Fungus: Introduction, Structure, Fungal Infections, Prevention and Control

Fungus:Yeast Vs mold-colonial and microscopic features


Mycology is the science that deals with the study of fungus. Medical mycology: The science that deals with the study of fungi that causes the disease is called medical mycology.

General Features Fungus

  • The fungi are a group of eukaryotic organisms.
  • They are found in soil, water, air, and decaying matter are the main sources.
  • They have a diversity of morphological appearances, depending on the species.
  • They are heterotrophic organisms that lack the definite root, stem, and leaves of higher plants.
  • Yeast is unicellular whereas mold is filamentous and multicellular.
  • Due to the lack of chlorophyll, they are differentiated from the algae and higher plants.
  • They are saprophytic or parasitic because of requiring prepared food.
  • Having more complex structures and greater size, they differ from bacteria.

They are mainly found in two forms.

a) yeast and

b) Mold


Unicellular form


Width: 1-5 µm

Length: 5-30 µm


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.


Molds are multicellular filamentous fungi consist 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-

a) Vegetative mycelium

b) Reproductive mycelium also called aerial mycelium.

They reproduce by the formation of different types of sexual and asexual spores that develop from the mycelium. Few examples of molds are Aspergillus, Dermatophytes, Penicillium, Rhizopus.

Dimorphic fungi

They exist as yeasts in the host tissue and the culture at 37°C and hyphal (mycelium) forms in the soil and in the culture at 22-25°C. 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

Sexual spores 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 further of two types-

  • Vegetative spores




  • Aerial spores





fungal classification chart

Taxonomical classification of fungus

Superkingdom: Eukaryota

Kingdom: Fungi

1. Phylum: Zygomycota

Class: Zygomycetes

Order: Mucorales

Family: Mucoraceae

Genus: Mucor

Species: indicus

2. Ascomycota:

e.g. Histoplasma



3. Basidiomycota:

e.g. Cryptococcus

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



Clinical classification of fungus

  • Superficial fungi
  • Cutaneous fungi
  • Subcutaneous fungi
  • Systemic fungi

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 clinical conditions 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 cutaneous and sub cutaneous tissue of wide of saprophytic fungi are Chromobalstomycosis, Mycetoma, Sporotrichosis, Rhinosporidiosis.
  4. Systemic mycoses: The fungi which cause the deep or systemic mycoses are Histoplasmosis, Blastomycosis, Cryptococcosis, Coccidioidosis, Paracoccidioidosis.
    Pathogenic group of fungi

Predisposing factors of fungal infection

  1. Diabetes
  2. Prolonged treatment with corticosteroids
  3. Immunosuppression
  4. Broad antibiotic therapy
  5. Injury


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

a.Trichophyton ( skin, hair, and nail)

b. Microsporum (Hair and skin)

c. Epidermophyton (skin and nail)



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 the groin
  • Tinea pedis: foot (Athlete’s foot)
  • Most common infection in humans by Trichophyton rubrum
  • Dermatomycosis
  • Another infection of 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 a common and lung infection.

Lab diagnosis


Selective media: Bird Seed agar

Other SDA  creamy white and mucoid colony


India ink preparation


Choice of dugs-

  • 5-fluorocytosine
  • Amphotericin B
  • Ketoconazole

 Candida albicans 

Normal flora of mouth and vagina


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 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


Potassium Hydroxide (KOH) Preparation


SDA: smooth creamy white colony with yeasty odor.


Germ tube test (GTT): Positive


Oral thrush:

  • Nystatin,
  • amphotericin B,
  • miconazole

Vaginal thrush

Topical- Imidazole

Oral-Fluconazole, itraconazole

Dimorphic fungi  

They occur in two forms-

  • Yeast form: parasitic phase

Culture at 37°C

  • Spores and filamentous form(mold)

Saprophytic phase

Culture at 22-25°C

Dimorphic fungi are

  • Sporothrix schenckii
  • Blastomycosis
  • Histoplasma
  • Candida albicans
  • Paracoccidiodes
  • Penicillium marnefii
  • Coccidioides

Lab Diagnosis of Fungi  

1. Microscopy

a. KOH (10%) preparation

For skin, nail, hair

b. India ink preparation: CSF

observation for the capsule of Cryptococcus neoformans

c. Lactophenol Cotton Blue (LPCB) for tease mount preparation

d. Stain

All fungi are Gram-positive

a. Sabouraud Dextrose Agar (SDA)

Commonly  used medium

Slightly acidic (pH 5.6-6.0)

b. Potato Dextrose Agar (PDA)

c. Dermatophyte test medium (DTM)

d. Mycosel agar

e. Czapek-Dox agar

f. Corn Meal Agar (CMA)

g. Birdseed agar (BSA)

 Prevention and control of fungal infection

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

Preventive measures are as follows-       

  1. Improvement of sanitary facilities e.g. improving living conditions i.e. hosting, fooding, and proper nutrition.
  2. 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.
  3. Environmental management e.g. proper disposal of decaying vegetation, removal of bird droppings, rotten woods, and so on from the living site.
  4. Improvement of health care facilities for example proper diagnosis and care facilities, early diagnosis, and treatment of cases.
  5. Reduction of predisposing factors that insist on infection e.g. reduce broad antibiotic therapy, reduce prolonged treatment with a corticosteroid, reduce stress, etc.

Treatment of fungal infections/ diseases

Fungal diseases are treated by anti-fungal 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


  • 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 1951 and abbreviated for New York State Institute.
  • They can also be classified as topical or systemic antifungal agents and on the basis of their route of administration.


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  6. Topley & Wilsons Medical Mycology. Editors: M.T. Parker & L.H. Collier, 8th ed 1990, Publisher Edward Arnold publication, London.
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  8. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  9. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.

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