Dimorphic fungi

Dimorphic fungi can define as a type of fungi which has a dual life cycle where Di means two and Morphic means morphology or structure. Therefore dimorphic fungi are the type of fungi which shows two kinds of morphology in its life cycle. This dual character of dimorphic fungi refers to “Dimorphism”.

On the basis of following factors like temperature, pH, oxygen concentration, nutrients availability etc. dimorphic fungi exist in two forms or structures: Mould phase and the Yeast phase. Mould phase occurs at a relatively low temperature (25-30 ֯ C) whereas the yeast phase occurs at high temperature (37 ֯ C).

Content: Dimorphic fungi

  1. Dimorphism of dimorphic fungi
  2. Phases of dimorphic fungi
  3. Examples of dimorphic fungi
  4. Transmission
  5. Diagnosis
  6. Treatment

Dimorphism of dimorphic fungi

As dimorphic fungi exist in two forms at a different environment, temperature, oxygen concentration etc. is known as dimorphism. As dimorphic fungi grow at two different temperature. Therefore, it also refers to “Thermally dimorphic fungi”.
DIMORPHISM OF DIMORPHIC FUNGI

Phases of dimorphic fungi

  • Mould phase
  • Yeast phase

The general growth cycle of mould

It includes the following steps:

  • First, a mycelial network forms.
  • Then, hyphae originate through the mycelial network.
  • After that, hyphae form fruiting structures known as “Spores”.
  • These spores then detach from the hyphae body during unfavourable conditions.
  • When favourable conditions return, the spore germinates and forms a new vegetative body.
    general life cycle of mould

The general growth cycle of yeast

It includes the following steps:

  • First yeast cell enlarges and forms a small protuberance extending outside the cell.
  • This small outgrowth refers to “Bud”, forms by the phenomena refers to budding.
  • After bud formation, nuclear divisions occur through mitosis.
  • Then, a copy of genetic material passes to the bud.
  • This bud then detaches during unfavourable conditions and forms a new yeast cell.
    general life cycle of yeast

But in dimorphic fungi these two growth cycle occurs one by one in its lifecycle:

In the mould phase :

  • First, the spores detach from the vegetative cell during adverse conditions.
  • Then, these spores remain in a dormant state in the soil.
  • Then through wind or air, it goes into the body, nowhere starts a yeast phase.

In the yeast phase:

  • A fungal spore goes into the body or tissues of the host cell.
  • This fungal spore then germinates into a yeast.
  • Then yeast grows through budding and multiplies within the host cell.
  • And when these get out of the host cell then it returns to the mould phase.

Therefore, these two phases switch to one another and below a whole diagram is given showing the life cycle of dimorphic fungi:
LIFE CYCLE OF DIMORPHIC FUNGI
Table showing growth conditions required in both mould and yeast phase:

Phase of dimorphic fungiMold phaseYeast phase
Conditions required for growthLow temperature
Optimal oxygen concentration
Improved nutrients availability
High temperature
Optimal oxygen concentration
Improved nutrients availability
Nutrition typeSaprobicParasitic
Temperature range<30degrees C35-40degrees C
ReproductionSporulationBudding

Examples of dimorphic fungi

Dimorphism of dimorphic fungi can be explained by taking examples which are given below:

Histoplasma capsulatum

HISTOPLASMA SP.

Culture characteristics:
In SDA media: 
Mould phase
Colony: Slow growing
Appearance: Cottony
Colour: White to brown in colour
Hyphae: Thin, septate
Conidiophore: Small
Conidia: Single-celled, circular

In BHI media: Yeast phase
Colony: Slow growing
Size: Small
Shape: Oval
Colour: Cream coloured

Distribution: Worldwide

Habitat: It founds in contaminated and nitrogenous soil enriched with bird droppings, chicken manure etc.

Hosts: Human, donkeys, cats, dogs, horses etc.

Target organ: lungs, bones, skin, reticuloendothelial system etc.

Disease: It causes
Histoplasmosis: It is an “Acute respiratory disease”
Symptoms: illness, chest pain, cough etc.

Coccidioides immitis 

COCCIDIOIDES SP.

Culture characteristics:
In SDA media: 
Mould phase
Colony: Slow growing, barrel-shaped
Appearance: Cottony
Colour: White
Hyphae: Filamentous and septate
Conidiophore: Small
Conidia: Also refers to ‘Arthroconidia’ which are oval in shape

In BHI media: Yeast phase
Colony: Slow growing, flat and exist as “Spherule”.
Size: Small
Shape: Oval
Colour: Cream coloured
Spores: Endospores present

Distribution: Cosmopolitan

Habitat: Soil of low elevation desert

Hosts: Human, cats, dogs, horses etc.

Target organ: Skin and soft tissues

Disease: It causes
Coccidioidomycosis: It is the pulmonary disease also known as “Valley fever”.
Symptoms: Fever, chest pain, cough, sputum secretion, sore throat etc.

Blastomyces dermatitidis

BLASTOMYCES SP.

Culture characteristics:
In SDA media: 
Mould phase
Colony: Slow growing
Appearance: Cottony
Colour: White to yellow in colour
Hyphae: Filamentous and septate
Conidiophore: Short and unbranched
Conidia:  Single-celled, circular

In BHI media: Yeast phase
Colony: Slow growing, broad base
Size: Small
Shape: Oval
Colour: Cream to tan coloured
Cells appear as a fusiform budding form

Distribution: Cosmopolitan

Habitat: Soil with low pH (Acidic soil)

Hosts: Human, cats, dogs etc.

Target organ: Skin, bones, kidneys, CNS, eyes etc.

Disease: It causes
Blastomycosis: It also refers to “North American Blastomycosis” and sometimes Blastomycosis dermatitis.
Symptoms: Fever, chest pain, cough, excessive sweating, muscle and joint pain, lethargy etc.

Paracoccidioides brasiliensis

PARACOCCIDIOIDES SP.

Culture characteristics:
In SDA media: 
Mould phase
Colony: Slow growing
Appearance: Cottony
Colour: White
Hyphae: Thin, septate
Conidiophore: Small
Conidia: Single-celled, oval to circular in shape

In BHI media: Yeast phase
Colony: Slow growing
Size: Large
Shape: Circular
Colour: Cream coloured
Cells exist with multiple daughter buds.

Distribution: Cosmopolitan

Habitat: Soil

Hosts: Human, cats, dogs etc.

Target organ: Skin, reticuloendothelial system, mucosal membrane etc.

Disease: It causes
Paracoccidioidomycosis: It also refers to “South American Blastomycosis” and sometimes Paracoccidioidomycosis granuloma.

Symptoms: Skin ulcers, localized lesions, abdominal pain etc.

Sporothrix schenckii

SPOROTHRIX SP.

Culture characteristics:
In SDA media: 
Mould phase
Colony: Fast-growing
Appearance: Velvety
Colour: Cream to orange grey in colour
Hyphae: Narrow and septate
Conidiophore: Slender and unbranched
Conidia:  “Rosette-like”

In BHI media: Yeast phase
Colony: Slow growing
Size: Small
Shape: Round to oval
Colour: Cream coloured
Cells appear as a fusiform budding form

Distribution: Worldwide

Habitat: Soil with decomposed plants, woods, peat moss etc.

Hosts: Human, cats, dogs, etc.

Target organ: Subcutaneous nodules, lymphatics etc.

Disease: It causes
Sporotrichosis: It also refers to “Rose handler’s disease”.
Symptoms: Bumps (on fingers, hands, arms etc.), cough, chest pain, fever, headache, seizures etc.

Transmission

The transmission of diseases caused by dimorphic fungi can occur through the following ways:

  • Inhalation of fungal spores through the air.
  • By direct contact with an infected person.

Diagnosis

Diseases caused by dimorphic fungi can be diagnosed by:

  • Cell culturing methods.
  • Serological testing like enzyme immunoassay, immunodiffusion test, complement fixation test etc.
  • Chest X-ray.

Treatment

For the treatment of diseases caused by dimorphic fungi, some antifungal agents like Itraconazole, Fluconazole, Triazoles are being used in mild cases. In severe cases Amphoterin-B is applicable.

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