Fungi: General characteristics

Question 1

Structure & growth

Answer 1

Fungi are eukaryotic organisms that exist in two basic forms: yeasts & molds

Yeasts:

• single cells
• reproduce by budding, a process in which the daughter cells are unequal

Molds:

• consists of long filaments of cells called hyphae
• reproduce by cell division (daughter cells are equal in size)

Some fungi are dimorphism (i.e. they can exist as yeasts or molds, depending on the temperature)

• at room temperature (e.g. 25oC), dimorphism fungi are molds
• at body temperature, they are yeasts (or some other form such as a spherule)

Question 2

Structure & growth 2

Answer 2

Fungal cell wall is made of chitin; bacterial cell wall is made of peptidogylcan

> therefore, antibiotics that inhibit peptidoglycan synthesis such as penicillins, cephalosporins, and vancomycin are not effective against fungi

> the fungal cell membrane contains ergosterol, whereas the bacterial cell membrane does not contain ergosterol
—> therefore, antibiotics that inhibit ergosterol synthesis (e.g. the azole drugs) are not effective against bacteria (ie. amphotericin B)

Question 3

Pathogenesis

Answer 3

Reduced cell-mediated immunity- predisposes to disseminated disease caused by systemic fungi, such as:

• Histoplasma
• Coccodioides

When a reduced number of neutrophils predisposes to disseminated disease caused by fungi such as Aspergilus & Mucor

Question 4

Microscopical diagnosis

Answer 4

• Microscopic examination of a KOH preparation can reveal the presence of fungal structures:
  —> the purpose of the KOH is dissolve the human cells, allowing visualization of the fungi
• Sabouraud’s agar is often used to grow fungi because it’s low pH inhibits the growth of bacterial, allowing the slowerA growing fungi to emerge
• tests for the presence of fungal antigens and fungal antibodies are often used

Question 5

Antifungal therapy

Answer 5

The selective toxicity of amphotericin B and the azole group of drugs is based on the presence of [ergosterol] in the fungal cell membranes, in contrast to the cholesterol found in human cell membranes and the absence of sterols in bacterial cell membranes

• Amphotericin B binds to fungal cell membranes at the site of ergostetol and disrupt the integrity of the membranes
• Azoks drugs (such as itraconazole, fluconazole, and ketoconazole) inhibits the synthesis of ergosterol

The selective toxicity of [echinocandins], such as Caspofungin, is based on the presence of a cell wall in the fungi, whereas human cells do not have a cell wall
—> Echinocandins inhibit the synthesis of D-glucan, which is component of the fungal cell wall you