Fungi, Osmosis, and Water Activity | Fungi of the Month: Torula
By Dr. Harriet Burge
Fungi are uniquely adapted to succeed in the earth’s natural ecosystem, and the characteristics that make them so successful also allow them to colonize man-made environments. Since the fungi are ubiquitous, it takes constant vigilance to keep them out of our buildings, off of our agricultural products, and out of our refrigerators. Approaches that are taken to keep fungi out of indoor environments include filtration of outdoor air, controlling food sources, use of biocides, and water control. Of these, only water control is practical for most environments. This article addresses how the fungi react to water in combination with other environmental conditions.
The semi-permeable membrane and osmosis
The basic unit of all fungi is the cell, a structure that, for the most part, is similar to all other animal and plant cells. The membrane that surrounds cells is complex. Of importance here is the fact that it allows passive transfer of water but not dissolved substances (for example salt molecules) to pass. This process of selective diffusion is called osmosis. Cells need to be wet to survive. When cells are placed in an environment where there are more dissolved molecules in the cell than in the environment, water diffuses across the semi-permeable membrane into the cell until a balance is achieved. On the other hand, when the environment has more dissolved molecules than the cell, water diffuses out of the cell. Figure 1 shoes what happens under different water/dissolved material conditions. The small circles are water molecules, and the spiky particles are molecules of dissolved solids.
Water activity and osmosis
Water activity is, essentially, the equilibrium relative humidity of a solid material. Virtually all materials on earth have at least one layer of water molecules attached to the surface. These water molecules are very tightly bound to the material, and do not contribute to water activity. As the number of layers of water molecules increases, the molecules become less tightly bound and eventually the water becomes sufficiently mobile for use by microorganisms. Water on surfaces also always contains some dissolved solids. These dissolved solids are equivalent to the spiky molecules in Figure 1.
The fungi and water activity
Fungi can be generally grouped into three categories with respect to water requirements:
1.Hydrophiles are fungi that can only grow in water with a low concentration of dissolved solids. These are the aquatic fungi and a few water tolerant mesophiles.
2. Mesophiles are fungi that can grow in water with moderate concentrations of dissolved solids plus a few tolerant hydrophiles and mesophiles.
3. Xerophiles are fungi that grow in water with high concentrations of dissolved solids plus a few dry-tolerant mesophiles that are tolerant of relatively high dissolved solids.
This all sounds complex, but in fact is a simplification of the actual fact that all of these conditions are not discrete but occur along a continuum.
The practical situation
Practically speaking, what does all this mean with respect to whether or not fungi will grow in a particular environment? In the first place, some water is always necessary. Secondly, the amount of water alone is not the only factor that determines the suitability of an environment for fungal growth. Of equal importance are the concentration of dissolved solids in whatever water is present, the type of nutrient available to the fungus, temperature and lighting conditions, inhibitors that might be present, and many other factors.
For example, water can sit on my ceramic tile basement floors for days with no apparent fungal growth. While water activity is essentially 100% in this location, the dissolved solids condition is suitable only for the water fungi (hydrophiles) and the nutrient conditions are not suitable for these fungi. The water that is sitting on my basement floor has diffused through a plaster wall. The wall is not moldy either, although it has a water activity in the 90-100% range. There are plenty of dissolved solids in the wall water, but there are also substances that inhibit fungal growth, and no good nutrient source for the fungi. Finally, there is a cardboard box sitting on the wet tile floor. The bottom of the box is soaked and is supporting the growth of Stachybotrys, a fungus that requires moderate amounts of water (it doesn’t grow under water), relatively low solute concentrations, and can use the cellulose that is abundant in the box for food. The water from the floor has climbed up the sides of the box through capillary action. There is an interface between the very wet cardboard that is supporting Stachybotrys growth, and the relatively dry cardboard on which some tolerant Penicillium species are growing. The rest of the box, which is surrounded by air at a relative humidity near 100%, has yet to support the growth of any mold. I would expect the xerophilic Aspergillus species to eventually colonize these surfaces. Even though the water activity level is probably suitable, the nutrient types available and the very cool conditions in the basement restrict the rate at which these fungi grow.
By Dr. Srivandana Kilambi
In the literary sense, Torula means a chain of special bacteria or a genus of budding fungi. True to its meaning, the spores (also called conidia) of Torula appear in simple or branched chains. The spores are round, ellipsoidal or sub-spherical, brown or olivaceous in color and the conidial walls range from smooth or minutely roughened to spiny and exhibit one to several strongly constricted transverse septa. The spores are dry and generally disseminated by wind.
Torula is an anamorphic Ascomycetous genus, and is generally classified as a dematiaceous (dark-walled) fungus. Hormiscium is an obsolete synonym of the genus. Torula was first described and established by Persoon in 1794. However, in later years some scientists described a group of beer yeasts and asporogenous yeasts under this genus name. As a result, many unrelated fungi have synonyms in this genus. Although the genus consists of only 12 species, over 400 names are found in the literature. Torula is sometimes confused with the yeast Torulopis. Torulosis is an old name for cryptococcosis, a disease in humans caused by the yeast Cryptococcus neoformans.
Torula is cosmopolitan in nature and is widely distributed from tropical to temperate regions. It is a saprophyte (lives on dead organic matter) commonly found on dead herbaceous stems, grasses, leaves, sugar beet roots, groundnuts and oats and is also isolated from air, soil and dung. In humid conditions Torula is often found indoors on cellulose containing materials such as jute, old sacking, wicker, wood and paper. The colonies appear as a velvety olive green to black on various surfaces. Torula has also been isolated from vegetable patches. Under laboratory conditions, Torula grows vegetatively on general fungal media but usually requires specialized media for sporulation.
Some indoor studies have shown that Torula is prevalent in wet and/or humid conditions. Torula sporulation peaks in the fall, and again during late spring or early summer. The conidia of Torula are readily identifiable on both spore traps and tape lifts.
1. Ellis, M. B., 1971. Dematiaceous Hyphomycetes. Common Wealth Mycological Institute, Kew, Surrey.
2. Kirk, P. M., Cannon, P. F., David J. C. and J. A. Staplers. 2004. Dictionary of Fungi. CABI Publishing, UK.
This article was originally published on October 2004.