Definition of Fungus | CHAPTER 4 | Microbiology & Parasitology for Nurses

Definition of Fungus – Basic microbiology, parasitology, and immunology; nature, reproduction, growth, and transmission of common microorganisms and parasites in Bangladesh; prevention including universal precaution and immunization, control, sterilization, and disinfection; and specimen collections and examination. Students will have an understanding of common organisms and parasites caused human diseases and acquire knowledge about the prevention and control of those organisms.

Definition of Fungus

A fungus (plural: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms.

or

Fungi are chlorophyll free unicellular or multicellular eukaryotic organisms reproduce by sexual and asexual spore formation.

General Characteristics of Fungus;

• Fungi are eukaryotic organisms.
• The fungal cell wall consists primarily of chitin, a polysaccharide.
• The fungal cell membrane consists of ergosterol.
• Most fungi are obligate aerobes, some are facultative anaerobes.
• There are two morphological forms of fungi- yeast & mould.
• All fungi require a pre-formed organic source of carbon.
• Fungi reproduce by forming sexual or asexual spores.

Classification of Fungus;

Morphological classification;

Clinical classification (According to site);

1. Superficial fungi; They infect the superficial layer of epidermis of skin. E.g Piedriaea hortae
2. Cutaneous fungi; They infect skin & skin appendages (e.g.- nail, hair etc.). E.g Candida species
3. Sub-cutaneous fungi. E.g Medurella mycetomatis 4. Systemic/deep fungi. E.g histoplasma capsulatum
5. Opportunistic fungus; e.g Candida albicans

According to reproduction:

1. Sexual reproduction (fungi perfecti):

• Ascomycotina (form ‘Ascus’)
• Basidiomycotina (form ‘Basidium’)
• Zygomycotina (form’Zygous’)

2. Non-Sexual reproduction (fungi imperfecti):

• Deutero-mycotina

Beneficial Effects of Fungus;

1. They reside in nature and are essential in breaking down and recycling organic matter.
2. Some fungi greatly enhance our quality of life by contributing to the production of food and spirits, including cheese, bread, and beer.
3. Other fungi have served medicine by providing useful bioactive secondary metabolites such as antibiotics (e.g. penicillin) and immunosuppressive drugs (e.g. cyclosporine).
4. Fungi have been exploited by geneticists and molecular biologists as model systems for the investigation of a variety of eukaryotic processes.

Opportunistic Fungus;

Differences between Fungus and Bacteria

Structure of Fungus:

The structural features of a fungal cell are the followings –
➤ Fungi are chlorophyll-free eukaryotic organisms.
➤ Cell wall: It is an essential component of fungal cell. It comprises 15-30% of the dry weight of a fungal cell.

• Composition

✓ Carbohydrate (80%): chitin, chitosan, glucan and mannan. Protein (10%): structural and

✓ enzymatic proteins.

✓ Glycoprotein (10%).

• Functions

✓ It protects the fungal cell from external environment.
✓ It gives rigidity and strength to the cell.
✓ It maintains shape of the cell.
✓ It takes part in reproduction.

• Clinical importance: Fungi are insensitive to cell wall inhibiting antibiotics due to presence of chitin instead of peptidoglycan.

➤ Cell membrane: This is also an essential component of fungal cell.

• Composition: Phospholipid bilayer incorporated with ergosterol & zymosterol.
• Functions

✓ It protects cytoplasm.

✓ Transport of materials to and from the cell.

✓ Synthesis of cell wall.

Clinical importance: Human cell membrane contains cholesterol. So, anti- fungal drugs selectively inhibit the fungal cell by acting on ergosterol.

➤ Cytoplasm: It contains mitochondria, 80S ribosome (60S + 40S), endoplasmic reticulum, Golgi apparatus and food storage vacuoles.
➤ Nucleus: It is eukaryotic.
➤ Capsule: It is a non-essential structure situated outside the cell wall. It is composed of polysaccharide.

Process of Fungal Reproduction:

Fungi reproduce by sexual and asexual spore formation. The processes are described below-

➤ Asexual reproduction: Producing spores or conidia usually by budding and binary fission. Asexual spores are

• Chlamydospore: In hyphae, one cell enlarges and specialized to form a spore. Example: Candida albicans.
• Arthrospore: Hyphae fragmented to individual cells. Example: Coccidioides immitis.
• Blastospore: Spore developing by budding. Example: Candida albicans.
• Sporangiospore: Example: Rhizopus & Mucor

➤ Sexual reproduction: Here, reproduction takes place by mitosis & meiosis. Sexual spores are-

• Zygospore: Fusion of tips of hyphae. Example: Rhizopus & Mucor.
• Ascospore: These spores are formed in a sac called ascus. Example: Histoplasma, Microsporum.
• Basidiospore: Spores are formed externally on the tip of a pedestal called a basidium. Example: Cryptococcus neoformans.

Lab. Diagnosis of Fungal Infection;

Principle:
Diagnosis of fungal infection is based on demonstration of fungal structure by direct microscopic examination and isolation from culture. Serological tests are also useful but have little importance.

Steps:

➤ Specimen collection: It depends upon the site of infection, e.g.-

• Skin scrapping
• Nail clipping
• Hair plucking
• Swabs: Throat swab, vaginal swab etc.
• Aspirated puss
• Biopsy material

➤ Microscopic examination:

• Wet film preparation after partial digestion of keratin material with 10-20% кон.
• Giemsa stain: to see the yeast of Histoplasma capsulatum.
• Gram stain: to see yeast.
• Periodic acid sciff (PAS).

➤ Isolation & identification from culture: Specimen is cultured in Sabouraud’s dextrose agar media at 25°C and 37°C for 2-3 weeks in aerobic condition.

• The main identifying structures are morphological, in particular the asexual and, if present, sexual reproductive structures.
• Biochemical tests are used mainly to identify yeasts and are generally not as important in mycology as they are in bacteriology.

➤ Serology: By the identification of antibodies to special fungal antigens in patient’s serum. The Interpretation of serological findings is quite difficult in fungal infections.
➤ Antigen detection: By finding of specific antigens in the diagnostic material by direct means using known antibodies, possible in some fungal infections (e.g., cryptococcosis).
➤ Cutaneous test: Cutaneous (allergy) tests with specific fungal antigens can be useful in diagnosing a number of fungal infections.
➤ DNA probe test: It is available for Histoplasma and Cryptococcus.

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