Chapter 4: The Fungi

Question 1

Which fungi are transmitted between humans? How are the rest transmitted?

Answer 1

Only Candida and the dermatophytes; the rest are environmental, including hospital-acquired

Question 2

What conditions does Candida sp. cause?

Answer 2

vaginal yeast infection and yeast infection of the mouth (oral thrush)

Question 3

What are dermatophytes and how are they acquired?

Answer 3

Dermatophytes are aerobic fungi that can invade and infect the keratinized layers of skin, hair, and nails. They are acquired from infected humans or animals and exposure to contaminated soil or fomites. Causes athlete’s foot and ringworm of the scalp and skin. (ringworm is not a worm!)

Question 4

Describe the filamentous growth form of fungi, including septate and nonseptate

Answer 4

Grow as multinucleate, branching hyphae, forming a mycelium. Nonseptate have protoplasm that is continuous and multinucleated. Septated have protoplasm that is interrupted by cross walls.

Question 5

Describe the yeast growth form of yeast.

Answer 5

Ovoid or spherical single cells multiply by budding and division

Question 6

Describe dimorphic forms of fungal growth, and what induces these.

Answer 6

They form hyphae (mycelium) at environmental temperatures, but occur as yeast cells in the body, the switch being temperature induced.

Question 7

What is an exception of the dimorphic form of fungal growth?

Answer 7

Candida forms hyphae within the body and yeast in environmental temperatures, whereas dimorphic forms usually occur as yeast in the body and as hyphae (mycelium) at environmental temperatures

Question 8

Fungal pathogens can be classified by the type of infection they cause. Describe the three types discussed in lecture and give examples.

Answer 8

1. In superficial mycoses, fungus grows on body surfaces (skin, hair, nails, mouth, vagina). Ex: tinea pedis (athlete’s foot) and vaginal candidiasis (thrush).
2. In subcutaneous mycoses, nails and deeper layers of skin are involved. Ex: mycetoma (Madura foot) and sporotrichosis
3. systemic or deep mycoses with involvement of internal organs, many infect the lungs. Endemic to US: endemic mycoses,Histoplasma capsulatum, Blastomyces dermatitidis,and
   Coccidioidesspecies
4. Indirectly: toxins produced by fungi are present in items used as food (e.g. aflatoxin, a carcinogen produced by Aspergillus flavus) or when their spores are inhaled, an immune response occurs and a hypersensitivity pneumonitis develops (allergic bronchopulmonary aspergillosis). Aflatoxin-producing fungi can contaminate crops in the field, at harvest, and during storage.

Question 9

Fungal infections can be controlled by targeting cell wall components and differences in cell membrane structures between mammals and fungi. What are these notable components?

Answer 9

Fungal cell membrane consists of ergosterol, whereas mammalian contains cholesterol.
Cell walls are predominantly made of chitin, mannan and glucan

Question 10

What is the mechanism of action of echinocandins, and what are three examples of echinocandins listed in lecture?

Answer 10

Echinocandins (caspofungin, micafungin, andilafungin) inhibit glucan synthesis in the fungal cell wall.

Question 11

What is the mechanism of action of polyenes, and what are the examples of polyenes listed in lecture?

Answer 11

Polyenes nystatin and amphotericin B bind ergosterol, creating a large hole in the cell membrane.

Question 12

What is the mechanism of action of azoles and allylamines, and what are the examples of azoles and allylamines listed in lecture?

Answer 12

Azoles (e.g. miconazole) and the allylamines (e.g. terbinafine) inhibit ergosterol synthesis to prevent its incorporation into cell membrane

Question 13

What is the mechanism of action of pyrimidines and what is the example of pyrimidines listed in lecture?

Answer 13

pyrimidines (e.g. flucytosine) inhibit nucleic acid synthesis