Host Microbes Interaction

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Course Details

  • Course: Introduction to Microbiology (BIOL-2026 E)

  • Term: Fall 2025

  • Instructor: A. Omri

  • Lecture Topic: Host-Microbe Interactions

  • Date: December 01

Introduction to Microorganisms

  • Definition: Microorganisms, or microbes, are small living organisms that include:

    • Bacteria

    • Viruses

    • Fungi

    • Protozoa

    • Algae

  • Host-Microbe Interactions: These organisms interact with animals and plants in various ways, forming relationships that are fundamental to life on Earth.

  • Types of Interactions: These interactions can be:

    • Beneficial

    • Neutral

    • Harmful

  • Importance: Understanding host-microbe interactions provides insights into:

    • Ecosystems

    • Disease processes

    • Potential applications in medicine, agriculture, and biotechnology.

Types of Host-Microbe Interactions

  • Definition of Symbiosis: A long-term biological relationship between two different organisms, with one organism referred to as a symbiont.

Types of Symbiotic Interactions

  1. Parasitism:

    • Characteristics: One organism (the parasite) benefits while the other (the host) is harmed.

    • Example: Plasmodium spp., which causes malaria, harms the human host by destroying red blood cells.

  2. Commensalism:

    • Characteristics: One organism benefits while the other is neither helped nor harmed.

    • Example: Certain skin bacteria consume dead skin cells without affecting the host.

  3. Mutualism:

    • Characteristics: Both organisms benefit from the relationship.

    • Example: Gut bacteria in humans assist in digestion while receiving nutrients and a stable environment.

Evolution of Microbes and Hosts

  • Animals and plants have evolved with microbes, forming intricate associations.

  • Some associations are healthy and beneficial, such as the catabolism of cellulose by bacteria in the guts of animals.

  • Other associations may be unhealthy to one of the partners.

Location of Symbionts

  • Categorization of Symbionts Based on Location:

    1. Ectosymbionts:

    • Live on the surface of the host.

    • Example: Fungi growing on plant roots.

    1. Endosymbionts:

    • Live inside the host's tissues or cells.

    • Example: Mitochondria, thought to originate as bacterial endosymbionts.

Microbial Endosymbionts in Plants

  • Table 15.1: Some Microbial Endosymbionts of Plants:

    • Frankia filamentous bacteria:

    • Symbiont of: Alder trees.

    • Description: Infection of the root. Host provides fixed carbon to support nitrogen (N₂) fixation by the endosymbiont.

    • Mycorrhizal fungi:

    • Symbiont of: Most plant families.

    • Description: Infection of the root. Host provides fixed carbon to the fungus, which aids in nutrient uptake.

    • Nostoc filamentous cyanobacteria:

    • Symbiont of: Gunnera, herbaceous flowering plants.

    • Description: Intracellular infection of glands at the base of leaves; host supports N fixation.

    • Rhizobia (e.g. Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium):

    • Symbiont of: Members of the Leguminosae family (peas, beans, clover, alfalfa).

    • Description: Infection of the root, supporting N fixation.

  • Summary: These microbial endosymbionts have evolved mutually beneficial relationships with their plant hosts, exchanging fixed carbon for nutrients, water, and nitrogen fixation services.

Host-Microbe Interactions in Plants

Rhizobia and Legumes

  • Rhizobia: N-fixing bacteria that form a mutualistic relationship with legume plants (e.g., beans, peas).

  • Benefits:

    • To the plant: Converts atmospheric nitrogen (N₂) into ammonium (NH₄⁺), usable by plants.

    • To the bacteria: The plant provides carbon compounds and leghemoglobin, a protein that protects nitrogenase enzymes from damage.

Root Nodule Formation Process

  1. Recognition: Rhizobia detect plant-released flavonoids.

  2. Attachment: Rhizobia attach to root hairs, causing curling.

  3. Invasion: Formation of an infection thread to penetrate root cells.

  4. Nodule Formation: Specialized structures (nodules) house the bacteria in a low-oxygen environment.

Lichens

  • Definition: Mutualistic associations between fungi (mycobionts) and photosynthetic algae or cyanobacteria (photobionts).

  • Benefits:

    • Fungi provide structural support and protection.

    • Algae or cyanobacteria contribute carbohydrates through photosynthesis.

  • Survival: Lichens can survive extreme environments (rocks, tree bark, nutrient-poor soils).

Structure of Lichens

  • Main body part: Thallus.

  • Types:

    • Crustose (flat)

    • Foliose (leafy)

    • Fruticose (branched)

  • Each cell type remains distinctly separate from the other type of cell, maintaining distinct identities.

Development and Reproduction of Lichens

  • Asexual reproduction involves soredia: groups of algal cells packaged in fungal hyphae.

  • Development is usually slow, and lichens can survive for thousands of years. Environmental conditions can halt metabolism, but they restart once conditions improve.

Host-Microbe Interactions in Animals

Symbionts in the Human Body

  • Microbes colonize both internal and external surfaces, contributing to health and disease prevention.

1. Skin Microbiota

  • Habitat: Hair follicles and oil glands.

  • Role: Beneficial microbes can prevent infections by crowding out harmful ones.

2. Oral Cavity Microbiota

  • Habitat: Warm, moist environment rich in nutrients.

  • Common Microbes:

    • Gram-positive bacteria (e.g., Streptococcus mutans associated with dental caries).

    • Anaerobic bacteria such as Fusobacterium spp. (beneath the gumline).

  • Biofilm Formation: Dental plaque is a biofilm that can lead to cavities and periodontal disease.

3. Gut Microbiota

  • Composition: Hosts 800–2,500 microbial species.

  • Roles:

    • Produce vitamins (e.g., Vitamin K).

    • Aid in digesting complex carbohydrates.

    • Regulate immune function, contributing to balanced immune responses (the "hygiene hypothesis").

Major Group

Phylum

Characteristics

Metabolic Activities

Firmicutes

Clostridia

Gram-positive anaerobic spore-forming bacilli

Fermentation of starch, glucose to butyrate and acetate

Bacteroidetes

Bacteroidaceae

Gram-positive non-spore-forming rods/cocci

Fermentation of plant-derived carbohydrates

Proteobacteria

Enterobacteriaceae

Gram-negative non-spore-forming bacilli

Fermentation of various sugars

Actinobacteria

-

Gram-positive non-spore-forming rods/cocci

Fermentation of starch to lactate

Archaea

Methanobrevibacter smithii

Only archaeon consistently present in the gut

Methane production from CO₂ and H₂

Benefits of Gut Microbiota

  • Produce vitamins and aid in digestion.

  • Crowd out pathogenic microbes, helping maintain immune function.

  • Routine exposure to these microbes may help in preventing autoimmune diseases and allergies (hygiene hypothesis).

Probiotics

  • Definition: Live microbes consumed to promote gut health.

  • Effectiveness: Studies show variable results, beneficial in conditions like lactose intolerance and antibiotic-induced diarrhea.

Host-Microbe Interactions in Herbivores

  1. Cecal Fermentation:

    • Microbes ferment food in the cecum, enabling breakdown of cellulose (e.g., rabbits practice coprophagia).

  2. Rumen Fermentation:

    • Microbes in the rumen digest plant material before absorption (e.g., cattle rely on rumen microbes for 70% of energy needs).

Symbionts of Termites

  • Description: Termites consume cellulose-rich materials but lack the enzymes to digest them.

  • Key Symbionts::

    • Protozoa, bacteria, and archaea collaborate to break down cellulose and recycle nitrogen.

  • Methanogens: Archaea produce methane as a byproduct of fermentation.

Host-Microbe Interactions in Corals

  • Description: Mutualistic relationship with zooxanthellae (photosynthetic algae).

  • Benefits:

    • Corals receive carbohydrates from algae.

    • Algae receive protection and nutrients from corals.

  • Coral Bleaching: Stress (e.g., heat) can lead to the expulsion of zooxanthellae, threatening coral health.

Emerging Topics in Host-Microbe Interactions

  • Microbes and Obesity: Changes in gut microbiota may influence obesity.

    • Evidence includes germ-free mice gaining weight when introduced to normal gut microbes.

    • Diet changes can alter gut microbiota to resemble that of lean individuals.

Conclusion

  • Host-microbe interactions are essential for health, evolution, and ecosystem sustainability. Studying these interactions enhances our understanding of biological systems, improving human health and informing sustainable practices in agriculture and medicine.