Summary of the part Mycology

HUMAN PARASITES: PART
MYCOLOGY
GENERAL INTRODUCTION

GENERAL CHARACTERISTICS OF TRUE FUNGI (EUMYCOTA)




 Antifungals focus on mostly on 2 aspects that are specific for fungi
 Cytoplasmic membrane contains sterol:
o Ergosterol is present VS humans which have cholesterol
 Cell membrane = outer membrane: chitin, glucan, mannan
 All fungi are eukaryotic
 Most are filamentous: filaments = hyphae, these exhibit apical growth and form a network of
hyphae called a mycelium
 Some are unicellular: e.g. yeasts
 Protoplasm of a cell or a hyphae is surrounded by a rigid wall
 Many reproduce both sexually and asexually: they both result in the production of spores
 Their nuclei are typically haploid and hyphal compartments are often multinucleate
 All are chemoheterotrophic (chemo-organotrophic): use carbon from the environment as
energy
 They may be free-living or parasitic or mutualistic (symbiotic)

GENERAL STRUCTURE


OF FUNGI
 Vegetative hyphae: feeding
o These are present IN the agar
o Can be segmented or non-
segmented
 Aerial hyphae: reproduction
 Mycellium = large mass of hyphae
 Fungael thallus (“body”) consists of hyphae


OF THE YEASTS


1

,  Daughter cell will be same size as parent cell
 Unicellular: oval to round
 Fission yeasts divide symmetrically
 Budding yeasts divide asymmetrically (e.g. Candida)
o Bud = knob = blastospore
o Parent cel  budding breaks of parent cell  ‘scar’ on parent cell
 Real hyphen= 1 structure >< pseudohyphen= no full separation (of the knob)


OF DIMORPHIC FUNGI
 Can grow both as fungus or as yeast
 Dependent on environmental factors: T°, CO2 and nutrients
o E.g. thermal dismorphism: Dimorphism in pathogenic fungi:
 37°C → yeast  pathogenic form
 25°C → fungus


SUBCELLULAR STRUCTURE
 Capsule (polysaccharides)  in some fungi!! (E.g. Cyptococcus neoformans)
o Protection against phagocytosis (because polysaccharides aren’t recognized) 
virulence factor
 Also a cell wall and cell membrane
 Cytoplasm
o Nucleus & nuclear membrane
o Endoplasmic reticulum
o Mitochondria
o Vacuoles
 Cholestrol (present in humans) is replaced by ergosterol (kind of specificity)
o Squalene produce ergosterol
 Therapeutic targets for antifungals
o Squalene
o Ergosterol!! (mostly this one)
o Chitin
o Glucan
o DNA/RNA synthesis
 Type and relative composition of chitin and glucan are
dependent on the fungal species 
o Antifungals focussing on glucan and chitin
have different effects on different fungal species and the susceptibility between the
species can vary

LIFE CYCLE (DON’T NEED TO KNOW THIS)
 Assexual
o Filamentous fungi: asexual by fragmentation of hyphae
o Asexual spores: Conidiospore (Aspergilus spp), Chlamydiaspore (C. albicans), Sporangiospore,
Macro-&micronidia


2

, o There are conidia + conidiophore  spores/conidia are released spores/conidia germinate
 Sexual
o Sexual spores: formed by fusion of nuclei of different haploid gametes
o + / - nucleus  come together = karyogamy 2n diploid
ISOLATIONoOF Haploid
FUNGI+ mitosis

Nutrients broths and agar’s (with addition of antibiotics (with no antifungal potential) to prevent
bacterial overgrowth)
 General: supports the growth of most fungi
o Sabouraud
o Potato Dextrose Agar (PDA): good for growing spores
 Selective: permits growth of a particular species
o Nickerson agar (bismuth-sulphate)  Candida spp. and other
yeasts
 Bismuth-sulphate: some yeast can reduce/metabolize
it  colour change
 But it’s still quite general: almost all candida

IDENTIFICATION OF FUNGI

 Morphological  not used a lot: there are other methods


STAINING METHODS
 KOH + lactophenol-blue staining
o Function of KOH: clears the sample, it solubilizes, makes it
translucent
o Done correctly  immediate diagnosis, but difficult interpretation
 Gram-staining
o Candida are Gram-pos
o Hyphen are Gram-neg
 Calcofluor
o Fluorescent dyes that bind specifically to the cell wall (to membrane
components)
o Fluorescence microscopic reading!


BIOCHEMICAL
 API Candida
o Principle: Enzyme reaction of candida to different
carbohydrates (substrates)
 This gives different colors depending on which
carbohydrates are fermented  depends on the species
o OPM: Fungi grow slower than bacteria
 ChomAgar
o Immediate identification and quantification  used a lot in labs
o Agar plate + substrate


3

, o Depending on species (enzymatic digestion of substrate )  different colors (e.g.: C.
albicans = green)
o Advantages: quantitative, different species
o Disadvantage: expensive

ANTIFUNGAL DRUGS

Fungal treatment is always long term!!

TARGETS




 Echinocandins, nikkomycin Z: Inhibition of fungal cell wall formation
 Sordarins: Interference with protein assembly
 Azoles, Polyenes, Terbinafine: Disruption of fungal cell membrane
 Flucytosine: Interference with DNA synthesis


TIMELINES FOR SYSTEMIC ANTIFUNGALS
 Antifungal treatment is relatively recent
 Antifungals are used a lot in the food/agriculture industry
o Other methodsd to preserve food: irradiation (it’s a bit
expensive)
 Azoles: o.a. Miconazole (=generic name = active compound)
 Amphotericin: works against Leishmania, it’s toxic  they make
liposomal formulations: these do cell targeting and integrate into
the cell wall
What to know:
POLYENES 1. Mechanism of action
2. Spectrum
 Structures 3. Action: killing, inhibiting multiplication + growth
o High molecular lactones
o Amphophilic compounds: has a lipophilic and ionic part  can solubilize to some
extent in water and is highly lipophilic as well
o MW above 500  (oral) absorption is a problem  combination of systemic (oral
absorption or IV) and topical (ointments)
 Action mechanism

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