Microbiology Lecture Notes: Protists, Lichens, Fungi, Helminths, and Viruses

Lichens: Symbiotic Pioneer Organisms

• Definition and Composition:

  • Lichens are unique, complex organisms resulting from a symbiotic relationship between two distinct partners: a fungus (chemoheterotroph) and a photosynthetic partner (photoautotroph).

  • Fungal Partner: Typically belongs to the phyla Ascomycota or Basidiomycota.

  • Photosynthetic Partner: Can be either Algae or Cyanobacteria (one of the first oxygen producers on Earth).

• Ecological and Clinical Significance:

  • Pioneer Organisms: They are often the first to colonize bare rock or harsh soil environments where neither fungi nor algae could survive independently.

  • Bioindicators: Extremely sensitive to air quality; they are used as fantastic indicators of environmental health, particularly in cities or sterile environments like HVAC systems.

  • Infection: Lichens generally do not cause infections in humans.

• Morphological Types (Exam Recognition):

  • Crustose: Crust-like growth that adheres closely to the surface.

  • Foliose: Leaf-like appearance; look like leaf structures but are neither plant nor fungus.

  • Fruticose: Shrub-like or strand-like growth, exhibiting more complex branch-like structures.

• Anatomy and Physiology:

  • Outer Cortex: The protective exterior layer formed by the fungus to house the partner.

  • Medullary Region: The inner region where the algae or bacteria are primarily located.

  • Lower Cortex: The bottom protective layer.

  • Metabolism: Primarily aerobic; the algae provide energy via photosynthesis while the fungus provides protection and absorbs nutrients.

Introduction to Mycology (The Study of Fungi)

• General Characteristics:

  • Eukaryotes: Possess membrane-bound organelles.

  • Cell Types: Can be unicellular (Yeasts) or multicellular (Mushrooms and Molds).

  • Nutritional Mode: Heterotrophic, specifically chemoheterotrophic. Most are saprozoic (decomposers), breaking down organic matter via the secretion of digestive enzymes into the environment and absorbing the resulting small molecules (monosaccharides, amino acids).

• Environmental Resilience:

  • Fungi are more environmentally resistant than bacteria.

  • pH Preference: Thrive in acidic environments, typically ranging from pH $4.0$ to pH $7.0$.

  • Moisture: Ubiquitous but prefer cold, damp, and high-moisture habitats.

• Fungal Anatomy:

  • Hyphae: Filamentous structures that make up the body of the fungus.

    • Septate Hyphae: Divided into individual cell compartments by walls (septa).

    • Aseptate (Coenocytic) Hyphae: Lack internal walls, allowing a continuous flow of cytoplasm and nutrients.

  • Mycelium: A massive network or bundle of hyphae.

  • Cell Wall: Composed of Chitin (rather than cellulose or peptidoglycan).

  • Sterols: Plasma membranes contain ergosterols instead of cholesterol.

  • Rhizoids: Anchor the fungus to its substrate.

  • Fruiting Body: The visible reproductive structure (like a mushroom) that releases spores.

• Reproduction and Spores:

  • Highly successful due to the production of millions of spores.

  • Dispersal: Spores are airborne and can stay in air currents, or be spread by water and animals.

  • Modes: Can reproduce both sexually (for genetic diversity and harsh conditions) and asexually (for rapid propagation in stable conditions).

Fungal Diversity and Phyla

• Phylum Microsporidia:

  • Early-diverging fungal branch.

  • Characteristics: Unicellular, non-motile, lacks typical hyphae and standard mitochondria (has modified mitochondria).

  • Pathogenesis: Obligate intracellular parasites. They use a unique polar tube (similar to a harpoon) to infect host cells (ants, fish, birds, humans).

• Phylum Zygomycota:

  • Multicellular with aseptate hyphae.

  • Examples: Rhizopus stolonifer (Black bread mold).

  • Reproduction: Produces sporangiospores (asexual) and zygospores (sexual).

• Phylum Basidiomycota (Club Fungi):

  • Includes mushrooms, puffballs, stinkhorns, and rusts.

  • Anatomy: Consists of a pileus (cap), stipe (stalk), and gills (where spores are released).

  • Spores: Produces basidiospores within a basidiocarp.

  • Clinical Relevance: Includes Amanita phalloides (The Death Cap), which produces potent neurotoxins, and various psychedelic/hallucinogenic species.

• Phylum Ascomycota (Sac Fungi):

  • The largest and most diverse fungal phylum.

  • Examples: Saccharomyces cerevisiae (Baker’s yeast used for beer/bread), truffles, morels, and the source of penicillin.

  • Pathogens: Causes dermatophyte infections (Athlete’s foot, Jock itch, Ringworm) and Valley Fever (respiratory infection from soil spores).

  • Vulnerability: Mostly problematic for immunocompromised individuals, the very young, or the very old.

Helminths: Parasitic Worms

• General Features:

  • Large, multicellular eukaryotic organisms.

  • Parasitism: The majority ($50\%$ to $80\%$) are parasitic.

  • System Reduction: Because they steal nutrients from hosts, they often have reduced or non-existent digestive, circulatory, and nervous systems. However, they have highly advanced reproductive systems.

  • Life Cycles: extremely complex, often requiring multiple hosts (intermediate and definitive).

• Phylum Nematoda (Roundworms):

  • Anatomy: Unsegmented, sleek cylindrical bodies with longitudinal muscles creating a characteristic "whip-like" motion. They possess a hydro-skeleton (pseudocoelom).

  • Pinworm: The most common parasitic worm in the United States.

  • Ascaris lumbricoides: The most common parasitic worm worldwide.

  • Vectors: Elephantiasis (Wuchereria bancrofti) is spread by mosquitoes, affecting the lymphatic system.

• Phylum Platyhelminthes (Flatworms):

  • Trematodes (Flukes): Leaf-shaped with oral and ventral suckers. They are named after the organs they infect (Liver flukes, Lung flukes, Blood flukes). Common life cycle involve snails and fish.

  • Cestodes (Tapeworms):

    • Structure: Consist of a Scolex (head with hooks/suckers for attachment) and Proglottids (segments).

    • Hermaphroditic: Each proglottid contains both male and female reproductive organs and can self-reproduce if broken off.

    • Size: Can reach lengths of $10$ meters ($30$ feet).

    • Species: Taenia saginata (Beef tapeworm) and Taenia solium (Pork tapeworm).

Acellular Microbes: Viruses

• Viral Composition:

  • Contain nucleic acids (DNA or RNA) and proteins (capsid).

  • Envelopes: Some are enveloped (lipid bilayer stolen from host); others are non-enveloped. Non-enveloped viruses are often more resistant to environmental stressors.

  • Spikes: Carbohydrate structures used for attachment.

• Biology and Life Cycles:

  • Obligate Intracellular Pathogens: No independent metabolism; must hijack a host cell to reproduce.

  • Lytic Cycle: Direct, rapid replication ending in host cell lysis ($1$ virus to thousands/millions).

  • Lysogenic/Latent Cycle: The virus integrates its genetic material into the host DNA and remains dormant (e.g., Shingles/Chickenpox hiding in the central nervous system for decades).

  • Oncogenic Viruses: Some viruses can trigger cancer by mutating host DNA or suppressing tumor-suppressor genes.

• Genome Classification:

  • Positive-sense single-stranded RNA ($ssRNA+$): Functions directly as mRNA; can be translated by host ribosomes immediately upon entry.

  • Negative-sense single-stranded RNA ($ssRNA-$): Must first be transcribed into positive-sense RNA using an enzyme.

  • Retroviruses: Use Reverse Transcriptase to go from RNA back to DNA, a process not found in any living organism. Example: HIV.

Viroids, Virusoids, and Prions

• Viroids and Virusoids:

  • Acellular entities consisting only of short strands of circular ssRNA ($220$ to $380$ nucleotides).

  • Lack a protein coat. Mostly identified as plant pathogens.

  • Ribozymes: Some RNA molecules act as enzymes (ribozymes), which may provide clues to how life originated from a "pre-viral" RNA world.

• Prions (Proteinaceous Infectious Particles):

  • Not living; consist only of misfolded proteins, specifically rich in beta-plated sheets.

  • Mechanism: A misfolded prion touches a normal protein and triggers a domino effect, causing the normal protein to misfold.

  • Disease: Causes Transmissible Spongiform Encephalopathies (TSEs), such as Mad Cow Disease (Bovine Spongiform Encephalopathy) and Creutzfeldt-Jakob Disease in humans. This results in sponge-like holes in the brain tissue.

  • Resistance: The most resistant of all microorganisms. They cannot be killed by standard boiling, UV light, or typical sterilization methods.

Questions & Discussion

• Student Question: Does the tapeworm segment have those hooks and stuff on each step?

• Instructor Response: No, the hooks/suckers are only on the Scolex (the head). However, every segment (proglottid) has its own male and female reproductive system. If a piece breaks off, it has the full DNA code to grow and differentiate. In theory, if you have the DNA code and a nucleus, you can make a head or any other part of the body by up-regulating gene expression.

• Instructor Closing/Logistics:

  • Next Tuesday is optional. The instructor will be in class to review the quiz and exam material.

  • The practice exam and actual exam will follow later. The actual exam is scheduled for Thursday at $12:30$.

  • The next unit covers Immunology: the body’s three lines of defense against these pathogens.