Morphology of fungi and Mycosis

Morphology of fungi.

On the basis of morphology, fungi, can be divided into yeast, molds, and dimorphic fungi.

Yeast: These are round, oval or elongated unicellular fungi varying in size ranging from 1.5 in width and (5-30) in length. They reproduce asexually by budding and in the budding process a new cell is formed as a small outgrowth of old cell. The bud gradually enlarges and then separates.

Elongated budding cells often linked together forming branching chains, which are referred to as pseudohyphae. The bud also is known as blastospore. The secondary bud may also form bud before it separates from the parent cells. These “bud on bud” phenomenon are thought to account for the occasional development of chains of elongated yeast cell called pseudohyphae which may be confused with the true hyphae of molds. Yeast cells are much larger than bacterial cells and can be distinguished microscopically from bacteria by their size and presence of internal cell structure such ass nucleus. Some yeast exhibit sexual reproduction by a process of matting in which two yeast cell fuse. The fused cell is called a ‘zygote’ within which Ascospores are eventually formed. The most important commercial yeasts as Bakers and Brewers yeast which are the members of genus saccharomyces.

Molds (Filamentous fungi)

Vegetative body or thallus of the mold is typically a mass of filament with many branches resembling a cotton bulk. This mass is called mycelium which grows by branching and tip-elongation. A single filament is called hyphae. New hyphae generally arise from a spore which on germination puts out a germ tube. The germ tube elongates and branch to form hyphae.

Hyphae occurs in three forms:

1. Non-septate or coenocytic: Nonseptate hyphae have no cross walls or septa and nuclei are scattered throughout.
2. Septate with uninucleate cells: Septate hyphae are filaments with cross wall called septa that septate hyphae into individual cellular compartments. Most septa have pores that allow migration of cytoplasm and many organelles. Each cell has only one nucleus.
3. Septate with multinucleated cells: Septate with multinucleate hyphae have cross walls. Each cell has more than one nucleus. When the hyphal filaments become densely packed. The mycelium may have the appearance of a cohesive tissue. An example of this is the body of a mushroom.

Mycelia can be either vegetative or reproductive. The vegetative hyphae are actively growing and form the main body of mycelium. Some hyphae of vegetative mycelium penetrate into the medium in order to obtain nutrient. The soluble nutrients are directly absorbed through the mycelium walls. Insoluble nutrients are first digested externally by extracellular enzymes. Reproductive mycelia are responsible for spore production.

1. Dimorphic fungi: These are fungi that are able to grow as yeast or molds depending on environmental condition and temperature. The yeast form is found in infected tissue and when the fungus is cultured at 35-37whereas the filamentous form is found in the soil and when the fungus is cultured at room temperature 20-30

Most of the dangerous human pathogenic fungi are dimorphic; one form i.e., a mold, spore adapted for entry into the host cell and the other form i.e., yeast capable of reproduction following the establishment in the host tissue.

Dimorphic fungi of medical importance

1. Trichophyton Microsporum
2. Histoplasma Capsulation
3. Blastomyces dermatitis
4. Cryptococcus Nneoformons
5. Candia Albicans
6. Sporothrix Schenckil

Mycosis (fungal infection)

Disease caused by fungi is collectively called mycosis. They are divided into four general categories on the basis of the site of the body affected.

1. Superficial mycosis
2. Cutaneous mycosis
3. Sub-Cutaneous mycosis
4. Systemic mycosis

Superficial mycosis:

These are infections limited to the hair and dead layers of the skin. They do not directly invade living tissue. These fungi possess proteolytic enzymes (keratinase) that can damage the horny layers.e.g., (1) Tinea vesicular: It is characterized by patch discolouration of the skin and caused by lipophilic yeast (Molasses furfur). The common sites involved include chest, abdomen, neck and upper arms.

(2) Tinea nigra: It is superficial asymptomatic skin disease. It is caused by CladosporiumWerneckii. It is characterized by light brown on muscular areas affecting thick keratinized regions of palmar and plantar stratum corneum.

(3) Piedra: It is characterized by the formation of nodules on the hair shaft. There are two varieties of piedra.

• Beigilli which affects hair shaft of the axilla.
• Black piedra caused by piedra Hortae

Cutaneous mycosis:

Most cutaneous mycosis is caused by a group of fungi called dermatophytes because of their affinity for tissues rich in the protein where they cause disease collectively known as dermatomycosis also known as tinea or ringworm. The fungi spread radially on deal kertaniced layer of skin of means of hyphae and occasionally spores. The dermatophyte belongs to 3 genera each with many species.

1. Trichophyton
2. Epidermophyton
3. Microsporum

Clinically ringworms are classified on the basis of the site involved.

Sub-Cutaneous mycosis:

This mycosis affects the subcutaneous tissue below the skin. The organisms are usually introduced to a puncture wound.

1. Sporotrichosis: It is caused by sporotheca Schenck which is a dimorphic saprophytic fungus. When conidia are inoculated beneath the skin often by the site of puncture and spread along the lymphatic system.
2. Chromoblastomycosis (chromomycosis): It is characterized by watery nodules specially on legs that spread slowly along the lymphatic system and developed access. It is caused by soil fungi belonging to three genera.

• Exophiala
• Fonseceae
• Cladosporium

1. Mycetoma: It is a localized swollen region with colored granules that are compact colonies of causative agent draining from tissues. It is caused by a variety of actinomycetes and filamentous fungi like Madurella sps, Nocardia sps, Actinomadora mature.

Mycetoma begins as a small subcutaneous region usually of the foot.

4. Systemic mycosis: It may involve any tissue or organ of the body and may spread throughout the host. Most of them are caused by dimorphic soil fungi. Only the mold form found in the soil produces spores capable of infecting a human. Infection of systematic fungi is acquired through lungs by inhalation and the primary focus of infector is the lungs. The respiratory infections in most cases are asymptomatic and result without serious form in immune compromised individuals.

Isolation of fungi:

Nearly all the fungi grow aerobically on the usual bacteriological culture media at temperature 28-30

(for saprophyte) and 30-37(for parasite). Most fungi are obligate aerobes. For the isolation of fungi; from the different sample it is good to use a medium that favors the growth of fungi but not optimal for growth of bacteria. Acidic media incorporated with the high concentration of sugar are tolerated by fungi but not bacteria.

The media most commonly used for the isolation of fungi are PDA (Potato Dextrose Agar) and SDA (Saboraced Dextrose Agar); the given sample is inoculated into PDA or SDA by spread plate technique or streak plate technique or pour plate technique and incubated for 3-5 days for required temperature conditions.

The fungal colonies are grown on the medium SDA is widely used for isolation of molds and certain yeast and especially useful for growing pathogenic fungi from infected body fluids.

Enumeration of fungi:

The enumeration of fungi is done especially by counting not of organisms provided in measured inoculated volume. The no. of the organism is expressed in colony forming the unit (CFU).

No. of organisms= (no. of colonies/inoculation size)dilution factor

Where dilution factor=1/dilution

First of all serial dilution of the given sample is done then from a particular dilution pour plate, streak plate and spread plate is performed the after incubation colonies are counted and no. of organisms are enumerated using the above relationship.

Reproduction of fungi

Fungi reproduce both sexually and asexually by means of spore. The reproductive structure elevated at the end of aerial hyphae produced specialized cells called spores. Spores are generally produced in large numbers, easily spread by wind currents and resistant to a relatively adverse condition. This property have selective advantages in fungal reproduction. The spores may be sexual or asexual.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.