Introduction To Mycology

mycology

concepts about fungi

Learning objective

• Definition of mycology and fungi
• Describe the structure of fungi
• Explain characteristics of fungi of medical importance
• Classify fungi basing on their modern phyla
• Identify fungi of ocular medical importance
• Explain diseases caused by fungi of ocular medical importance

List drugs of choice used for treatment fungi of ocular medical importance

Defition of related terms

Mycology is a branch of microbiology which deals with the study of fungi, including their genetics , ecology , physiology and uses (such as in medicine ,food, and industry) as well as the disease they may cause in animals, plants and humans .

Fungi are the eukaryotic organisms that include yeast, molds and mushrooms that live heterotrophically

Structure of fungi

Fungi are eukaryotic organisms , meaning that they have true nucleus and membrane bound organelles Their structure varies depending on their cellular nature e.g. single cellular or multicellular

Structural components of fungal cells

Cell wall made up of chitin (a tough, flexible and nitrogen containing polysaccharide , which provides shape, protection ,prevents desiccation and acts a barrier against environmental stress They have at least one nucleus and nuclear bound membrane within their cellular structure They have cytoplasm containing cellular organelles such as endoplasmic reticulum, Golgi apparatus and Mitochondria responsible for specific functions They do not have chlorophyll or chloroplast in their structure

Structural components of fungal cells

• Vegetative structure, fungi posses the body structure called thallus which may be :
• unicellular (yeast)
• Single celled , oval shaped cells
• Reproductive by budding or fission,
• example saccharomyces cerevisiae

Multicellular ( molds and mushrooms), made up of long , thread like filaments called hyphae and a mass of hyphae is called mycelium

Structural components of fungal cells

They have specialized structures for reproduction or fruiting process; the fungi reproduce by forming spores (asexually and sexually) Asexual spores

Sporangiospores –produced inside a sac called a sporangia (sporangiosphore) in Zygomycota Conidiospores (conidia)-asexual spores formed externally (not in sac) on a specialized hyphae called conidiophore in Ascomycota Chlamydospores–thick-walled spores formed from hyphal cells in Zygomycota and blastopores-the spores formed by budding of the parent cells (sexual spore) Zoospores –produced in the sporangium in Chytridiomycota with flagella

General characteristics of fungi cont…

Most fungi are obligate or facultative aerobes.

They are chemotrophic organisms (source of energy are chemical compounds rather than sunlight) Fungi are heterotrophic organisms; they secrete enzymes into their surroundings to break down organic matters and absorb nutrients. Some have the structural parts that are specialized for nutritional acquirement e.g. hyphae, mycelium, rhizoids and haustoria They are Saprophytic, parasitic, or mutualistic:

• Saprophytic: Decompose dead organic matter.
• Parasitic: Feed on living hosts, sometimes causing disease.

Mutualistic: Form beneficial relationships, like mycorrhizae with plant roots

General characteristics of fungi

Fungi are Eukaryotic cells: Fungi have a true nucleus enclosed in a membrane.

Their cell walls contain chitin: Unlike plants (which have cellulose), fungal cell walls are made of chitin, a tough, flexible nitrogenous containing compound.

They are non-vascular: They lack xylem and phloem, so they don’t transport water or nutrients like plants do.

Unicellular or multicellular: Yeasts are unicellular, while molds and mushrooms are multicellular.

• Reproduce via spores: Both sexually and asexually.
• Asexual spores: Sporangiospores, conidia, etc.

Sexual spores: Zygospores, ascospores, basidiospores

General characteristics of fungi

Achlorophyllous: They lack chlorophyll and cannot photosynthesize.

Typically non-motile: Most fungi do not move, although some spores may be motile during their reproduction

Taxonomic classification of fungi

Earlier fungi were classified mainly by their morphological features and reproduction but in phylogenic classification , fungi are grouped into phyla like .

• Ascomycota
• Zygomycota
• Basidiomycota

Chytridiomycota

This classification bases on the genetic sequence analysis and evolutionary relationship of fungi in phyla

Fungi of medical importance

Fungi of medical importance are the fungi that causes diseases , produce toxins or allergies to humans, examples of fungi include the following Candida albicans

• Microsporum
• Sprothrix spp
• Trichophyton spp
• Cryptococcus neoformans
• Histoplasma
• Coccidiodomides
• Mushroom
• Malassezia furfur
• Epidermophyton
• Aspergillus spp

Blastomyces

Major fungal Diseases of ocular importance and their Drugs

• Keratomycosis ( fungal keratitis)
• Caused by aspergillus, fusarium
• Features : Eye pain ,redness,blurred vision, cornea ulcer with feathery edges
• Drugs: natamycin( first line for filamentous fungi)
• Amphotericin B (for yeast infection)
• Voriconazole (broad-spectrum, severe cases)
• Endophthalmitis (fungal) candida albicanus
• Features: sever pain ,decreased vision, floaters ,inflammantion inside in eye
• DRUGS: Amphotericin B, Fluconazole, voriconazole
• Dacryocystitis ( fungal). Caused by aspergillus
• Features: infection of lacrimal sac, swelling near inner eye, dischage

Drugs : Itraconazole , fluconazole and surgical drainage may be needed

Major fungal Diseases of ocular importance and their Drugs cont

• Conjunctivitis (fungal –Rare) : caused candida , aspergillus
• Features: Redness, Irritation, discharge

Drugs: Amphotericin B (Topical), Natamycin

• Blepharitis (Fungal- Rare): caused candida
• Features: Eyelid inflammation, itching, crusting

Drugs: Ketoconazole topical/system, fluconazole

Candida albicans is a common fungus that lives in the human body, but an overgrowth can lead to infections.

Diseases ; Candidiasis

• Oral thrush: white, curd-like plaques in the oral cavity.

Vaginal candidiasis: itching, discharge described as “cottage cheese”.

Invasive candidiasis: bloodstream infections (candidemia), affecting organs like the eyes, kidneys, heart, and brain.

Cutaneous candidiasis: skin infections, often in warm, moist areas such as armpits and groin.

Diseases caused by fungi of medical importance

Oral thrush of candida albicans

Diseases caused by fungi of medical importance

2. Tinea means Ring worm infections and dermatologists use special terms to refer infections in different body areas.

• Tinea corporis – lesions occurring anywhere on the body.
• Tinea pedis – Infection of toe webs and soles of feet called athlete's foot.
• Tinea unguium – affects nails (Onychomycosis).

Tinea Capitis – Infection of the scalp, leading to hair loss and scaling.

Diseases caused by fungi of medical importance cont…

• Tinea Cruris (Jock Itch) – Itchy, red rash in the groin area.
• Tinea barbae-infection on beard area

Tinea faciel-infection on facial area Note: this infection is caused by trichophyton , microsporum, Epidermophyton (dermophytes)

Tinea infection

3. Cryptococcus neoformans

Disease caused are

Cryptococcal meningitis: affects immunocompromised patients, especially those with HIV/AIDS; characterized by fever, headache, neck stiffness, and altered mental status.

• Pulmonary cryptococcosis: respiratory symptoms similar to pneumonia.

Diseases caused by fungi of medical importance

4. Histoplasma capsulatum

Diseases ; Histoplasmosis:

Acute pulmonary histoplasmosis: flu-like symptoms, cough and chest pain.

Chronic pulmonary histoplasmosis: mimics tuberculosis, leading to progressive lung damage.

Disseminated histoplasmosis: systemic involvement, typically affecting the liver, spleen, lymph nodes, and bone marrow.

Diseases caused by fungi of medical importance

5. Coccidioides spp

Diseases ; Coccidioidomycosis (Valley Fever)

• Primary pulmonary infection: mild respiratory symptoms, can include fever and rash.
• Chronic pulmonary coccidioidomycosis: lung nodules or cavities, similar to tuberculosis.
• Disseminated coccidioidomycosis: affects the skin, brain (meningitis), bones, or joints.

Diseases caused by fungi of medical importance

6. Mushrooms (Toxic groups, e.g., Amanita species)

Diseases; Mushroom Poisoning (Mycetism)

Amatoxin poisoning: severe liver, kidney damage and gastrointestinal system damage .

potentially fetal, associated with Amanita phalloides (“death cap”) and other related species.

Diseases caused by fungi of medical importance

APPEARANCE OF CANDIDA ALBICANS

Cryptococcus neoformans in Indian ink

Drugs of choice used for treatment of fungi of medical importance

• Topical Antifungals (for skin, nail, and mucosal infections)
• Clotrimazole – Cream, lotion, vaginal tablet
• Miconazole – Cream, powder, vaginal suppository
• Terbinafine – Cream, gel, spray

Nystatin – Cream, ointment, powder (also oral suspension for oral thrush)

Drugs of choice used for treatment of fungi of medical importance

• Intravenous (IV) Antifungals (for severe systemic infections)
• Amphotericin B – IV infusion
• Broad-spectrum, used for life-threatening systemic fungal infections
• Voriconazole – IV and oral forms
• Used for invasive aspergillosis and serious Candida infections
• Posaconazole – IV and oral suspension
• Used for prophylaxis in high-risk immunocompromised patients
• Echinocandins (e.g., caspofungin, micafungin, anidulafungin) – IV only

Used for candidemia and invasive candidiasis

Drugs of choice used for treatment of fungi of medical importance

• Oral Antifungals (for systemic or stubborn infections)
• Fluconazole – Tablet, capsule, oral suspension
• Used for candidiasis, cryptococcosis, and prophylaxis in immunocompromised patients
• Itraconazole – Capsule, oral solution
• Effective for histoplasmosis, blastomycosis, and onychomycosis

Terbinafine – Tablet

• Preferred for nail fungal infections (onychomycosis)
• Griseofulvin – Tablet, suspension

Used for tinea capitis and other dermatophyte infections

There are three basic forms of fungi

• Yeasts
• Unicellular fungi that reproduce asexually by budding or fission.
• Commonly used in baking and brewing (e.g., Saccharomyces cerevisiae).
• Some species can cause infections like candidiasis, cryptococcus neoformans
• Molds
• Multicellular fungi with filamentous structures called hyphae.
• Grow on food, damp surfaces, and decaying matter.
• Examples include Penicillium (used to make penicillin) and Aspergillus
• Dimorphism refers to the ability of certain fungi to switch between:

Yeast form: unicellular, typically found in host tissues at body temperature (35–37°C).

Mold form: multicellular with filamentous hyphae, usually found in the environment at cooler temperatures (25–30°C)

Yeasts

• Yeasts are single fungal cells
• They reproduce asexually by the process called budding.

Examples of yeasts include ,Cryptococcus neoformans, candida albicans.

Yeast Cells

Molds

• Moulds are fungi which grows in multinuclear filamentous forms
• Are composed of branching cylindrical filaments called hyphae
• The mass of intertwined hyphae that accumulates during active growth is a mycelium.

There are two types of hyphae

septate their hyphae are separated by cross-walls or septa, typically forming at regular Intervals during hyphal growth aseptate their hyphae are not separated by cross-walls or septa Hyphae that penetrate the supporting medium and absorb nutrients are the vegetative or Substrate hyphae.

Septate and aseptate hyphae

Dimorphic Fungi

These are fungi that can grow both as yeast and as moulds depending on temperature and other environmental conditions. Example histoplasma capsulatum.

Spores

Classification of Fungi basing on their phyla

Fungal species is assigned to a phylum, as well as the appropriate class, order, and family, based on its mode of sexual reproduction, phenotypic properties such as morphology and physiology, and phylogenetic relationships.

Fungi are classified in four phyla

• Ascomycota
• Zygomycota
• Basidiomycota

Chytridiomycota

Ascomycota (Ascomycetes)

Is the largest group, which includes more than 60% of the known fungi and about 85% of the human pathogens Sexual reproduction involves a sac or ascus in which karyogamy (nucleus division) and Meiosis occur, producing ascospores Asexual reproduction is by conidia have septate hyphae and formed on special structure called conidiophores Examples

• blastomyces and histoplasma
• microsporum and trichophyton

coccidioides, and yeast (saccharomyces and candida)

Zygomycota (Zygomycetes)

• Sexual spores are thick-walled resting spores called zygospores.
• Produces hyphae that are aseptate
• Asexual reproduction is via sporangia/sporangium
• The sporangia contains the sporangiospore in their ball- like structure
• The sporangia are held by the sporangiophores up for better dispersal

Examples: Mucor, Rhizopus, absidia

Basidiomycota (basidomycetes)

Sexual Reproduction: Involves fusion of compatible hyphae, formation of basidia, and release of basidiospores.

• Asexual Reproduction: Less common, but may occur via conidia or budding (in yeasts).
• Hyphae have complex septa

Examples: mushrooms, cryptococcus neoformans

Chytridiomycota

• They are none of medical significance.(they are mainly aquatic fungi)
• Involves the formation of zoospores inside a sporangium in asexual reproduction

And the sexual reproduction involves zygospores (resting spores )

Key Points

Fungi are eukaryotic organisms that exist in two forms; yeasts (cells) and moulds (Filaments).

The cell wall of fungi is made of chitin Fungi are classified into four phyla namely; ascomycota, zygomycota, basidiomycota, Chytridiomycota.

Evaluation questions

• Explain on the structural components of fungi
• Classify fungi basing on their host body locations
• Classify fungi basing their phyla

List five drugs of choice used in fungal treatments Mention three laboratory specimen which can be collected for tinea and candida investigations

References

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Dennis Kunkel Microscopy Inc. (2010). Microscopy Photographs (science images, electron microscope images, photomicrographs, microscopy photos, microscope photos, microscopic pictures). Retrieved from http://www.denniskunkel.com/.

Greenwood, D. Richard, C.B.S., John, F.P. (1992). Medical Microbiology (4th ed). Hong Kong: ELBS with Churchill Livingstone, Medical Division of Longman Group, UK Ltd.

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