What is Dimorphic Fungi? Dimorphic Life Cycle, Examples & Transmission

Dimorphic fungi can define as a type of fungi, which has a dual life cycle. We can understand the meaning of the term dimorphic just by breaking it into two, in which Di means two and Morphic means morphology or structure. Therefore, dimorphic fungi possess two kinds of morphology in its life cycle. This dual character of dimorphic fungi is known as “Dimorphism”.

Based on the factors like temperature, pH, oxygen concentration, nutrients availability etc. the dimorphic fungi exists in two forms or structures, namely filamentous and yeast-like phase. Temperature is the primary factor of phase transition, and based on it, thermal and non-thermal are the two forms of dimorphic fungi.

The thermal and non-thermal dimorphic fungi infect humans, animals, rodents and insects and the latter are mainly phytopathogens. In this context, we will discuss the dimorphic behaviour and the two different phases of dimorphic fungi along with some important pathogens that are included in this group.

Definition of Dimorphic Fungi

Dimorphic fungi possess a property of phase transition, in which they can reversibly change to the mould and yeast phase relative to the temperature. Mould phase occurs relatively at a low temperature (25-30 degrees C), whereas the yeast phase occurs remarkably at a high temperature (37 degrees C). As the temperature reaches 37 degrees C, the yeast phase transforms into a filamentous phase and vice versa. The species of dimorphic fungi belong to the phylum (Ascomycota), and all are thermally dimorphic except Coccidioides immitis.

Dimorphic Life Cycle

The dimorphic fungi exist in two forms at different environment, temperature, oxygen concentration etc. and the property exhibited by them is known as dimorphism. Filamentous and yeast-like phases are seen in their life cycle, which mainly depends upon the temperature and can be thermally dimorphic. Except for Coccidioides immitis, all are thermally dimorphic.

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

The general growth cycle of yeast

It includes the following steps:

First yeast cell enlarges and forms a small protuberance, which extends outside the cell.
This small outgrowth is called as “Bud”, and it forms by the phenomena called budding.
After bud formation, the nuclear division occurs 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.

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 on the soil.
Then through wind or air, the spores go into the body and eventually, it turns into the 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, the yeast grows through budding and multiplies within the host cell.
When yeast releases out of the host cell, it eventually returns to the mould phase.

Therefore, these two phases switch to one another, and the whole diagram is given below that is showing the life cycle of dimorphic fungi:

Table showing the growth conditions required in the mould and yeast phase:

Phase of dimorphic fungi	Mold phase	Yeast phase
Conditions required for growth	Low temperature Optimal oxygen concentration Improved nutrients availability	High temperature Optimal oxygen concentration Improved nutrients availability
Nutrition type	Saprobic	Parasitic
Temperature range	<30degrees C	35-40degrees C
Reproduction	Sporulation	Budding

Examples of Dimorphic Fungi

Dimorphism of dimorphic fungi can be explained by taking a few examples like:

Histoplasma capsulatum

Culture characteristics:
1. In BHI media: Yeast phaseColony: Slow-growing
  Size: Small
  Shape: Oval
  Colour: Cream coloured
2. 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
Distribution: Worldwide
Habitat: It is found in the 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 (an acute respiratory disease), which shows symptoms like illness, chest pain, cough etc.

Coccidioides immitis

Culture characteristics:
1. 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
2. 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 (a pulmonary disease) that is also known as “Valley fever”.
Symptoms: Fever, chest pain, cough, sputum secretion, sore throat etc.

Blastomyces dermatitidis

Culture characteristics:
1. 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
2. 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, which is also called North American Blastomycosis and sometimes Blastomycosis dermatitis.
Symptoms: Fever, chest pain, cough, excessive sweating, muscle and joint pain, lethargy etc.

Paracoccidioides brasiliensis

Culture characteristics:
1. 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
2. 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, which is also called South American Blastomycosis and sometimes Paracoccidioidomycosis granuloma.
Symptoms: Skin ulcers, localized lesions, abdominal pain etc.

Sporothrix schenckii

Culture characteristics:
1. 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
2. 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 or Rose handler’s disease.
Symptoms: Bumps (on fingers, hands, arms etc.), cough, chest pain, fever, headache, seizures etc.

Transmission

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

Inhalation of fungal spores through the air.
By the 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.