunocompromised individuals.

Proto-cooperation (Synergism) (Positive Interaction)

• Both populations benefit from each other.
• Allows for metabolic activities, with nutrient exchange between species.
• Association is NOT obligatory.
• Example 1: Desulfovibrio bacteria and Chromatium bacteria.
  • Desulfovibrio supplies hydrogen sulfide and carbon dioxide.
  • Chromatium provides sulfates and organic material.
• Example 2: Azotobacter and Cellulomonas.
  • Azotobacter fixes nitrogen.
  • Cellulomonas provides glucose.

Syntrophism (Positive Interaction)

• Special symbiosis between two metabolically different microorganisms.
• Cooperation through short-distance metabolite transfer.
• Both organisms can carry out a metabolic function that neither can alone
• The growth of one organism depends on or is improved by another's substrate.
• Example: Methanogenic ecosystem in sludge digester.
  • Methanogenic bacteria produce methane using hydrogen transferred from fermentative bacteria.
  • Anaerobic fermentative bacteria utilize carbohydrates for CO2 and H2, which Methanobacter uses for methane production.
• Example: Lactobacillus arobinosus and Enterococcus faecalis
  • E.faecalis requires folic acid from L.arobinosus.
  • L.arobinosus requires phenylalanine from E.faecalis

Parasitism (Negative Interaction)

• Characterized by a long-term physical or metabolic contact between host and parasite.
• Ectoparasites live outside the host cell, while endoparasites live inside.
• Examples:
  • Fungi: Mycoparasitism (necrotrophic, biotrophic)
  • Viruses: Obligate intracellular parasites with host specificity (bacteriophage, fungi, algae, protozoa).
  • Bdellovibrio: Ectoparasite to gram-negative bacteria.
    • Penetrates the host's outer membrane, entering the periplasmic space (not the cytoplasm).

Predation (Negative Interaction)

• One organism (predator) engulfs or attacks another (prey), resulting in the prey's death.
• Short-duration interactions.
• Examples:
  • Protozoa-Bacteria in soil: Protozoa feed on bacteria.
  • Predatory bacteria: Bdellovibrio, Vampirococcus, Daptobacter.