32. intro to fungi and antifungal drugs

in what forms do fungi grow?

• molds and yeasts

• some are dimorphic → can grow as mold or yeast

what is the general structure of fungal cell walls?

• multi-layered — polysaccharide subunits

  • chitin

  • glucans

  • mannans

  • protein fibrils

what is a major source of fungi?

soil (found everywhere)

how do molds grow?

• growth as filaments (hyphae)

  • septate hyphae (crosswalls)

  • aseptate hypahe (no crosswalls)

how do fungi obtain nutrients?

• get carbon from organic sources

• hyphal tips release enzymes → enzymatic breakdown of substrate

• products diffuse back into hyphae

how do molds reproduce?

• asexually

  • spores or fragmentation of hyphae

  • usually seen in diagnostic labs

• sexually (specialized spores)

  • important for taxonomy but not usually seen in infected animals or diagnostic lab

how do yeasts grow? how do they reproduce?

• single-cell growth

• reproduce by budding

microscopic fungi ID

• direct microscopy

  • look for fungal elements in tissue or fluid

  • stain with new methylene blue, giemsa

  • gram stain not helpful

• histopathology — need to use special stains to visualize fungal elements (not H&E)

  • silver stain

  • PAS

how are fungi cultured?

• many fungi grow on blood agar, but usually use fungal media

• molds often grow better at room temp than incubator temp

what makes fungi difficult to treat?

• antibiotics don’t work → target bacterial molecules

• similarity of fungal and mammalian cells pose toxicity problems

• few effective antifungal drugs available

• in general, therapy is prolonged, expensive, and can have significant side effects

what is the mechanism of action for azole drugs?

• inhibit enzyme* required for ergosterol synthesis (part of fungal cell wall)

  • *lanosterol 14a-demethylase = P450 enzyme known as CYP51

  • CYP51 found in fungi, animals, plants, and mycobacteria

• some also inhibit aromatase (P450 enzyme involved in synthesis of estrogens vs. androgens)

side effects of azole drugs

• usually minor (more with ketoconazole) → liver, inappetence, pruritis

• inhibit certain P450 dependent pathways (i.e. CYP3A4)

• can cause significant drug interactions with other drugs that are metabolized by CYP3A4

what are examples of polyene drugs?

amphotericin B, nystatin

what is the mechanism of action for polyene drugs?

binds to ergosterol in fungal cell membrane; increase permeability

how are polyenes administered? what are the side effects?

1. poorly soluble; slowly infuse IV

2. toxicity → absorbs to renal tubules → loss of renal function

   • greater efficacy and less side effects with liposomal versions

what is the mechanism of action for terbinafine (lamisil)?

inhibits squalene epoxidase (enzyme involved in ergosterol synthesis pathway)

how are iodides used in fungal treatment?

• KI given orally or NaI by IV injection

• effective for various chronic granulomatous infections (sporotrichosis, mycetoma)

• mechanism unknown

• cause iodide toxicity, but is reversible

• low cost → used in large animals

is susceptibility testing a routine part of fungal treatment?

no → very difficult, especially for hyphal fungi

antifungal resistance

• some fungi naturally resistant

• some acquire resistance to azole drugs

  • genetic → point mutation in 14a-demethylase (i.e. CYP51)

  • phenotypic → greater efflux of azole drugs out of fungal cell

• increasing resistance in certain aspergillus species

  • CYP51 mutation