Dimorphic Fungi: What Are They?

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Dimorphic fungi are those fungi that exist either in yeast form or as mold (mycelial form) depending on environmental conditions, physiological conditions of the fungus or the genetic characteristics. As yeasts, dimorphic fungi exist as single cells and multiply by old cells producing daughter cells. Dimorphism is common with some members of the major divisions of the fungal kingdom, i.e., Zygomycota, Ascomycota, Basidiomycota, and mitosporic fungi.

Some dimorphic fungi will switch to yeast or mold form depending on the prevailing temperature. These fungi are referred to as thermally dimorphic. Others will switch to yeast if they grow inside host tissues.

Examples of Dimorphic Fungi in Indoor Environments

Examples of dimorphic fungi in indoor environments are Aureobasidium pullulans, Histoplasma capsulatum and Sporothrix spp.

Aureobasidium pullulans

Aureobasidium pullulans is both a soil and leaf (phylloplane) fungus. It is common in indoor environments, especially on wet wood, windowsills, and bathroom tiles in wet buildings. A. pullulans has also been isolated from floor, carpet, and mattress dust; damp walls; and in humidifier water. A. pullulans is a spoilage organism, especially a deteriogenic agent of painted surfaces. It has occasionally been found on a wide range of stored foodstuffs and cereals (such as wheat, barley and oats). Sometimes it occurs on meat in cold stores as it can grow at very low temperatures.

A. pullulans is associated with occupational disease in wood processing. It has also been isolated from human skin and nails.

Some strains of A. pullulans are used in various standard tests for resistance to microbial degradation.

Histoplasma capsulatum

Histoplasma capsulatum is a good example of a thermally dimorphic fungus. This fungus grows in its yeast form at 37°C, but switches to mold form at 20-25°C. H. capsulatum is found throughout the world in substrates rich in nitrates such as bird droppings, chicken manure and bat guano. It is also common in soils enriched by bird droppings. It causes a disease called histoplasmosis. Humans are infected by inhalation of spores in the air or carried in fine dust. In indoor environments, H. capsulatum is only likely to be found in old dried bird droppings, chicken manure or bat guano. When the droppings have dried, massive amounts of spores can be released, especially if the droppings are disturbed. H. capsulatum may cause chronic infection resulting in pneumonia, blindness, and even death. Although it is not always present, it is good to assume it is there and take the necessary precautions if handling bird droppings or guano.

Sporothrix spp

Species of Sporothrix are also thermally dimorphic. At 25°C they grow as mold and at 37°C as yeast. They also grow as yeast in host tissue. Sporothrix has a worldwide distribution and is commonly isolated from soil, living and decomposing plants, woods, and peat moss. One species of Sporothrix – Sporothrix schenckii – is a primary pathogenic fungus and will occasionally cause human infections, especially in individuals with weak immune systems. Factors such as malnutrition and alcoholism also predispose individuals to infection by this fungus. It is a well known cause of "Rose handler's disease", an occupational disease of farmers, gardeners and horticulturists. Route of infection is mainly through wounds or scratches.

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References

1. Flannigan B, Samson RA, Miller JD (2001). Microorganisms in home and indoor work environments: Diversity, Health Impacts, Investigation and Control. Taylor & Francis, London. 490 pp. Reprinted with revisions 2005.

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About the Author

Dr. Jackson Kung’u is a microbiologist who has specialised in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analysed thousands of mould samples from across Canada. Jackson provides how-to advice on mould and bacteria issues.

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