Mycology Flashcards by Karen Bourns

How are fungi classified?

Unicellular (yeasts), filamentous (moulds), dimorphic

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Unicellular fungi are

yeasts

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Filamentous fungi are

moulds

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Unicellular fungi are aerobes/anaerobes?

facultative anaerobes

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Filamentous fungi are aerobes/anaerobes?

aerobes

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Unicellular fungi/yeasts reproduce

asexually by budding

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Filamentous fungi/moulds reproduce

by conidia (asexual spores)

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On agar, unicellular fungi form _______ while filamentous fungi form _______

colonies (similar to bacteria); mycelia (do NOT look like colonies)

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T/F yeasts are the same size as bacteria on gram stain

false; they are 2-3um whereas bacteria are 1-1.5um; ie yeasts are LARGER

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Most fungi are _________ and live in ________

saprophytes (can live off organic matter) and live in soil or grow on organic material

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Virulence of fungi (descending order)

dimorphic
dermatophytes
agents of subcutaneous mycoses
opportunists
toxic fungi

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What makes dimorphic fungi so virulent?

They are best adapted to grow in human tissues because they can grow as filamentous fungi (non-pathogenic, do not grow well at 37’) in the environment and then as yeasts 9grow well at 37’) inside the body

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Dimorphic fungi have the capacity to

switch between unicellular and filamentous forms

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Why are filamentous fungi not pathogenic?

They do not grow well at 37’ and they are aerobic; they may sometimes infect the skin

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Histoplasma capsulatum is a ______ fungi that lives in ______ and causes what disease?

dimorphic (very virulent); caves; caves disease

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Primary pathogenic fungi are

dimorphic and dermatophytes

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T/F ringworm fungus is caused by a ringworm

False; caused by dermatophytes

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Dermatophytes grow on __________ - why?

hair, skin, nails - they love keratin

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T/F dermatophytes invade deeply

False

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T/F Microsprum canis is a dermatophyte commonly found in dogs

False; more common in cats but affects both cats and dogs

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Dermatophytes grow in which layer of skin?

Dead cells - keratin

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T/F reaction to dermatophytes occurs in response to the growth of the fungi

False; it is the biproducts they release

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Infection is caused by dermatophytes is called

tinea (not to be confused with the tapeworm taenia)

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When taking a skin sample, what is preferential?

A skin scale from the edges where the fungi are growing

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Tinea curis

tinea of the groin/jock itch

Tinea pedis

athletes foot

Tinea capitis

scalp

Tinea unguium

nails

Mycetoma (maduromycosis) commonly infects

feet

Mycetoma can be caused by

bacterium OR fungus

Bacterial cause of mycetoma (acintomycetoma) is

Nocardia spp

Fungal cause of mycetoma (eumycatoma) is

Madurella spp

What is characteristic of mycetoma?

sulfur granules

Actinomycetoma vs eumycetoma is distinguished by

diagnosis on sulfur granule samples

Mycoses refers to

fungal infections

Candidiasis is an example of what type of mycosis?

Opportunistic

Candidiasis is caused by

Candida albicans

How can antibacterial antibiotics eg ampicilin lead to vaginal thrush?

normally lactobacilli produce lactic acid to keep the vaginal pH low which fungi do not like; after AB Tx for an infection it will suppress these lactobacilli and raise the pH of the vagina predisposing to infection by candida fungi already present

Why do we react less to fungi (which allows more of them to grow before causing death)?

no PAMPs to induce cytokine response like with bacteria, therefore there can be much higher numbers of fungi than bacteria present

Which form of cryptococcus is more virulent? neoformans or gattii?

gattii - causes cryptococcal/fungal meningitis in well people as well as IC (tf not necessarily an opportunistic pathogen)

Fungal meningitis is most commonly caused by

Cryptococcus gattii

_______________ is an AIDS-defining fungal illness

Cryptococcal meningitis

Fungal meningitis is fatal if untreated because

it grows in the skull and raises ICP

Like other common causes of meningitis, cryptococcus fungus has

a massive capsule

On X-ray, what can be confused for lung cancer?

saphrophytic aspergillosis growing in a TB cavitation

The worst form of aspergillosis is

systemic - occurs in immunocompromised patients eg little boy with leukaemia who had it in his brain

T/F the type of fungal infection is able to be determined from a tissue slide sample

False; all fungi look the same in tissues

How are fillamentous fungi differentiated?

By the conidia; fillamentous fungi do not produce conidia in tissues but may do so in a fungus ball (saprophytic)

How are fungal infections diagnosed initially?

Microscopy

The quick and dirty test for C. albicans involves

formation of germ tubes (pseudohyphae) in horse serum

Other methods of fungal infection diagnosis

microscopy, culture, biochemical tests, Ag detection (latex agg), PCR, mass spec (MALDI-TOF like for bacteria)

Ag detection is used commonly in which fungal infection?

Cryptococcus - capsular Ag can be detected in the CSF (same as for bacteria)

What is the other application of Ag detection in fungal meningitis?

Montioring of treatment - titrating down the level of Ag to see if tx is working even though symptoms are not improving

Systemic anti-fungal chemotherapies:

polyenes (amphotericin B) triazoles (fluconazole, voriconazole) echinocandins (capsofungin) 5-fluorocytosine

Topical anti-fungal chemotherapy:

polyenes (nystatin) | imidazoles (clotrimazole)

T/F Polyenes taken orally are not absorbed by the GIT

True

When polyenes are used systemically they must be administered by ________ and are _________

injected; very toxic

What are the targets of anti-fungals?

Ergosterol (cytoplasmic membrane) | Nucleic acids

Which is the most important target of anti-fungals?

ergosterol in the cytoplasmic membrane

Ergosterol is found in _________ and is akin to _________

fungal cytoplasmic membranes; cholesterol in human membranes

Why are polyenes so toxic?

They bind to cholesterol in human cell membranes (less affinity than for ergosterol)

Polyenes bind with high affinity to ________ and lower affinity to ________

ergosterol; cholesterol

Polyenes have what affect on ergosterol?

Affect integrity of ergosterol and therefore the membrane

What is critical about patients on polyene tx eg systemic amphotericin B?

they must be hospitalized to monitor toxicity of the polyene drug (binds human cholesterol as well as fungal ergosterol)

Allylamines, triazoles, morpholines affect

synthesis of ergosterol

T/F Using polyenes (amphotericin B) with allylamines, triazoles, morpholines etc. is antagonistic

False; while polyenes target integrity (bind to ergosterol) and the others affect the synthesis they seem as though they should act antagonistically but they are in fact often used together as there is no evidence of antagonism in the clinic

In serious fungal infection, ________ and ________ are often used together (ergosterol targeting drugs)

polyenes and triazoles

5-fluorocytosine targets _______ and therefore affects synthesis of ___________

nucleic acids; DNA and RNA

5-fluorocytosine is a ______ which is converted to ________

prodrug; 5-fluorouracil (5-FU)

5-FU is also used as a drug in

cancer tx

Cytosine deaminase is an enzyme found in

fungi but not humans

What is the function of cytosine deaminase and therefore its use as an anti-fungal target?

activates the prodrug 5-fluorocytosine to 5-fluorouracil - therefore activation of the drug can only happen in yeasts

The downside of 5-fluorocytosine as an anti-fungal tx is

that fungi readily become resistant

Candida spp are tx with

fluconazole, amphotericin B, capsofungin

Cryptococcus are tx with

amphotericin B + 5-fluorocytosine (5-FU)

Aspergillus are tx with

Voriconazole, amphotericin B

Dimorphic fungi are tx with

amphotericin B

Why isn't amphotericin B used for all fungal infections?

It is very effective but it is toxic

Why are dermatophytoses NOT treated with amphotericin B?

they are not life-threatening infections

Non-specific tx for dermatophytosis is

reducing fungal load

Topical tx for dermatophytosis is

terbinafine (lamisil) clotrimazole (canesten) amorolfine (loceryl)

Oral tx for dermatophytosis is

terbinafine fluconazole griseofulvin (most toxic)

Oral anti-fungals are used

when it is difficult to tx topically OR infection is in the nails because they grow very slowly and drugs don't penetrate the nail