INTRODUCTION TO MEDICAL MYCOLOGY

ByProf Ashraf MOGAHED

Medical mycology is the study of mycoses of man and their etiologic agents. Mycoses are the diseases caused by fungi. Of the several thousands of species of fungi that are known, less than 100 are pathogenic to man.

In addition to those species which are generally recognized as pathogenic to man it is firmly established that under unusual circumstances of abnormal susceptibility of patient, or the traumatic implantation of the fungus, other fungi are capable of causing lesions. Those are called (Opportunistic Fungi.)

These circumstances may be :

1. A debilitating condition of the host, as Diabetes.

2. A concurrent disease such as leukaemia.

3. Prolonged treatment with corticosteroids.

4.Immunosuppressive drugs or an antibiotic for long duration.

Systemic and subcutaneous mycoses are caused by fungi which are essentially free-living saprophytes in nature. These mycoses are not contagious and infection in man follows inhalation of spores or traumatic implantation of fungi.

Medical Importance of fungi

A) Antibiotics. (Penicillin& Streptomycin)

B) Ergot (Alkaloids).

C) Toxins. (Amanita & Aflatoxin)

D) Mycoses. (Superficial, subcutaneous &Systemic)

Antibiotics

SIR ALEXANDER FLEMING

In 1928, he observed that mould had developed accidently on a staphylococcus culture plate and that the mould had created a bacteria-free circle around itself. He found that a mould culture prevented growth of staphylococci, even when diluted 800 times. He named the active substance penicillin.

Penicillium notatum

Ergot alkaloidsSpecies of the fungus Claviceps parasitizes wheat, corn, rye, and other cereals. These fungi produce toxins in rye and wheat and toxicity caused by this group of fungi is referred to as ergotism and their toxins ergot alkaloids. Ergot contain five or more alkaloids. The levo-rotatory isomers are active in inducing uterine contractions and controlling bleeding. It has also an important role in treating migraine.

Claviceps

ToxinsThere is no natural division of fungi into edible “mushroom” and toxic fungi. The most generally recognized toxic genus is Amanita, or toxic mushroom. These fungi induce intoxication and hallucination

“Mushroom”

Amanita

Aflatoxin B1

Aflatoxin B1 is the most potent liver cancer-forming chemical known. It is a product of a mold called Aspergillus flavus, which is found in food that has been stored in a hot and humid environment. This mold is found in such foods as peanuts, rice, soybeans, corn, and wheat.

Aflatoxin B1 has been implicated in the development of liver cancer in Southern China and Sub-Saharan Africa. It is thought to cause cancer by producing changes (mutations) in the p53 gene. These mutations work by interfering with the gene's important tumor suppressing (inhibiting) functions.

Aspergillus flavus

Mycoses Certain fungi cause disease

and death in man and these diseases vary from superficial skin infections to subcutaneous or generalized systemic deep mycoses

Superficial mycoses

Subcutaneous mycoses

Systemic Mycoses

Classification

Traditionally the fungi have been classified as primitive plants and the subdivision Thallophyta of plant kingdom include the autotrophic algae and the heterotrophic fungi. Today, there is a strong and increasing tendency among taxonomists to exclude the fungi from the plant kingdom (Protista) to the fungi kingdom (Fungi).

There are many classification systems for the fungi, but most of them are complicated and difficult to use for practical purposes.

The following classification is relatively simple and can be used to classify these organisms:

Classification• KINGDOM: Protista • DIVISION: mycota (fungi)• SUBDIVISION:Eumycotina (true fungi)• CLASSES: 1. Phycomycetes.

» 2. Ascomycetes 3. Basidiomycetes

» 4. Deuteromycetes» (fungi imperfecti)

Class Phycomycetes 1.Contain mainly filamentous fungi that

usually lack cross walls or septa.

2.They reproduce asexually by production of sporangiospores.

3.Sexual reproduction occurs through the formation of thick walled zygospores.

Aseptate mycelium

Asexual Sporangiospores

Sporangia SEM

Zygot formation

Class Ascomycetes 1.Fungi may be filamentous and have

septa e.g. Molds or may be unicellular e.g. Yeasts.

2.Asexual reproduction may be through budding, cell division or the production of conidiospores.

3.Sexual reproduction, ascomycetes produce what is known as ascospores

Filaments & Yeasts

Asexual spores (Conidia)

Ascus

Ascospores

Class Basidiomycetes These fungi produce sexual spores in a

base called basidium which carry basidiospores.

This group includes mainly fleshy mushrooms.

They play no role in disease production in humans except in mushroom poisoning

Basidium

Basidiospores

SEM

Mushrooms

Class DeuteromycetesFungi imperfecti

1.Fungi imperfecti are fungi that have no demonstrable sexual reproductive cycle.

2.All fungi that seem to lack sexual process are gathered in this group.

3.Deuteromycetes are important because they include many known disease producing fungi. (e.g.) Dermatophytes

Deuteromycetes

MORPHOLOGY

Morphology of Fungi1 .Filamentous fungi (molds)

2 .Yeasts

3 .Yeast-like fungi

4 .Dimorphic fungi

Filamentous Fungi 1.The basic morphological elements of

filamentous fungi are long branching filaments or hyphae, which intertwine to produce a mass of filaments or mycelium

2.Colonies are strongly adherent to the medium and unlike most bacterial colonies cannot be emulsified in water.

3. The surface of these colonies may be velvety, powdery, or may show a cottony aerial mycelium.

4. Pigmentation of the colony itself and of the underlying medium is frequently present.

Mycelia & Conidia

Colony Morphology

Yeasts1. These occur in the form of round or oval

bodies which reproduce by the formation of buds known as blastospores.

2. Yeasts colonies resemble bacterial colonies in appearance and in consistency.

3. The only pathogenic yeast in medical mycology is Cryptococcus neoformans.

Yeast colonies

Mucoid colonies

Cryptococcus neoformans

Yeast-Like1.These are fungi which occur in the form of

budding yeast-like cells and as chains of elongated unbranched filamentous cells which present the appearance of broad septate hyphae. these hyphae intertwine to form a pseudomycelium.

2. The yeast like fungi are grouped together in the genus Candida.

Candida Colonies

Candida albicans

SEM

Dimorphic Fungi These are fungi which exhibit a

filamentous mycelial morphology (saprophytic phase) when grown at room temperature 27oC, but have a typical yeast morphology (parasitic phase) inside the body and when grown at 37oC in the laboratory (e.g. Histoplasmosis).

Histoplasma capsulatum 27oC

Histoplasma capsulatum 37oc

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