6. Microorganisms: fungi

Question 1

What are the two types of moulds?

Answer 1

• Plasmodial slime moulds
• Cellular slime moulds

Question 2

What are the two types of yeast?

Answer 2

• Budding yeast
• Fission yeast

Question 3

What are the types of fungi?

Answer 3

Fungi grow as yeast / filamentous fungi:
- Yeast
- Filamentous fungi

Slime moulds previously classified as fungi but not anymore after genome sequencing

Question 4

Explain slime moulds

Answer 4

Slime moulds - several kinds of unrelated eukaryotic organisms - life cycle includes a free-living single-celled stage and the formation of spores

Question 5

What is the difference between cellular and plasmodial slime moulds?

Answer 5

Plasmodial slime moulds - form a single-celled (one giant cell) form fusion, multinucleate mass

Cellular slime molds (“social amoebae”) - form an aggregated mass of separate amoebas that are able to migrate as a unified whole, differentiate into spore bearing structures

Question 6

How can cellular extension be induced in amoeba?

Answer 6

By providing cyclic AMP (cAMP) - amoeba have receptors - chemotaxis

Question 7

What is the structure of yeast cells?

Answer 7

Question 8

What is the difference between budding and fission yeast?

Answer 8

Different modes of preproduction:
- Budding yeast - parent cell creates an outgrowth that eventually becomes a daughter cell
- Fission yeast - parent cell reproduces by splitting in half

Question 9

Give the latin name of a budding yeast

Answer 9

Saccharomyces cerevisiae

Question 10

Give the latin name of a fission yeast

Answer 10

Saccharomyces pombe

Question 11

Explain what are filamentous fungi

Answer 11

Filamentous fungi - producers of a wealth of secondary metabolites with various biological activities

Filamentous fungi grow by apical extension of their filaments, known as hyphae - hyphal growth can occur with / without cell-wall separation of cellular compartments, which is known as septation

Question 12

How do hyphae grow?

Answer 12

Tip growth - apical extension: polarised extension that increases cell length

Question 13

What cells exhibit tip growth?

Answer 13

• Fungal hyphae
• Pollen tubes
• Root hairs
• Algal and ferm rhizoids
• Moss cell filaments

Question 14

Explain the cell structure of a fungal hypha

Answer 14

Question 15

What are fungal spores? How are they spread?

Answer 15

Fungal spores - microscopic biological particles that allow fungi to be reproduced

Fungal spores spread: so small + lightweight - can easily move unseen in the air currents, and most fungal spores are spread by the wind

Question 16

What is fungal spore germination?

Answer 16

Germination - the mechanism that under favourable conditions converts the spore from a dormant biological organism to one that grows vegetatively and is capable of either sexual / asexual reproduction

Germ tubes - specialised hyphae emerge during spore germination

Some spores are able to germinate after extended periods and extreme conditions (freezing temp, vacuum, electron beam / radiation)

Question 17

What is a mycelium?

Answer 17

Mycelium are fungi’s vegetative structures composed of white or cream-colored long threads called hyphae

Mycellium a 3D network - in agar spread in 3D not only on the surface

Question 18

What is a rhizomorphic mycelium?

Answer 18

Rhizomorphic mycelium - thick strands of hyphae which have adhered - helps fungus cover area, grow quickly through substrates and establish a feeding network

Question 19

What is the apical body / apical vesicle cluster?

Answer 19

Hyphal tip - has apical body which contains apical vesicles, mcirovesicles, ribosomes, actin and microtubules

Also called Spiztenkorper

New apical body forms at the site of new branch formation

Question 20

What happens if hyphae experience stress?

Answer 20

• Cessation of growth / retraction of apical body
• Hypha burst - spores burst - due to osmotic stress

Question 21

What are hyphal septa?

Answer 21

Hyphal septa - cross walls with pores - allow passage of organelles and cytoplasm between adjacent hyphal compartments - septal pores can be blocked if hyphae are damaged

Septum formation compartmentalises hyphae and governs cytoplasmic flow - provides damage limitation mechanism in mechanical injuries

Question 22

What is phototoxicity?

Answer 22

**Phototoxicity **- live cells stained with fluorescence - exposed to intensee light - can result in cell death - mitochondria turn off - potentially due to the loss of membrane potential

Question 23

What is different about Golgi in filamentous fungi?

Answer 23

Golgi in filamentous fungi don’t form stacks as in animal, plant cells - Golgi donuts

Question 24

How are the nuclei arranged in fungal hyphae?

Answer 24

Fungal hyphae are multinucleate - nuclei and other organelles pass through open septal pores

Usually nuclei divide synchonously in different hyphae - directional waves of nuclear division

Regions of hyphal extension usually without nuclei

Many nuclei needed -> spore production

Question 25

Explain hyphal fusion

Answer 25

**Hyphal fusion** - **hyphal branches grow towards** each other **in older parts** of fungal colony Occurs during **colony establishment** by germinating spores + during the f**ormation of cross connections** within mature mycelial colonies - for improving the chances of colony establishment by allowing **heterogeneously distributed nutrients / water** within the environment to be shared - **cytoplasmic flow** Developed new technique - differential staining of colonies allows to observe hyphal fusion and cytoplasmic flow

Question 26

What is the model organism for studying fungal mycelium?

Answer 26

Neurospora crassa

Question 27

Examples of cell signalling mechanisms in growth of slime moulds, yeast and filamentous fungi

Answer 27

Slime molds, yeast, filamentous fungi - use diff signaling mechanisms to regulate their growth and development: - **Slime molds**: extracellular **signalling molecules** (ex: cAMP) - coordinate their multicellular development - **Yeast**: **signaling pathways** to regulate growth and response to environment (ex:. MAPK pathway, TOR pathway) - **Filamentous fungi**: several **signaling pathways** to coordinate their growth and development (ex:. cAMP-PKA pathway)

Question 28

What is the process of insect pathogenesis from fungi?

Answer 28

Question 29

What are the fungal parasites of nematode worms?

Answer 29

**Nematrophs** - parasites of nematode worms - adhesive knobs - nematodes **trapped by specialised hyphae and adhesive knobs**

Question 30

What are the human mycoses?

Answer 30

Only ~600 fungi are known to infect humans - disease - mycoses **Pathogenic fungi** - **opportunistic**, cause mild symptoms in healthy adults **Human mycoses** (fungal diseases): - Dermatophytes - Commensals - Opportunistic pathogens of the lungs - Opportunistic pathogens of wounds and traumatised tissues - Naturally live in lungs

Question 31

Explain dermatophytes

Answer 31

**Dermatophytes** - human mycoses of **pathogenic fungi**: - superficial mycoses - grow on dead, **keratinised** tissue of **skin, hair, nails** - use keratin as sole carbon and E source because produce keratinases - cause ringworm, athlete's foot, nail infections - Reproduce asexual spores - **conidia** - Hyphae can develop structures which aid infection - "perforating organ"

Question 32

Explain commensal fungi

Answer 32

**Commensals** - grow harmlessly on mucous membranes but **can become invasive** under particular conditions Ex **Candida albicans yeast**: - usually harmless in mucous membranes (mouth, gut, vulvo-vagina tract) - **dimorphic** under appropriate conditions fungus **changes from budding yeast to hyphal growth** - can become pathogenic - **newborns** can get thrush from the birth canal

Question 33

Explain opportunistic fungal pathogens of the lungs

Answer 33

**Opportunistic fungal pathogens of the lungs** - normally found as **saprotrophs** but can grow in the **lungs** and invade other tissues of **immunocrompromised** - Fungi usually in soil / plants / animal remains - Don't spread host-to-host - Ex: **black mould** - Aspergillus fumigatus - spores developed on hyphae - conidia - airborne asexual spores - Ex: **Cryptococcus neoformans** - infect central nervous system causing meningitis - cryptococcosis - fatal if untreated

Question 34

Example of an opportunistic fungi infecting through wounds / traumatised tissues

Answer 34

Opportunistic fungi of wounds or traumatised tissues - **Sporothrix brasiliensis** - cause **sporotrichosis** - a subcutaneous (deep skin leayers, fat, muscle) fungal infection

Question 35

What are the stages of systematic infection of Candida albicans?

Answer 35

Question 36

What are the stages of systematic infection of Aspergillus fumigatus?

Answer 36

- **Lung invasion**: through ciliated epithelium lining of respiratory tract / through alveoli - Cells of epithelium **engulf conidia** - not engulfed conidia enter alveoli where germinate and cause infection - In healthy individuals conidia killed by host defences / in **immunodeficient Aspergillus fumigatus can be fatal**

Question 37

What are the stages of systematic infection of Cryptococcus neoformans?

Answer 37

Question 38

What are the pathogenicity factors of Cryptococcus neoformans?

Answer 38

Pathogenicity factors: - grow in human **body temp** - can grow in **5% CO2** - have a **capsule** - not as easily phagocytosed or killed - **melanin** - protects against UV and active oxidants

Question 39

What are the main antifungal compounds used to treat fungal diseases in humans?

Answer 39

Antifungal compounds: - **Azoles** - inhibit ergosterol synthesis - **Polyene antibiotic**s - disrupts ion homeostasis in fungal cells - **Echinocandins** - inhibit synthesis of beta (1,3)-glucan - **Flucytosine** - inhibits DNA and protein synthesis

Question 40

What are the uses of fungi?

Answer 40

- Fungal textiles: cell wall based on chitin instead of cellulose/keratin - Medical products: can be made into absorbent pads for personal hygiene; medicinal compounds from fungi (not fully approved by Western medicine - ex Cordyceps, Trametes versicolor) - Food: mycoprotein from fungi potential to solve global food crisis - sustainable - Used to produce secondary metabolites - penicillin - Mycotoxins: aflatoxin, coprine, muscimol, muscarine, amanitins, orellanin - Psychedelic compounds: LSD, psilocybin