Introduction To Mycology

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mycology

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the study of fungi

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some properties of fungi

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are more than 50,000 species

reside in nature

are essential to in breaking and recycling organic material

are all eukaryotic organisms

most are NOT disease-causing in humans

are gram-positive

most are aerobic or facultative anaerobes

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pathogenic fungi

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are mostly exogenous (meaning they originated outside of the tissues they infect) with two main exceptions:
1) candidiasis
or
2) tinea versicolor

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mycoses

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another name for fungal infections

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inhalation
or
traumatic implantation

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how most fungal infections originate

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fungal cell membrane contains

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ergosterols and zymosterol
and human cell membranes contain cholesterol...most of the drugs out there that are anti-fungal target the biosyntesis of these steroids

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KOH

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is used to identify fungi...will hydrolyze the polysaccharides found in the fungal cell wall (chitin, glucan, mannan) except cellulose and chitosan (are resistant)..the presence of these will help identify the micro-organism as Fungi

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polysaccharide capsule and cell wall of fungi

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determine virulence factor and play a role in eliciting host immune responses

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cryptococcus neoformans

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causative agent of cryptococcal meningitis

has a prominent capsule

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fungi--staining methods

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methenamine silver and periodic-acid Schiff

are useful because they stain the glucans and complex polysaccharides that make up the cell wall

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two basic morphological forms of fungi

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yeast and hyphae

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yeast

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one of the two basic morphological forms of fungi

are unicellular

reproduce asexually by processes termed budding or fission

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hyphae

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a long, branching filamentous cell of a fungi (as opposed to yeast which are unicellular)

grow at their tips by a process called an apical extension

can branch and extend to form a mass called mycelium (synonymous with mold)

can be coenocytic (hollow and multi-nucleate) or seperate (divided by partitions/septae)

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vegetative hyphae

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grow submerged or on the surface of culture medium and are responsible for absorbing nutrients

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aerial hyphae

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project above the surface of the medium

often produce conidia (asexual reproductive elements) that are easily airborne and disseminated into the environment

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candida albicans

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has a unique morphology

is a part of the normal flora of the mouth, GI tract, and membranes lining the mucosa of other cavities and tissues

they can be yeast-like or filamentous

can assume a psuedo-hyphal morphology where the cells are elongated and linked like sausages

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pseudohyphae

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they look like sausages and at the end a chlamydospore....are characteristic of Candida Albicans

buds that characteristically fail to detach and become elongated continuation of the budding process

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what does it mean when you say some fungi are dimorphic?

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some may exist in a mycelial or yeast morphology depending on the environmental conditions of growth

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how can fungi reproduce?

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asexually, sexually and parasexually

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spores/conidia

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specialized reproductive structures that all fungi generate during reproduction

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most fungi that are pathogenic and disease-causing in humans reproduce in what manner?

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asexually

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perfect state

versus

imperfect state

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the sexual form of a fungus is often called it's perfect state


the asexual form of a fungus is often called it's imperfect state

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most of the fungi that cause human disease are:

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imperfect fungi

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what is the name of the growth media that fungi are cultured in?

Describe it!

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Sabouraud's agar

adjusted to pH 5.0 because it favors the growth of mycotic agents but discourages the growth of bacteria

can be supplemented with cyc lohexamide and choramphenicol and this makes the membrane highly selective (only pathogenic fungi will grow)

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dimorphic fungal agents will grow in what phase at 37 degrees C? 25 degrees C?

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yeast phase at 37 degrees

mold phase at 25 degrees

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asexual spores arise from two mechanisms

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1) thallic conidia are formed by altering the structure of the hyphal segment (come from body of the fungus)...put in a cross wall and then the sectioned off cell becomes a spore

2) blastic conidia arise from a specialized region of the hyphae (the long branching filamentous cell of a fungi)...the spore is evident before it separates from the hypha giving rise to it...

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aleuriospores

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called microconidia and macroconidia
occur singly or in groups from short lateral branches off the hyphae
or can come directly off the hyphae

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what is a dermatophyte?

and how is it identified?

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common name for fungi that cause skin disease in humans


are identified by the aleuriospores they form on agar

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arthrospores

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occur when hyphae form double septa

are released with fungal hyphae breaks apart

can be highly infectious

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chlamydospores

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thick-walled spores that form at the end of a hyphae or between hyphal segments

is formed by candida albicans

are uniquely resistant to heat and drying

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blastospores

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are formed by yeast during budding

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conidiospores

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occur singly or in groups at the end of specialized structures called conidiophores

aspergillus and penicillum are two medically important fungi that reproduce conidiospores

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sporangiospores

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form within sac-like structures called sporangia

found at the end of specialized hyphae called sporangiophores

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6 types of asexual spores

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aleurisopores

arthrospores

chalamydospores

blastospores

conidiospores

sporangiospores

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sexual spores of medical importance?

describe them

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ascospores

are enclosed within a small sac called an ascus and many asci are contained within a larger sac called cleistothecium

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wood's lamp

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small hand-held UV light that can be used to shine on skin surfaces or the scalp

useful because some dermatophytes glow in the dark

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India Ink Preparation

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CSF samples suspected of containing Cryptococcus neoformans are stained with this ink

the yeast in the dark background has a large clear polysaccharide capsule

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antifungal antibiotics

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amphotericin B

Nystatin

Griseofulvin

5-flurocytosine

Azole antibiotics

Allylanine antibiotics

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amphotericin B

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a polyene antibiotic
used for most systemic fungal agents
binds to steroid (ergosterol) in the membrane of fungal agents
also binds to cholesterol in human cell membranes so it is TOXIC when used intravenously and at high doses, is damaging to the renal basement membrane

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nystatin

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also a polyene antibiotic

used topically for candida albicans infections

binds to membrane steroids (ergosterol)

disrupts membrane structure and function

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griseofulvin

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used for dermatophytosis

is taken orally...in time becomes incorporated into the keratin layers of the skin and interferes with DNA replication of fungi

is well tolerated and a headache is the most common side effect

acts like colchicine (inhibiting fungal mitosis because it interferes with microtubule polymerization)

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5-fluorocytosine

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a nucleotide analog currently utilized in treating fungal diseases

converted in susceptible fungi to uriacil derivatives which gets incorporated into fungal RNA and inhibits proteins synthesis

used to treat candida albicans infections of the urinary tract

is toxic, some patients become leukopenic and thrombocytopenic

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azole antiobiotics

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inhibit the synthesis of ergosterol by inhibiting the activity of a cytochrome-P450 enzyme (a demethylase)

include imidazoles and bistriazoles

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allylanine antibiotics

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relatively new

used topically to treat dermatophyte infections such as ringworm and athelet's foot

inhibits squalene epoxidase (an enzyme in the biosynthesis of ergosterol)

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bristriazole antibiotics

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are less toxic than other azoles when administered intravenously

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griseofulvin

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used for dermatophytosis

is taken orally...in time becomes incorporated into the keratin layers of the skin and interferes with DNA replication of fungi

is well tolerated and a headache is the most common side effect

acts like colchicine (inhibiting fungal mitosis because it interferes with microtubule polymerization)

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5-fluorocytosine

Back 47

a nucleotide analog currently utilized in treating fungal diseases

converted in susceptible fungi to uriacil derivatives which gets incorporated into fungal RNA and inhibits proteins synthesis

used to treat candida albicans infections of the urinary tract

is toxic, some patients become leukopenic and thrombocytopenic

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azole antiobiotics

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inhibit the synthesis of ergosterol by inhibiting the activity of a cytochrome-P450 enzyme (a demethylase)

include imidazoles and bistriazoles

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allylanine antibiotics

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relatively new

used topically to treat dermatophyte infections such as ringworm and athelet's foot

inhibits squalene epoxidase (an enzyme in the biosynthesis of ergosterol)

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bristriazole antibiotics

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are less toxic than other azoles when administered intravenously