Mycology:
General Properties of fungi:
Fungi are a diverse group of saprophytic and parasitic
eukaryotic organisms that varies in complexity and size,
ranging from the single-cell microscopic yeasts to multicellular
molds.
Fungi can be distinguished from other infectious organisms
by:
1- They are Eukaryotes (that is, they have a membraneenclosed nucleus, and other membrane bounded organelles.
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2- Cell wall and membrane components:
Fungal cell wall are composed largely of Chitin, a polymer
of N-acetylglucosamine, rather than peptidoglycan.
The fungal membrane contains ergosterol, rather than the
Cholesterol found in mammalian membranes.
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3- All fungi are Heterotrophes ,and Chemotrophic
organisms.
Fungi secrete degradative enzymes ( cellulases,
proteases, nucleases) into their immediate environment.
4- Fungi are Facultative anaerobic or obligatory aerobic
organisms.
5-Most fungi are Acidophilic organisms.
6- Fungi reproduce and spread through the environment
by
Spore formation which may be sexual or asexual.
Fungi Morphology:
Fungi exist into two main forms yeasts or hyphae (Moulds). Some
fungi may occur in both the yeast and mycelial forms. These are
called dimorphic fungi.
Hyphae are multicellular filamentous structures, constituted by
tubular cells with cell walls.
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Yeasts are Unicellular non-branched, oval or rounded cells,
measuring 3- 15 µm in diameter.
Yeasts reproduce asexually by budding (blastospore, and
chlamydospores).
Dimorphic fungi
These fungi are changing their morphology from mould to yeast
phase, or from yeast to mould depending on the growth conditions.
1. Yeast : (parasitic or pathogenic form): This is the form usually seen in
tissue, in exudates, or if cultured in an incubator at 37ºC.
2. Mycelium :( saprophytic or mold form): The form observed in nature
or when cultured at 25ºC.
Characteristics of Fungal Hyphae: Septate versus Coenocytic
Fungal Diseases:
Fungal diseases can be classified into the following groups:
1-Hypersensitivity (allergy):
It is an allergic reaction to molds and spores ( Indoor air pollution).
2-Mycotoxicoses:
Poisoning of man and animals due to accidental ingestion of food
contaminated by toxic compounds produced by fungi.
Examples:
A- Ergotism: caused by Claviceps purpurea ; (Ergot alkaloids) .
B- Aflatoxicosis: caused by Aspergillus flavus (Aflatoxins).
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3-Mycetismus:
Due to ingestion of Amanitins (a toxin produced by a specific
type of mushroom ; Amanita verna ).
4-Infection (Mycosis):
A-Superficial (Hair, skin, nail, cornea) mycosis.
B-Subcutaneous mycosis.
C-True systemic (endemic) mycosis.
D-Opportunistic mycosis.
SUPERFICIAL MYCOSIS:
The superficial mycoses are usually limited to the outermost
layers of the skin, hair, and nails, and do not invade living
tissues.
Most common Types:
1- Dermatophytosis.
2- Pityriasis versicolor.
Dermatophytosis: (Tinea or Ringworm):
It is a type of superficial skin infection of cutaneous layer
(mainly epidermis).
Causes:
*Epidermophyton (Skin and nail)
*Microsporum ( Skin and hair)
*Trichophyton ( Hair.Skin & nail)
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Pityriasis Versicolor:
It is a superficial chronic infection of stratum corneum.
This fungal infection is
caused by: Malassezia furfur.
(a lipophilic yeast organism).
Other superficial infection of skin:
1-Tinea Nigra:
Exophiala werneckii :
Infection of Stratum corneum
(Outermost layer of epidermis)
2-Black Piedra:
Piedraia hortae
Infections of scalp hair.
3-White Piedra
Trichosporon beigelii :
Fungal infection of facial,
axillary or genital hair.
Types of superficial Mycosis:
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Subcutaneous Mycosis:
Sporotrichosis:
Caused by Sporothrix schenkii.
At 25°C: Septate hyphae, rosette-like clusters of conidia
at the tips of the hyphae.
At 35 C: yeast.
Systemic Mycosis:
Respiratory system infection:
Chronic granulomatous pneumonia.
Causes:
Histoplasma capsulatum (dimorphic fungi).
Coccidioides immitis (dimorphic fungi).
Opportunistic mycosis:
Candidiasis:
Candida albicans infection.
Example:
1-Oral Candidiasis.
2-Vaginal Candidiasis.
Laboratory Diagnosis of Fungal Infections:
(1) Specimens:
According to the site of infection as, skin scales, hairs, nails, respiratory
secretions, blood, …
(2) Direct Detection:
A- Direct microscopy of unstained preparations (mounting
method):
Examination of unstained preparations to demonstrate hyphae, spores
or yeast cells. Skin scraping, nails or hairs are mounted with 10-20%
KOH to digest the keratin layer so that hyphae and spores can be
seen.
B- Direct microscopy of stained preparations:
Different stains are used, e.g. India ink, Periodic Acid Schiff (PAS),
silver stain and Lactophenol cotton blue stain (specific fungal stain).
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(3) Culture:
Sabouraud`s dextrose agar (SDA): the standard media for
most fungi.
Chloramphenicol added to inhibit bacterial growth &
cycloheximide added to inhibit saprophytic fungi.
Incubation temperature is 22°C.
If systemic mycosis is suspected, Enriched media is used and
incubated at 37°C.
(4) Serodiagnosis:
Detection of specific antibody help in diagnosis of systemic
mycosis.
(4) Cutaneous delayed type hypersensitivity test:
Example: histoplasmin skin test and blastomycin skin test. For
identification of systemic mycosis.
Dermatophyte morphology:
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