Mycology: Introduction, Structure, Classification of Medically Important Fungi, Fungal Infection, Prevention, and Control of Fungal Infections

Introduction of Mycology

Mycology is the science that deals with the study of fungi whereas medical mycology is the science that deals with the study of fungi that causes disease is called medical mycology.

General Features-

1. Fungi are a group of eukaryotic organisms.
2. They are found in soil, water, air, and decaying matter main sources.
3. They have a diversity of morphological appearances, depending on the species.
4. They are heterotrophic organisms that lack the definite root, stems, and leaves of higher plants.
5. Yeast is unicellular whereas mold is filamentous and multicellular.
6. Due to a lack of chlorophyll, they are differentiated from algae and higher plants.
7. They are saprophytic or parasitic because of requiring prepared food.
8. Having a more complex structure and greater size, they differ from bacteria.
9. They are mainly found in two forms. a) yeast and b) Mold

Yeast

Unicellular form
Size-
Width: 1-5 µm
Length: 5-30 µm
Shape-Commonly oval shape but some may be elongated or spherical. Each species has a characteristic shape, but even in pure culture, there is considerable variation in the size and shape of individual cells, depending on age and environment. Non-motile due to lacking flagella or organ of locomotion.

Mold

Molds are multicellular filamentous fungi consisting of mycelium and spores. The mycelium is composed of branching filaments called hyphae, which interface to form a mycelium hyphae are usually 2-10 µm composed of an outer tube-like wall surrounding a cavity, lumen which is filled or lined by protoplasm.
The protoplasm is surrounded by plasmalemma. The hyphae may be aseptate i.e. without walls or separate with a central pore in each cross wall. Mycelium has two forms- Aa)Vegetative mycelium, and b) Reproductive mycelium is also called aerial mycelium. They reproduce by the formation of different types of sexual and asexual spores that develop from the mycelium. A few examples of molds are Aspergillus, Dermatophytes, Penicillium, and Rhizopus.

Dimorphic Fungi

They exist as yeasts in the host tissue and the culture at 37⁰C and hyphal (mycelium) forms in the soil and in the culture at 22-25⁰C. Most of them cause systemic infection. e.g. Blastomyces dermatitidis, coccidioides immitis, Histoplasma capsulatum.

Reproduction and sporulation

Fungal spores are of 2 types, sexual and asexual spores.

Sexual spores: They are of 4 types-

1. Oospore
2. Zygospore
3. Ascospore
4. Basidiospore

-Formed by the fusion of cells and meiosis as in all forms of higher life
Asexual spores: They are produced by mitosis and are further of two types-
a)vegetative spores and aerial spores.

Vegetative spores are-

1. Arthrospores
2. Blastospores
3. Chlamydospores

Aerial spores are-

1. Conidiospores
2. Microconidia
3. Macroconidia
4. Sporangiospores

Morphological Classification of Fungi

Taxonomical Classification of Fungi

• Superkingdom: Eukaryota
• Kingdom: Fungi
• Phylum: (1) Zygomycota
• Class: Zygomycetes
• Order: Mucorales
• Family: Mucoraceae
• Genus: Mucor
• Species: indicus

2. Ascomycota: They are-

• Histoplasma
• Candida
• Trichophyton

3. Basidiomycota: e.g. Cryptococcus

4. Deuteromycota: Sexual reproduction is absent and asexual reproduction occurs by conidia. e.g. Penicillium and Aspergillus

Clinical Classification

1. Superficial fungi
2. Cutaneous fungi
3. Subcutaneous fungi
4. Systemic fungi

Fungal Infections

The infection of fungi is called mycoses.

It is divided into four groups and they are-

1. Superficial mycoses: The filamentous fungi which cause superficial disease in humans may be broadly divided into two groups. Primarily the dermatophytes, natural group-related fungi causing the disease tinea or ringworm in various forms. Secondarily a miscellaneous group of unrelated filamentous fungi may be saprophytes or plant pathogens which produce a clinical condition of skin, hair, nail, eye, or ear, causing  Tinea Nigra (Hortaea werneckii), Piedra (Trichosporon and Piedraia hortae) and Malassezia infections.
2. Cutaneous mycoses: It is caused by dermatophytes a group of three genera Trichophyton, Microsporum, and Epidermophyton.
3. Subcutaneous mycoses: It is localized, spreading infections that result from the inoculation into the cutaneous and subcutaneous tissue of a wide of saprophytic fungi are Chromobalstomycosis, Mycetoma, Sporotrichosis, Rhinosporidiosis.
4. Systemic mycoses: The fungi which cause deep or systemic mycoses are Histoplasmosis, Blastomycosis, Cryptococcosis, Coccidioidosis, and Paracoccidioidosis.

Pathogenic Group of Fungi

They are two types:

1. True pathogens and
2. Opportunistic pathogens

True pathogens are of four genera-

1. Histoplasma
2. Blastomyces dermatitidis
3. Coccidioides immitis
4. Paracoccidioides brasiliensis

Opportunistic pathogens are-

• Candida
• Aspergillus
• Pneumocystis jirovecii
• Mucor
• Penicillium

Some Fungal Diseases and Their Causative Agents

Some fungal diseases and their causative agents are mentioned in a table.

S.No.	Diseases	Causative agents
1.	Candidiasis	Candida albicans
2.	Meningitis	Cryptococcus neoformans
3.	Ringworm	Dermatophytes (Trichophyton)
4.	Aspergillosis	Aspergillus species
5.	Histoplasmosis	Histoplasma capsulatum
6.	Blastomycosis	Blastomycosis dermatitidis
7.	Coccidioidomycosis	Coccidioides immitis

Predisposing Factors of Fungal Infection/Disease

• Diabetes
• Prolonged treatment with corticosteroids
• Immunosuppression
• Broad antibiotic therapy
• Injury

Dermatophytes

Dermatophytes infect only superficial keratinized tissues and infection of the skin, hair, and nail. They are groups of three genera.

1. Trichophyton ( skin, hair, and nail)
2. Microsporum (Hair and skin)
3. Epidermophyton (skin and nail)

• Causative agents of ringworm and tinea
• Not involved in living tissue
• Culture media: Sabouraud Dextrose agar (SDA)-General/non-selective, Dermatophyte test medium(DTM)-Selective Medium

Features	Trichophyton	Microsporum	Epidermophyton
Site of Infection	Hair, skin, and nail	hair and skin	skin and nail
Colony	Powderly pigmented	Cotton like pigmented	Powered greenish yellow
Spores- a)Microconidia	Abundant	Relatively scanty	Absent
b)Macroconidia	Pencil or cylindrical shaped	Spindle-shaped	Club or pear-shaped

Ringworm

Clinically ringworm is classified depending on the site of the body involved. e.g.

• Tinea axillaries: axilla
• Tinea barbae: beard area
• Tinea corporis: Non-hair skin
• Tinea capitis: Hair and scalp
• Tinea corporis: in groin
• Tinea pedis: foot (Athlete’s foot)

Most common infection in humans by Trichophyton rubrum

Dermatomycosis: Another infection than dermatophytes is termed dermatomycosis.

Otomycosis: Fungal infection of the ear is called otomycosis.

Oculomycosis: Fungal infection of the eye

Cryptococcus neoformans

• Yeast-like fungi and monomorphic
• Reservoir: bird dropping
• Most common infection in immunocompromised patients e.g. AIDS
• It contains a prominent polysaccharide capsule
• It does not produce pseudohyphae
• Antigenically 4 types-A, B, C, and D
• A and D are the most common infection type
• It causes meningitis which is common and lung infection.

Lab diagnosis

Culture: SDA-creamy white and mucoid colony

Selective media: Bird Seed agar

Microscopy: India ink preparation

Treatment

Choice of drugs-

• 5-fluorocytosine
• Amphotericin B
• Ketoconazole

Candida albicans

Normal flora of mouth and vagina

Pathogenicity

• Oral thrush

Oral thrush also called oral candidiasis is a condition in which the fungus Candida albicans accumulates on the lining of your mouth. Candida is a normal organism in your mouth, but sometimes it can overgrow and cause symptoms. Oral thrush causes creamy white lesions, usually on your tongue or inner cheeks. Sometimes oral thrush may spread to the roof of your mouth, your gums or tonsils, or the back of your throat. Although oral thrush can affect anyone, it’s more likely to occur in babies and older adults because they have reduced immunity; in other people with suppressed immune systems or certain health conditions; or in people who take certain medications. Oral thrush is a minor problem if you’re healthy, but if you have a weakened immune system, symptoms may be more severe and difficult to control.

• Vaginal thrush

Vaginal thrush is a common infection caused by an overgrowth of Candida albicans yeasts. This yeast lives naturally in the bowel and in small numbers in the vagina. It is mostly harmless, but symptoms can develop if yeast numbers increase. About 75 % of women will have vaginal thrush in their lifetime. Other names for this infection are candidiasis or monilia. Symptoms can include vaginal itching or burning, a white discharge, and stinging or burning while urinating. Vaginal creams or vaginal tablets (pessaries) can help reduce the symptoms of thrush.

• Infection of skin and nails
• Systemic infection

Lab Diagnosis

Microscopy: Potassium Hydroxide (KOH) Preparation

Culture: SDA-smooth creamy white colony with a yeasty odor

Identification: Germ tube test (GTT)-Positive

Treatment

Oral thrush: Nystatin, amphotericin B, miconazole

Vaginal thrush: Topical- Imidazole

Oral-Fluconazole, itraconazole

Lab Diagnosis of Fungi

• Microscopy

1. KOH(20%) preparation: For skin, nail, and hair
2. India ink preparation: CSF-observation for the capsule of Cryptococcus neoformans
3. Lactophenol Cotton Blue (LPCB) for tease mount preparation
4. Stain: All fungi are Gram-positive.

• Giemsa stain– Histoplasma
• PAS stain
• Grocott-Gomori’s methenamine silver stain
• Mucicarmine stain

• Culture

1. Sabouraud Dextrose Agar (SDA): Commonly used medium and it is slightly acidic (pH 5.6-6.0).
2. Potato Dextrose Agar (PDA)
3. Dermatophyte test medium (DTM)
4. Mycosel agar
5. Czapek-Dox agar
6. Corn Meal agar (CMA)
7. Bird seed agar (BSA)

Prevention and Control of Fungal Infection

Fungal infections result from the direct invasion of tissue and organs or direct inhalation of fungal spores or their hyphae. Most infections are acquired through exposure. So, fungal disease can be prevented by applying measures that prevent or reduce exposure to fungi and are controlled by proper treatment of cases.

Preventive measures are as follows-

• Improvement of sanitary facilities e.g. improving living conditions i.e. hosting, flooding, and proper nutrition.
• Personal protection e.g. hand and foot protection while working on the field covering the site where any cut or scratch is found, wearing a mask while working in an old building or area where fungal spores are readily found.
• Environmental management e.g. proper disposal of decaying vegetation, removal of bird droppings, rotten wood, and so on from the living site.
• Improvement of health care facilities for example proper diagnosis and care facilities, early diagnosis, and treatment of cases.
• Reduction of predisposing factors that insist on infection e.g. reduce broad antibiotic therapy, reduce prolonged treatment with corticosteroids, reduce stress, etc.

Treatment

Fungal diseases are treated by antifungal agents and those agents are categorized into three-

1. Polyenes e.g. Amphotericin B, nystatin, griseofulvin
2. Azoles e.g. clotrimazole, ketoconazole, fluconazole, miconazole, itraconazole, voriconazole
3. Nucleoside derivatives e.g. 5-fluorocytosine

Keynotes

• Most of these drugs are fungistatic except a few like amphotericin B. allylamine, benzylamine, and morpholines, which are fungicidal.
• Nystatin is the first discovered antifungal drug in 1951and is abbreviated for New York State Institute.
• They can also be classified as topical or systemic antifungal agents on the basis of their route of administration.