L23: Introduction to Mycology

Question 1

Ways in which fungi are agents of human disease?

Answer 1

1. ) Mycotoxins: ergot alkaloids, aflatoxin, stachybotrys

2. ) Allergens

Question 2

Fungi are eukaryotes – True / False

Answer 2

• True

Question 3

Differences between fungal eukaryotic cells and human eukaryotic cells?

Answer 3

1.) Cell wall (not peptidoglycan): chitin (from NAG), chitosan, cellulose and alpha-glucan (from glucose) and mannan (from D-mannose)
2.) Cell membrane is composed of ergosterol, whereas this sterol is cholesterol in humans
• Cryptococcus neoformans possesses a capsule

Question 4

Function of fungal cell wall

Answer 4

1. ) Shape
2. ) Rigidity
3. ) Strength
4. ) Protection from osmotic shock

Question 5

Function of fungal cell membrane

Answer 5

1. ) Contain cytoplasm

2. ) Regulate nutrient flow

Question 6

Difference between fungal structures yeast and molds

Answer 6

1.) Yeast = unicellular, white
2.) Molds = fuzzy appearance with different colors
• some fungi can grow as both and are known as dimorphic

Question 7

How do yeast reproduce?

Answer 7

• Budding

Question 8

What are pseudohyphae and germ tubes?

Answer 8

• Pseudohyphae are false hyphae formed by some yeast

- Germ tubes are structures formed by almost always, Candida albicans, when cultured in serum

Question 9

How do molds produce?

Answer 9

• Asexually: produce conidia (micro, macro, chlamydo/arthro where spore forms within a strand)
• Sexually: produce spores
  • think of spores and conidia as similar – both aerosolized

Question 10

What are hyphae and mycelium? Two types of mycelium?

Answer 10

• Hyphae are long filamentous outgrowths of fungi. As a cluster they are known as mycelium. These protrude into the air (aerial mycelium) or into the substrate (vegetative mycelium)

Question 11

What are superficial mycoses? Examples of these infections?

Answer 11

• Infection of outermost layers of skin and hair

- Example: pityriasis versicolor, tinea nigra, black and white piedra

Question 12

What are cutaneous mycoses? Examples of these infections?

Answer 12

• Infection which extends deep into epidermis as well as invasive hair and nail infections.
• Examples: dermatophyte infections such as ringworm, athlete’s foot and jock itch

Question 13

What are subcutaneous mycoses? Agents that cause these infections?

Answer 13

• Infections involving the dermis, subcutaneous tissues, muscle and fascia
• Agents: sporotrichosis, chloroblastomycosis

Question 14

What are systemic mycoses? Causative agents of infections?

Answer 14

• Infection that originate primarily in the lung but may spread to any organ in the body
• Agents: dimorphic fungi such as Histoplasmosis, Blastomycosis, Paracoccidioidomycosis, Coccidioidomycosis and yeast: Cryptococcus

Question 15

What are opportunistic mycoses? Causative agents of infections?

Answer 15

• Infection associated primarily with immunosuppressed individuals.
• Agents: Candidiasis, Aspergillosis, Zygomycosis, Pneumocystis carinii pneumonia

Question 16

Which arm of the immune system is essential for controlling and eliminating fungal infections? Which patient populations do we therefore become concerned for?

Answer 16

• Cell-mediated/adaptive immune system

- Concern for: immunosuppressed pt (d/t steroids and radiation), AIDS pts, pt with T-cell and neutrophil deficit

Question 17

Targets of antifungal medication

Answer 17

1. ) Cell wall synthesis (glucan and chitin synthesis)
2. ) Cell membrane synthesis (inhibitors of ergosterol synthesis)*
3. ) Direct membrane damage (binds to ergosterol and disrupts them)*
4. ) Disruption of microtubules and inhibition of mitosis
5. ) Nucleic acid synthesis
   * most common

Question 18

MOA of polyenes

Answer 18

• direct damage of fungal cell membrane by binding to ergosterol

Question 19

MOA of azoles

Answer 19

• disrupts ergosterol biosynthesis

Question 20

MOA of nucleoside analogues such as flucytosine

Answer 20

• inhibits DNA/RNA synthesis

Question 21

MOA of grisans

Answer 21

• inhibits microtubular function in fungi

Question 22

MOA of allylamines

Answer 22

• Inhibits ergosterol synthesis

Question 23

MOA of thiocarbamates

Answer 23

• inhibits ergosterol synthesis

Question 24

MOA of morpholines

Answer 24

• inhibits ergosterol biosynthesis

Question 25

MOA of amphotericin B

Answer 25

• this is a polyene class of drug

- causes small pore formation in antifungal cell membrane by binding to ergosterol, cells are disrupted osmotically

Question 26

MOA of echinocandins

Answer 26

• inhibit beta-glucan synthesis and therefore disrupting fungal cell walls, used primarily in treating Candida and Aspergillus which are typically resistant to other antifungals

Question 27

How are fungi typically diagnosed/identified in laboratory?

Answer 27

• Direct visualization of fungal elements typically

Question 28

Function of KOH prep

Answer 28

• KOH may be used to examine hair, nails, skin, fluids, exudates or biopsies for identification of fungi
• 15% KOH dissolves specimens greater rate than fungi d/t chitinous cell wall

Question 29

Specialized stains to visualize fungal cell wall

Answer 29

1. ) chlorazol black

2. ) calcofluor white

Question 30

Do fungi stain via gram staining?

Answer 30

• Yes, all stain gram positive

Question 31

Special stain for Cryptococcus neoformans

Answer 31

• India ink – this fungi has thick polysaccharide capsule

Question 32

What organism is the germ tube test looking to identify?

Answer 32

• Likely candida albicans