Fungi

Symbiosis (Quality of Microbe–Host Relationships)

Eukaryotic Microbes & Life-Cycle Fundamentals

  • Include fungi and protozoa (plus arthropods & helminths briefly mentioned).

  • Shared trait with plants: complex life cycles involving both diploid and haploid stages.

    • Organisms unable to “seek a mate” must survive as both genome states.

    • Humans are exclusively diploid; fungi/protozoa can alternate.

  • Reproduction via standard eukaryotic processes → mitosis and meiosis.

Fungi – General Characteristics & Morphology

  • Study = Mycology.

  • Enormous morphological diversity:

    • Single-celled yeasts (e.g., Saccharomyces spp.).

    • Multicellular forms (molds, mushrooms).

    • Dimorphic fungi – can alternate between yeast-like (single) and mold-like (multi) forms depending on environment; most pathogenic fungi fall here.

  • Metabolism

    • Non-photosynthetic → heterotrophic (ingest food)

      • Many are saprophytic – eat dead organic matter (fallen trees, carcasses, feces, etc.) and drive decomposition.

Therapeutic & Industrial Importance of Fungi

  • Therapeutics

    • Antibiotics: Penicillin (Penicillium) – first true antibiotic. was from fungi

    • Cholesterol drugs: statins (Crestor, Liptor) from fungi metabolites

    • Immunosuppressants: transplants

  • Homeostasis- Candida alicans if grow too much can make us sick

  • Food & Beverage

    • alcohol & Bread leavening

    • cheese (blue cheese, feta, gorgonzola) (swiss and cheddar= bacteria)

    • Cheese: bacterial cheeses (cheddar, Swiss) vs. fungal cheeses (blue, feta, Gorgonzola – the blue veins are fungal, not cheese!).

    • Food spoilage- majority fungi are psychotropic→ love to grow in refrigerator

  • Agriculture

    • plant pathogens- smut and rust are primary sources of harming plants

    • Mycorrhizae: symbiotic mutualistic fungi relationship with the alfalfa plant. they enhance nutrient uptake in plants, improving growth and resistance to pathogens by 2x

  • Biochemistry of fungal cell wall → chitin

Fungal Phyla Overview (Phylogenetic “Big Four”)

  • Ascomycetes (75% of all fungi)

    • Can be single-celled, multicellular, or dimorphic (only phylum with all three options).

  • Basidiomycetes

    • Home of true mushrooms/toadstools.

    • Mostly multicellular, some single-cellular

    • Reproduce sexually; very few asexual species.

  • Zygomycetes

    • Strictly multicellular.

    • source of molds

  • Chytridiomycetes

    • Only aquatic fungi.

    • Sexual spores have flagella → only motile fungal spores.

Asexual Reproduction via Spores

  • Fundamental contrast with bacteria

    • bacteria: 1 spore → not considered reproduction

    • fungi: thousands of spores→ replication and genetic variation capabilities

  • Spores are the primary means of fungal reproduction and dissemination.

  • Seasonal pattern for many species: massive releases in spring and fall maximize chances of finding suitable conditions.

Major Spore-Producing Strategies

Sporangiospores

  • Spores formed inside a membranous sac (sporangium) → rls upon rupture and is airborn

  • pro: Timed release in favorable seasons increases successful colonization.

  • con: Vulnerable before maturity (an animal steps on it → entire season’s output lost).

Arthroconidia

  • Hyphal fragments breaks off and become spores (thallus)

  • Usually not wind-borne; drop and moved by water flow.

Conidiospores ("Conidia" = Latin dust)

  • No surrounding membrane. Spores "bud" continuously from hyphae tips.

  • pro: can’t be "cut off"; as long as the mold is alive, spores are produced.

  • con: continuous production ⇒ majority spores ever successful (energetic waste).

Fungal Anatomy

(note- Yeasts are unicellular and lack these structures.)

  • fungi can grow from:

    • A spore (sexual or asexual reproductive structure)

    • A thallus: a fragment of the fungus that continues growing like a plant cutting.

      • hyphae- branching filaments formed from the spore/thallus (fungus structural filaments)

        • Vegetative hyphae – mass of growth: absorb nutrients

        • Reproductive (fertile) hyphae – make spores

        • Rhizoids- anchors to structure for support only

          • Haustoria – specialized parasitic rhizoids that penetrate host cells.

          • Clinical significance: infections with haustoria often require tissue removal and to antifungals because the fungus is bound to tissue.

  • Mycelium – visible mass = sum of all hyphae.

Clinical Priorities with Fungi

  • minor infections very common, severe infections not common bc must enter thru an opening to the body

  • numbered based on highest concern

  1. Hypersensitivity- Allergy / Asthma

    • Airborne spores

    • Spores can reach \approx 7\ \text{miles} (≈ 11.3\ \text{km}) altitude and spread hundreds of miles laterally.

  2. Intoxication – fungi produce many toxins

    • Aflatoxin produced by Aspergillus – food-borne carcinogen; constant concern for grains, and crops

      • important for agriculture

    • Ergot produced by Claviceps purpurea on rye –

      • pro: drug ergotamine → used in surgery b/c cuts off blood supply

      • con: powerful hallucinogen (salem witch trials)

    • Amanita toxins – one mushroom cap may cause fatal liver damage.

  3. Mycosis (actual infection) – fungal rare and opportunistic

Categories of Mycoses

Level

Typical Examples

Key Points

Superficial (Dermal) infection

Athlete’s foot, ringworm

On surface; barely serious. most common

Subcutaneous infection

Sporothrix spp. – "Rose-gardener’s disease"

Fungus introduced traumatically into the skin (rose thorn); fungus can’t invade deeper or leave; spreads laterally; often requires surgical cleaning + topical antifungal.

  • most rare

Systemic infection

Pulmonary, blood-borne, CNS forms

mostly opportunistic and dimorphic fungi (ascomycetes)

  • middle common

(opportunistic fungi)

candida albicans

Aspergillus

fungi will only cause a disease under certain conditions

Immunocompromise – Broader Than You Think

  • Any breach or physiologic change that opens a route into tissue counts (wet, macerated feet in soldiers → “jungle rot” / trench foot).

  • Classic severe immunosuppression (AIDS, leukemia, chemotherapy) still poses the highest risk for deep/systemic disease (e.g., invasive aspergillosis).

Dimorphic Fungi

  1. Mold Phase

    • Mold grows in the environment

    • favorable growth conditions: lots of nutrients of Oxygen

    • produces thousands & thousands of spores (airborne particles)

    • spores inhaled → the infectious particle

  2. Yeast Phase

    • body provides drastically diff conditions: low O_2 & nutrients and high temp. cannot become multicellular

    • cannot become multicellular so becomes yeast → causes pathogenic illness

Phylum Ascomycota

  • 75% of known fungi.

  • yeasts or molds: Only phylum containing dimorphic species.

  • Industrial & medical importance:

    • includes most medically important fungi-

      • Penicillium (source of penicillin & food molds).

      • also largest pathogenic contributor bc contains dimorphic

    • Culinary morels & truffles (not true mushrooms technically mold).

3 Pathogens discussed

1. Histoplasma capsulatum

  • dimorphic fungus; endemic to eastern USA (east of the Rockies – wetter climate).

  • sporangia erupts and can cause outbreaks via pulmonary route

    • allergies in spring, sometimes a cold, even more rare: pnemonia or blood borne infection

2. Aspergillus sp.

  • dimorphic

  • most common airborne allergen worldwide! present on 6 of 7 continents.

  • Produces aflatoxin; also a top airborne allergen.

  • Typical diseases

    • (ABPA) Allergies & hypersensitivity.

    • sinusitis

    • Pulmonary aspergilloma- (“fungus ball” pneumonia).

      • especially w/ immunocompromised ppl, when breath spore in, starts as pneumonia and the yeast grows into mold in the lungs

      • golf-ball-sized masses obstruct bronchi.

      • absolutely worst: sporulate in lungs and spread to brain (cerebral aspergillus)

3. Candida

  • Normal human skin & mucosal flora.

  • 2 species

    • C. albicans

      • majority

      • have overtime evolved to become opportunistic pathogens bc lived with humans so long

      • infection sites:

        • Oral- “thrush” (neonatal, old ppl, STI)

        • Vaginal- “yeast infection”

        • Invasive (systemic)- “Candidemia” (deathly, fungi enter blood) often occurs from surgical procedures

        • Babies- diaper rash

    • C. aurus

      • multi drug-resistant fungus (some resistant to all)

      • often contracted in hospitals

      • more contractible → spreads easily between ppl and surfaces w/o too much energy compared to normal fungi which need lots of energy to do so

Phylum Zygomycota

  • multicellular rarely pathogenic

  • coenocytic hyphae: no septum in their hyphae (no places for fragmentation)

    • Rhizopus: bread molds

      • digest starch and sugar (cannot digest cellulose)

      • the only pathogenic type

Phylum Basidiomycota

  • called “club fungi” → spore containers

  • always have septate hyphae

  • includes: mushrooms, toadstools, rusts & smuts

  • pathogen

    • human: few→ Cryptococcus neoformans (affects immunocompromised)

    • plants: many→ Rusts and Smuts which infect the plants.

      • they don’t harm us but the things they carry and produce do

      • toxic: Deathcap mushrooms

Eukaryotic Parasites: Vectors

  • ectoparasites- attach to outside of host

    • arthropods like ticks and lice and they function as vectors

  • endoparasites- attach to inside of host

    • fungi, protozoa, helminths (worms)

  • Vectors:

    • usually multicellular carriers of disease

      • Mechanical vector A → pathogens ‘hitches’ a ride and you eat

        • very common, not so severe

      • Mechanical vector B → pathogen inside arthropod so need to enter us via blood (mosquito, west nile virus)

        • less common, more severe