Introduction to Mycology

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Introduction to MycologyGeneral Properties of fungiFungi are a diverse group of saprophytic and parasitic eukaryotic organisms

They varies in complexity and sizeranging from the single-cell microscopic yeasts to multicellular molds

Fungi can be distinguished from other infectious organisms by

1- They are Eukaryotesthat is, they have a membrane-enclosed nucleus, and other membrane bounded organelles

2- Cell wall and membrane componentsFungal cell wall are composed largely of Chitina polymer of N-Acetylglucosamine, rather than peptidoglycan

The fungal membrane contains ergosterol, rather than the Cholesterol found in mammalian membranes

3- All fungi are Heterotroph, and Chemotrophic organismsFungi 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 asexual5Fungi MorphologyFungi exist into two main forms yeasts or hyphae (Moulds)

Some fungi may occur in both the yeast and mycelial formsThese are called dimorphic fungi

Hyphae are multicellular filamentous structures, constituted by tubular cells with cell walls

Hyphae of Penicillium

Yeasts are Unicellular non-branched, oval or rounded cellsmeasuring 3- 15 m in diameter

YeastYeast of the species Saccharomyces cerevisiae

Yeasts reproduce asexually by budding (Blastospore, and Chlamydospore)

Chlamydospores of the yeastCandida albicans11Dimorphic fungiThese 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 37C

2. Mycelium ( saprophytic or mold form)The form observed in nature or when cultured at 25C

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Fungal DiseasesFungal diseases can be classified into the following groups

1-Hypersensitivity (allergy)It is an allergic reaction to molds and spores ( Indoor air pollution)

2-MycotoxicosesPoisoning of man and animals due to accidental ingestion of food contaminated by toxic compounds produced by fungi

ExamplesA- Ergotism: caused by Claviceps purpurea; (Ergot alkaloids)B- Aflatoxicosis: caused by Aspergillus flavus (Aflatoxins)

ErgotismErgotism3-MycetismusDue to ingestion of Amanitins (a toxin produced by a specific type of mushroom ; Amanita verna )

4-Infection (Mycosis)A-Superficial (Hair, skin, nail, cornea) mycosisB-Subcutaneous mycosisC-True systemic (endemic) mycosisD-Opportunistic mycosis

SUPERFICIAL MYCOSISThe 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- Dermatophytosis2- Pityriasis versicolor

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Dermatophytosis (Tinea or Ringworm)It is a type of superficial skin infection of cutaneous layer (mainly epidermis)

Causes: EpidermophytonMicrosporumTrichophyton

Pityriasis VersicolorIt is a superficial chronic infection of stratum corneum.

This fungal infection is caused by Malassezia furfur

Other superficial infection of skinTinea Nigra, Black Piedra, and White Piedra are caused by Exophiala werneckii, Piedraia hortae, and Trichosporon species respectively

Tinea Nigra

Black Piedra

White Piedra201-Tinea NigraExophiala werneckiiInfection of Stratum corneum

2-Black PiedraPiedraia hortaeInfections of scalp hair

3-White Piedra Trichosporon beigelii Fungal infection of facial,axillary or genital hair

Types of superficial Mycosis

Subcutaneous MycosisSporotrichosisCaused by Sporothrix schenkiiAt 25C: Septate hyphae, rosette-like clusters of conidia at the tips of the hyphaeAt 35 C: Yeast

Systemic MycosisRespiratory system infectionChronic granulomatous pneumoniaCausesHistoplasma capsulatum (dimorphic fungi)Coccidioides immitis (dimorphic fungi)

Opportunistic mycosisCandidiasisCandida albicans infectionExample:1-Oral Candidiasis2-Vaginal Candidiasis

Laboratory Diagnosis of Fungal Infections(1) Specimens

According to the site of infectionSkin scalesHairsNailsRespiratory secretionsBlood

(2) Direct Detection

A- Direct microscopy of unstained preparations (mounting method)

Examination of unstained preparations to demonstrate hyphae, spores or yeast cellsSkin scraping, nails or hairs are mounted with 10-20% KOHto digest the keratin layer so that hyphae and spores can be seen

B- Direct microscopy of stained preparations

Different stains are usedIndia inkPeriodic acid-Schiff stain (PAS)Silver stainLactophenol cotton blue stain (specific fungal stain)

(3) CultureSabouraud`s dextrose agar (SDA)The standard media for most fungiChloramphenicol added to inhibit bacterial growth & Cycloheximide added to inhibit saprophytic fungi

Incubation temperature is 22C

If systemic mycosis is suspected, Enriched media is used and incubated at 37C

(4) SerodiagnosisDetection of specific antibody help in diagnosis of systemic mycosis

(5) Cutaneous delayed type hypersensitivity testExampleHistoplasmin skin testHistoplasma capsulatumBlastomycin skin testBlastomyces dermatitidisFor identification of systemic mycosis

Dermatophyte morphology