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Water and Fungi | Paecilomyces species

Water and Fungi

By Chris Karch, M.S., EMLab P&K Analyst

Fungi, like all living organisms, require three essential factors for life: air, food and water. Fungal growth in the indoor environment is dictated by the availability of these three factors. Just like humans, what most fungi "breathe" is oxygen, which is generally abundant in indoor environments. Fungi can digest human food, but also many materials that are commonly used in buildings. Water availability, however, is a factor that can be controlled to prevent or reduce fungal growth.

Fungi do not ingest their food like animals. Instead, they release enzymes into their surrounding environment to break down complex materials into simpler ones that can be absorbed by the fungus in a process known as absorptive nutrition. For these enzymes to leave the fungus, remain functional and to break down complex substances, water is necessary. Once the complex substances have been broken down and dissolved in water they are absorbed by the fungus.

For fungal growth to occur, a certain level of free water (the liquid water found inside plant cells, not in their walls, which is not bound to other molecules) needs to be present. The water that is available to support microbial life in building materials is commonly expressed as water activity (aw). Water activity compares the physical properties of water in the material in question with that of pure water to generate a scale from 0 - 1.0. Pure distilled water has a value of 1.0 and as the value moves closer to 0, more and more solutes are present in the water and less and less water is available for an organism to use, thereby preventing the growth of many fungi.

There is no sampling device that directly tests for water activity; instead the equilibrium relative humidity (ERH) is reported. For this measurement, the material is enclosed in a small container, allowed to equilibrate, and the relative humidity in the air above the material is measured. The ERH assumes that the air and materials in an enclosed space have the same concentration of water. The ERH is reported as a percentage, which is the aw decimal multiplied by 100.

A definitive aw threshold does not exist for fungal growth. Studies have indicated that fungal growth can occur at and above aw=0.65. The most basic factor to consider when contemplating the potential of growth is the specific type of fungus being examined. Different fungi have different water requirements. These varying requirements allow them to be grouped into three broad categories. Fungi that require aw higher then 0.9 are known as hydrophilic fungi. Some common examples are many yeasts, Stachybotrys chartarum, Fusarium species, and Chaetomium globosum. Fungi that can grow between 0.8 and 0.9 are known as mesophilic fungi, which include species of Alternaria, Cladosporium, some species of Penicillium and Aspergillus, and many other fungi. Finally, fungi that can grow at aw below 0.8 are known as xerophilic fungi. Examples include Wallemia species, Aspergillus restrictus, A. repens and many others, as well as some species of Penicillium.

Many physical factors in the environment can affect the water requirements of fungi. Two of the most important factors in the indoor environment are temperature and the type of food that is present. For each species of fungi there is an optimal temperature range for growth to occur. Outside of that optimal temperature range more water is necessary for growth. The same is true if the food source is not something the fungi normally would grow on in nature. Many fungi have evolved to break down plant compounds, thus fungi tend to grow more frequently and quickly on plant-derived materials such as wood and paper. When building materials do not come from a plant origin, or have been altered in some way, fungi require more water in order to grow.

Studies have indicated that some fungi can survive periodic drying and rewetting, continuing their growth once free water is available again. Fungi are also susceptible to the rate of drying for a given material or substrate that they are growing on. This process is more detrimental to the fungus when the drying occurs rapidly. The slower the drying process occurs, the more likely the fungus will be able to continue its growth once water is reintroduced to the environment.

Besides allergic reactions to the presence of fungi and their spores, another area of concern is the production of mycotoxins. Mycotoxins are secondary metabolites only produced and released under certain conditions that can cause various health effects in humans and animals. Many studies have indicated that Stachybotrys does not start to produce mycotoxins until the aw reaches 0.90, with the level of production increasing once the aw reaches 0.95. Other toxigenic fungi have different water activity requirements for the production of toxins.

As with all living organisms on earth, water plays an important role in the growth and life cycle of a fungus. Without water, fungi are not able to grow or reproduce. The key to preventing and controlling fungal growth in buildings is to reduce the availability of free water by removing the water source such as repairing leaks, running a dehumidifier in damp areas and removing wet building materials.

1. Alexopoulos, C.J., Mims, C.W., Blackwell, M. 1996. Introductory Mycology. John Wiley & Sons Inc.

2. Deacon, Jim. 2006. Fungal Biology. Malden, MA: Blackwell Publishing.

3. Neilsen, K.F., Holm, G., Uttrup, L.P., Nielsen, P.A. Mould growth on building materials under low water activities influence of humidity and temperature on fungal growth and secondary metabolism. International Biodeterioration & Biodegradation. 54(2006): 325-336.

4. Nielson, Kristian Fog. Mycotoxin production by indoor molds. Fungal Genetics and Biology. 39(2003): 103-117.

5. Park, Ju-Hyeong, Cox-Ganser, Jean M., Kreiss, Kathleen, White, Sandra, K., Rao, Carol Y. Hydrophilic Fungi and Ergosterol Associated with Respiratory Illness in a Water Damaged Building. Environmental Health Prospectives 116 (2008) 45-50.

6. Pasanen, Anna-Lissa, Kasanes, Jukka-Pekka, Rautiala, Sirpa, Ikaheimo, Marko, Rantamaki, Jouko, Karriainen, Kalliokoski, Pentti. Fungal growth and survival in building materials under fluctuating moisture and temperature conditions. International Biodeterioration & Biodegredation 46(2000) 117-127.


Microorganism of the Month: Paecilomyces species

By Karen A. Santo-Pietro, M.S., EMLab P&K Analyst

Paecilomyces species are fungi common throughout the world. They are found in substrates such as soil, food, optical lenses, paper and tobacco. They are important industrially as producers of citric and gluconic acids, and antifungal antibiotics (e.g. variotin). Some species are also important agriculturally as they can stimulate the growth of important crop seedlings like barley and corn. They are significant in the indoor environment as possible causative agents of Type I (hay fever, asthma) and Type III hypersensitivity pneumonitis.

In indoor environmental testing, microscopic differentiation of this group from other small chain-forming spores in air samples such as Penicillium and Aspergillus is not possible. (However, DNA based techniques such as PCR can be used for the detection of viable and non-viable Paecilomyces fungi.) They are also readily identifiable with culture-based methods if the spore-bearing structures (conidiophores) are present. The conidiophore may be branched in a simple arrangement with one phialide (the cell that produces the spores) or into whorls (multiple phialides that arise from the same base). The phialide is swollen at the base, gradually narrows into a long beak, and gives rise to chains of colorless or slightly pigmented conidia (spores). Depending on the species, the spores appear ellipsoidal to spindle-shaped, are smooth-walled to slightly roughened, and are about 2.5 to 8 µm in length. Thick-walled chlamydospores (spores that develop from the hyphae) may be present as a single spore or in short chains. They may also appear smooth-walled or ornamented. Paecilomyces species are the asexual forms of some ascomycetes, such as Thermoascus sp. and Talaromyces sp.

Microphoto of Paecilomyces

Microphoto of Paecilomyces.
Copyright © 2008 EMLab P&K

In fungal cultures, Paecilomyces colonies grow rapidly on malt extract agar (MEA) at 25°C, although they can grow at various temperatures. They grow in a variety of colors and appear white, brownish or in bright shades of pink, violet, or lilac. P.variotii, one of the more important species to humans, is a thermophilic (heat-loving) species and can grow between 50 to 60°C. It is initially buff, to tan and then develops into a powdery yellowish brown colony. P. variotii has been documented to cause infections of the eyes, skin, bones, and pulmonary system in immunocompromised patients. Ingestion of contaminated feed has also been reported to be toxic in ducks, rabbits, swine and chicken.

1. De hoog, GS, J. Guarro, J. Gene, and M.J. Figueras. 2000. Atlas of Clinical Fungi, 2nd ed. Pp. 794-809.

2. Domsch, K.H., W. Gams, and T.-H. Anderson. 1993. Compendium of Soil Fungi. Volume I. P. 525-536.

3. Samson, R.A., E.S. Hoekstra, J.C. Frisvad (eds). 2004. Introduction to Food- and Airborne Fungi. Pp. 170-171.

4. Sutton, D.A., A.W. Fothergill, and M.G. Rinaldi. 1998. Guide to Clinically Significant Fungi. Pp. 290-293.


This article was originally published on February 2008.