DIMORPHIC FUNGI: GENERALITIES AND EXAMPLES - BIOLOGY - 2025

The dimorphic fungi are those having two anatomical forms or different morphological: a mycelial form and another yeastlike. This property of dimorphism is exhibited by only some fungal species and is called fungal dimorphism.

In the morphological phase of mycelium, the dimorphic fungus appears as a mass formed by a set of hyphae or cylindrical filaments. The function of the hyphae is to nourish the fungus, as they have the ability to absorb nutrients. The mycelium constitutes the so-called vegetative body of a macroscopic multicellular fungus.

Figure 1. Candida albicans yeast phase. Source: David Arqueas, from Wikimedia Commons

In the yeast phase, the dimorphic fungus appears as a microscopic unicellular organism, with spherical or ovoid cells. It also has the ability to break down organic matter, sugars and carbohydrates through fermentation processes.

A small group of fungi within the Ascomycota phyllum is considered dimorphic; these fungi have the ability to infect mammals, plants and insects as parasites.

Figure 2. Candida albicans in mycelial phase. Source: Garnhami, from Wikimedia Commons

Examples include the human pathogens (disease causing), Candida albicans and Histoplasma capsulatum. Also the phytopathogenic fungus Ophiostoma novo-ulmi, which causes Dutch elm disease.

Other examples are Ophiocordyceps unilateralis, an entomopathogenic fungus that presents dimorphism and secretes chemical compounds that alter the behavior of infected ants. It is called "the fungus of zombie ants."

There is also Malassezia furfur, a dimorphic fungus that is both phytopathogenic and entomopathogenic.

Dimorphism and pathogenicity

Fungal dimorphism is related to the ability to cause fungal disease or pathogenicity.

The process by which a fungus passes from a unicellular state in the form of yeast (yeastiform) to a multicellular state of hyphae or mycelium, is called phase transition. This transition is essential for the pathogenicity and virulence of the fungus.

The pathogenic fungus receives signals with information from the environment that surrounds it, and according to its convenience it responds by transforming itself into one of the two phases. For example, there are fungi that change their state depending on the temperature of the environment, being then thermodependent.

This is the case of fungi that grow in the soil at a temperature of 22 to 26 ° C, remaining in a mycelial state. These mycelia can fragment and become suspensions in the air or aerosols due to the effect of alterations such as natural disasters or human intervention (construction, agriculture, among others).

When inhaled by a mammalian host, airborne fungi colonize the lungs, where the temperature is maintained at 37 ^° C. At this temperature, mycelial hyphae act as infectious propagules, becoming pathogenic yeasts, and causing pneumonia.

Once the infection is established in the lungs, the yeasts can spread to other organs, such as skin, bones, and brain.

Factors that determine the phase change or fungal dimorphism

Among the environmental factors that generate the transformation of the fungus from one state to another in a reversible way are the following.

Changes in temperature

The change in temperature generates a transition or morphological phase change in the fungal species Talaromyces marneffei. When the ambient temperature is between 22 and 25 ^° C, the fungus presents filamentous morphology (hyphal), and when the temperature rises to 37 ^° C, it acquires yeast morphology.

Other human pathogenic fungal species with temperature-dependent dimorphism are Histoplasma capsulatum, Blastomyces dermatitides, Sporothrix schenkii, Paracoccidioides brasiliensis, Coccidioides inmitis, Lacazia laboi, and Emmansia sp.

Change in nutrient availability

In the species Candida albicans, the following phase transition occurs: in the presence of nutrient-rich media the morphology is yeast, while in nutrient-poor media the growth form is mycelial filamentous.

Joint changes in temperature and nutrient availability or presence of toxic substances

Although temperature seems to be the predominant environmental stimulus that directs the transition from hypha (at 22-25 ° C) to yeast (at 37 ^° C) and vice versa, there are additional stimuli that influence the morphological change, such as the concentration of carbon dioxide (CO ₂), the presence of cysteine, estradiol or toxic substances in the medium.

Some fungal species require changes in both environmental factors (temperature and nutrient availability) to express dimorphism. Also, other environmental changes, such as the presence of metals or chelating agents, can trigger morphological phase transitions.

Human pathogenic dimorphic fungi

Three examples of human pathogenic dimorphic fungi are briefly described below.

Talaromyces marneffei

It is a pathogenic fungal species that belongs to the Ascomycota phyllum. It shows temperature-dependent dimorphism: at 25 ^° C it grows in its filamentous phase as saprophyte, and at 37 ^° C it shows parasitic yeast morphology.

The fungus T. marneffei can cause a deadly infection of the whole organism; penicillosis, named for its old taxonomic name as Penicillium marneffei.

Morphological forms or phases

The fungus T. marneffei in the hyphal or filamentous phase, grows in grayish-white colonies, with a smooth and smooth surface. These colonies change to a reddish-brown color with yellow tones, while their surface acquires a radiated relief, with the underside of a salmon color.

In the yeast phase, T. marneffei develops small ivory-colored colonies with a rough-looking relief.

Reservoirs

The reservoirs of T. marneffei are the soil (in the tropics and subtropics, in rainy seasons, from May to October), and several species of bamboo rats (Cannomis badius, Rhizomis sinensis, Rhizomis sumatrensis and Rhizomis pruinosis).

Hosts

Common hosts for the pathogenic fungus T. marneffei are rats, humans, cats, and dogs.

The fungus T. marneffei enters the body mainly through the respiratory tract. It can also penetrate through any other route other than the digestive one.

Clinical manifestations

The fungus T. marneffei causes opportunistic generalized or systemic infection in immunodeficient humans. It initially affects the lungs and then different organs via the bloodstream. It produces lesions in the form of papules on the skin of the neck, face and trunk.

Candida albicans

The fungus Candida albicans belongs to the phyllum Ascomycota and presents dimorphism dependent on the availability of nutrients.

In Candida albicans, yeast cells seem to be the most suitable for blood dissemination and virulence factor. While the hyphal phase has been proposed as the most invasive in tissue penetration and organ colonization.

The transition from yeast to hypha is a rapid process, induced by environmental factors such as levels of carbon dioxide, lack of oxygen, changes in the nutrient medium and in temperature.

Through pleomorphism or multiple phase changes, this fungus can survive the immune defense mechanisms of its host. In the yeast phase, the morphology is spherical or ovoid cells in small groups. In the hyphal phase or filamentous fungus morphology, the cells appear elongated, stretched in the form of filaments.

Additionally, in the yeast phase it acquires a symbiotic life form and in the hyphal phase it becomes a pathogenic parasite.

Reservoir

The reservoir for Candida albicans is the human body. It is present in the microflora of the skin, in the gastrointestinal tract, in the oral cavity and in the genitourinary system.

Hosts

The human organism functions as a host for Candida albicans, whose route of entry is the skin and mucous membranes.

The Candida albicans fungus produces candidiasis or moniliasis, which affects the skin, nails, mucous membranes of the mouth and gastrointestinal mucosa. In immunosuppressed people, the infection can become systemic or generalized throughout the body.

Candida albicans is capable of crossing the blood-brain barrier. Mortality rates of 40% are reported in severe infections with this pathogenic fungus.

Histoplasma capsulatum

Histoplasma capsulatum belongs to the phyllum Ascomycota. It is a fungal species that is pathogenic for humans and exhibits temperature-dependent dimorphism. The fungus grows in soil and on mixtures of faeces from starlings (Stumus vulgaris), blackbirds (Turdus merula) and various species of bats.

The Histoplasma capsulatum fungus is prevalent in bird roosting areas and in caves, attics, or tree holes that inhabit bats.

This fungus has a wide distribution throughout the planet, except in Antarctica. It is frequently associated with river valleys. It is found particularly in the valleys of the Mississippi and Ohio rivers in the United States.

Morphological forms or phases

Histoplasma capsulatum shows filamentous, mycelial growth in the form of a saprophytic life in the soil. When infecting animals or humans, it develops the growth phase in the form of parasitic yeast at a body temperature of 37 ^° C.

The morphological phase of mycelium is made up of hyphae. Colonies are initially white, cottony, and later turn dark brown with a yellow to orange underside.

The yeast phase has ovoid cells, slow growing at 37 ^° C, which form gray to beige colonies with a moist and creamy appearance.

Reservoirs

The reservoirs of Histoplasma capsulatum are soil contaminated with nitrogen-rich bird and bat droppings.

Hosts

Histoplasma capsulatum hosts include human organisms, some birds (starlings, blackbirds, thrushes, chickens, turkeys, geese), bats, dogs, cats, rodents, horses, and cattle.

This fungus enters the human body through the respiratory, percutaneous (through the skin) and mucous membranes.

Clinical manifestations

Cases of acute lung infection with Histoplasma capsulatum are very common, with symptoms such as fever, cold, chills, headache, chest pain, fatigue, erythema, and rash.

References

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