JC

Mycology: Trash men of the microbial world

Mycology: Overview and Importance

  • Mycology: the study of fungi, often described as the trash men of the microbial world.
  • Fungi are divided into:
    • Filamentous/fleshy fungi (molds and mushrooms)
    • Non-filamentous (yeasts)
  • Relevance:
    • Increasing importance in medicine: rising incidence as nosocomial infections; problematic for immunocompromised individuals
    • Important in agriculture: many devastating pathogens of commercial crops; nearly all plants rely on mycorrhizae for nutrient uptake
    • Balancing note: fungi can be beneficial (e.g., nutrient cycling) and harmful (pathogens, allergies)

Fungi vs. Bacteria: Key Differences

  • Fungi are eukaryotic cells.
  • Fungi have sterols in their plasma membrane.
  • Fungi lack peptidoglycan in their cell walls.
  • Fungi are heterotrophic.
  • Fungi have both sexual and asexual life cycles.

Filamentous Fungi: Structure and Growth

  • Filamentous fungi possess hyphae that form the thallus (the body of the fungus).
  • Thallus: composed of long, thin filaments that are chains of cells.
  • Hyphae (singular: hypha) are the basic filaments; they can grow to very large extents.
    • Example scale: the hyphae of a single fungus in Michigan covered 40\,\mathrm{acres}.
    • Example scale: a single mushroom’s hyphae extended over 4\,\mathrm{miles} in Oregon.
  • Hyphae may be:
    • Septate hyphae: cross-linked walls present, dividing the hypha into cells.
    • Coenocytic hyphae: lack septa, appear as one continuous cell with multiple nuclei.

Hyphal Structure: Vegetative vs Reproductive

  • Hyphae are functionally divided into two regions:
    • Vegetative hypha: responsible for nutrient absorption.
    • Reproductive (aerial) hypha: responsible for reproduction; commonly bears spores.
  • Spores are often produced on the reproductive hyphae.

Fungal Hyphae: Visual References

  • Septate hypha: has septa (dividing walls) between cells.
  • Coenocytic hypha: lacks septa; multiple nuclei inside a continuous cytoplasm.
  • Visual cues include labeled structures: cell wall, pore, nuclei, septum (from figures in lecture).

Types of Hyphae

  • Aerial hyphae: extend into the air; typically involved in spore production.
  • Vegetative hyphae: grow within the substrate; involved in nutrient uptake.
  • Example organism: Aspergillus niger (on agar and in lab images show both aerial and vegetative hyphae).

Hyphae Growth and Mycelium Formation

  • Hyphae elongate at the tips; both tips are capable of growth.
  • Under favorable conditions, hyphae form a filamentous mass called a mycelium.
  • Mycelium is visible to the unaided eye when it matures.
  • Practical lab note: cultures are often started from hyphae of thallus.
  • Summary: growth at tips → filamentous mass → mycelium, visible as a network.

Yeast: Overview

  • Yeast are unicellular, nonfilamentous fungi; typically spherical or oval.
  • Ubiquitous in nature; common on fruit and leaves as a white powder.
  • Yeasts are divided into two classes based on their division method: budding and fission.

Budding Yeast

  • Budding process:
    • A small protrusion (bud) forms on the surface of the parent cell.
    • The nucleus divides and is sequestered into the growing bud.
    • Cellular components are also sequestered into the bud; cell wall material is distributed.
    • The bud usually breaks away to become a new cell.
  • Reproductive capacity: one yeast cell can produce up to 24\,\text{daughter cells}.
  • Budding can fail to separate, forming short chains of cells called pseudohyphae.
  • Clinical note: some pathogens (e.g., Candida albicans) require pseudohyphae to invade deeper tissue.

Fission Yeast

  • Division resembles bacterial binary fission in principle.
  • Three general stages:
    • Cell elongates.
    • Nucleus and organelles are replicated.
    • Cell is pinched in the middle, producing two daughter cells.
  • On solid media, fission yeast colonies resemble bacterial colonies.

Oxygen Dependence

  • Yeast exhibit facultative anaerobic growth.
  • With oxygen: they respire sugars to CO₂ and H₂O.
  • Without oxygen: they ferment sugars to CO₂ and ethanol.
  • This metabolic flexibility allows survival in diverse environments.

Dimorphic Fungi

  • Some fungi, notably pathogens, exhibit dimorphism: they grow as yeasts or as molds depending on conditions.
  • Rule of thumb: for many pathogens, temperature-regulated dimorphism: at 37°C, yeast-like growth; at 25°C, mold-like growth.
  • For non-pathogenic fungi, dimorphism can also depend on CO₂ concentration.

Life Cycle of Filamentous Fungi

  • Asexual reproduction via fragmentation of hyphae is common in labs; hyphae can be moved to new locations.
  • Filamentous fungi can also reproduce by spore formation, both sexually and asexually.

Spore Differences: Fungi vs Bacteria

  • Fungal spores are reproductive; bacterial spores are dormant and not reproductive.
  • Fungal spores can undergo sexual recombination.
  • Fungal spores are hardy, but generally not as protective as bacterial endospores.
  • Spores are typically more resistant than vegetative cells.

Asexual Spores

  • Asexual spores are more common than sexual spores.
  • Produced from the hyphae of a single organism; offspring are genetically identical to the parent.
  • Two main types:
    • Conidiospore: spores not enclosed by a sac.
    • Sporangiospore: spores enclosed by a sac (sporangium).

Conidiospores: Classification and Formation

  • Conidiospores can be classified by mode of formation:
    • Arthroconidia: formed by fragmentation of hyphae into single cells with thickened walls (example: Coccidioides immitis).
    • Blastoconidia: formed by budding off of the parent cell (example: Cryptococcus).
  • All conidiospores form at the end of a stalk-like structure called a conidiophore.

Chlamydoconidium

  • Chlamydoconidium is a thick-walled spore formed by rounding and enlarging of a hyphal segment; produced by important fungi such as Candida albicans.

Sporangiospore

  • Sporangiospores are enclosed in a sac called a sporangium.
  • The sporangium is located at the end of an aerial hypha called a sporangiophore.
  • Terminology parallels conidiospores but uses the prefix sporangio- instead of conidio-.

Conidia and Sporangiophore: Visual References

  • Conidia and sporangiophores are often depicted in SEM images (e.g., scale bars 5\,\mu\mathrm{m} for conidia, 10\,\mu\mathrm{m} for sporangiophores).

Sexual Spores

  • Result from sexual reproduction occurring in three phases:
    • Plasmogamy: haploid nucleus from donor cell penetrates the cytoplasm of the recipient cell.
    • Karyogamy: nuclei fuse to form a diploid zygote nucleus.
    • Meiosis: diploid nucleus gives rise to haploid nuclei, some of which may be genetic recombinants.
  • A representative sequence shows zygospore formation within a zygosporangium, leading to sporangia that release spores.
  • Life cycle can alternate between vegetative mycelium and sexuales structures, depending on species and conditions.

Nutrition and Environmental Adaptations

  • Fungi occupy niches different from bacteria; notable features include:
    • Preference for a mildly acidic environment: growth around pH = 5.0.
    • Most molds are aerobic; yeasts are typically facultative (can grow with or without oxygen).
    • Fungi are more resistant to osmotic pressure and can grow in higher sugar or salt concentrations.
    • Fungi can grow on substrates with lower moisture content than many bacteria.
    • Fungi require less nitrogen for equivalent growth and can metabolize complex carbohydrates, which many bacteria cannot.

Economic and Practical Aspects

  • Economic uses:
    • Food and beverage production: bread, beer, wine.
    • Antibiotics: penicillin originally harvested from molds; other antibiotics isolated from molds.
    • Enzymes: cellulase from molds used to clarify fruit juices.
    • Pharmaceuticals: Taxol (paclitaxel) produced by Taxomyces spp. as an anticancer agent.
    • Biocontrol: some molds repelling or killing select insects.
  • Negative economic aspects:
    • Fungi can infect plant roots, causing crop damage.
    • Spores can cause allergic reactions in sensitive individuals.
    • They are natural decomposers and can affect aging items, including foods.
    • Fungi can cause severe, long-lasting, potentially fatal infections in humans and animals.

Fungal Diseases (Mycoses)

  • Not all fungi cause disease; those that do cause mycoses.
  • Mycosis often chronic due to slow fungal growth.
  • Four classifications of mycoses based on host tissue involvement and entry route:
    • Systemic
    • Subcutaneous
    • Cutaneous
    • Opportunistic

Systemic Mycoses

  • Deep-seated infections that can affect multiple tissues and organs.
  • Transmission typically via inhalation of soil-dwelling fungal spores.
  • Initial infection usually in the lungs and may spread to other tissues.
  • Generally not contagious from person to person.

Subcutaneous Mycoses

  • Infections beneath the skin, often through puncture wounds that implant spores or mycelial fragments.
  • Common in gardeners or farmers due to soil exposure and wounds.

Cutaneous Mycoses

  • Involve epidermis, hair, or nails; caused by dermatophytes.
  • Disease: dermatophytosis; contagious between humans and animals.
  • Dermatophytes produce keratinase, which degrades keratin in hair, nails, and skin.

Opportunistic Pathogens

  • Opportunistic fungi are generally harmless in their normal habitats but can cause disease in immunocompromised hosts.
  • At-risk groups include:
    • People on long-term drug therapies
    • Individuals with advanced HIV/AIDS
    • Cancer patients undergoing chemotherapy
    • People with immune disorders
  • Many mycoses are caused by opportunistic fungi.