An Insight into Mold Growth and Sporulation in Buildings | Stachybotrys chartarum
A driving factor in building construction is the continuous pressure to save time and money. These pressures usually result in gradual shifts in how buildings are made. Beginning in the late 1940’s, these gradual shifts have resulted in better and better conditions for fungal growth. This is perhaps nowhere more apparent than in schools. In the 1940’s and before, schools were built of stone, brick and marble. Floors (at least in urban schools) were either hardwood (nutrient poor for most fungi) or marble. Walls were tile or plaster. Windows were operable, and the buildings were not tight, resulting in plenty of ventilation. Now, we have floors covered with carpeting, which holds water and nutrients. Walls are built of gypsum board that is filled with nutrients. Windows are sealed, which requires costly energy to ventilate, leading to low ventilation rates and accumulation of water. We can live with these changes if we understand how fungi grow so that we can efficiently limit their growth.
Molds are spread by spores, each of which contains all the genetic material to make a new colony. While traveling through the air or on your clothes or other carriers, they are dormant, and chemical reactions in the spore are going on very slowly. When a spore lands on a dry surface, it remains dormant and will eventually die. If water is present, it is drawn into the spore by a process known as osmosis. With the water are dissolved nutrients. The concentration and type of nutrients depends on the material on which the spore lands. If there is enough water and if the nutrient content is appropriate, then the spore will swell, and a germ tube will appear. The germ tube releases enzymes (catalysts) that help in the digestion of insoluble nutrients (e.g., starch, cellulose, etc). If the appropriate nutrient is available, the enzymes will break it down into soluble fragments, which will be absorbed into the germ tube, stimulating continued growth. The size the colony reaches depends on both environmental conditions and the genetics of the fungus. Given ideal conditions, it will grow to its genetically pre-determined size as long as sufficient water and nutrients are available and the temperature is appropriate or until the colony encounters competition from other fungi. If the proper nutrients are not available, or if the water supply disappears, then the germ tube and the spore die. Given proper conditions, fungi will generally grow vegetatively until some environmental variable becomes limiting. Water or nutrients may be depleted, or temperature or lighting conditions could change. When these changes occur, fungi stop vegetative growth and may begin to produce new spores.
Each stage of fungal growth (spore germination, vegetative growth, sporulation) has a specific set of conditions that is optimal. Important conditions in this set are nutrient types and concentrations, light, temperature, oxygen and water availability. Water availability (i.e., water activity) is one of the most important of these. Each fungus has optimal water activities for spore germination, radial growth and sporulation. At optimal water activity, temperature changes will affect each of these processes. Optimum germination, growth and sporulation occur only when all environmental conditions are ideal. For example, Stachybotrys chartarum spores will swell and begin to germinate in distilled water, while Penicillium chrysogenum spores will germinate only if some soluble nutrients are present in the water. These properties have evolved as part of the fungal defense mechanism. The ability of Stachybotrys chartarum spores to germinate in pure water enables this fungus to grow under conditions that are not suitable for many other fungi. The ability of Aspergillus restrictus to grow at a very low water activity (0.69) was measured under optimal conditions for all of the other variables. As these other conditions depart from the optimum, more water is needed to allow continued growth. Practically speaking, this means that most xerophilic fungi will not grow at very low water activities under normal building conditions.
There are sensors on the market that are being used to test the potential for fungal growth on building surfaces. They consist of specific spore types sandwiched between water vapor permeable membranes. They are placed on a surface and read after a number of hours. If germination occurs, then growth is considered possible. The sensors are available with different kinds of fungi, recognizing the fact that different fungi have different optima for spore germination. One should remember, however, that optimal germination conditions may not be the same as those for growth and sporulation, and a positive test does not necessarily mean that conditions are suitable for growth. On the other hand, if germination does not occur, it still may be possible for some other spore type to germinate under the given conditions.
Now, how do we use this information on fungal characteristics to prevent fungal growth? We usually talk about controlling water activity in buildings, and this is the most important and best approach for preventing fungal growth. Controlling water activity in buildings is accomplished by controlling relative humidity and keeping temperatures high enough so that condensation does not occur. This does work. Dry buildings do not become moldy even when really delicious fungal food is available. There are wall-surfacing materials available now that will not hold water. Although still relatively expensive, these materials provide time following water incidents before fungal growth occurs. Where water is inevitable, the next step is to control nutrients. Thus, leaks onto plaster walls rarely result in fungal growth, and hard flooring (tile, marble, hardwood) provide little intrinsic nutrient and can be kept free of accumulated dirt and dust.
Biocides that are included in some paints seek to prevent either germination or growth or both. Some do prevent most spores from germinating but have no effect on the growth rate of those spores that are biocide resistant. Others allow germination but slow or prevent subsequent growth. The important point to remember with biocides is that fungi are adaptable. Optimal requirements for germination, growth and sporulation are genetically determined, and they can change with genetic recombination. Resistance to biocides also changes so that, eventually, some spore is likely to be able to resist even the most potent biocide.
By Subu Thiagarajan, and Payam Fallah, and Dave Gallup
Stachybotrys chartarum (atra) is a greenish-black fungus that colonizes particularly well in high-cellulose based materials, such as dry wall paper, straw, hay, and building materials. Stachybotrys chartarum was first described as S. atra, by Corda in 1837, from wallpaper collected in a home in Prague. S. alternans and S. atra are obsolete species names, and hasve been renamed as S. chartarum.
Spores (conidia) are produced in a slime droplet initially, then eventually become dry at which time they might readily disseminate in the air compared toalong with other fungi such as Aspergillus or Penicillium species.
Stachybotrys is a slow-growing fungus on media and on natural substrates. It does not compete well with other rapidly growing fungi. Common areas for growth include dry wall paper, wall board, wood, ceiling tiles, wall paneling, unpainted plasterboard surfaces and building materials. It grows on building material with high cellulose content and low nitrogen content. Areas with relative humidity above 55% and that are subject to temperature fluctuations are ideal for toxin production. Appropriate media for the growth of this organism will have high cellulose content and low nitrogen content.
There have been reports in the popular media stating that Stachybotrys is responsible for severe adverse health effects. It is also true that Stachybotrys is capable of producing toxins. Stachybotryotoxicosis is the disease that results from ingestion of the toxins produced by Stachybotrys and it is most common in horses, where they encounter the fungus and its toxins through feed contaminated with Stachybotrys.
SS.tachybotrys chartarum, is capable of producing several mycotoxins including macrocyclic trichothecenes (satratoxins H, G, F, roridin E, verrucarin J, and Trichoverrols A and B). Trichothecenes are toxins that are produced by at least five genera of fungi, including Fusarium, Stachybotrys, Tricothecium, Myrothecium, and Cephalosporium. Many epidemics have resulted upon crops being infected with trichothecene producing fungi. The mode of action of trichothecenes is by functioning as potent inhibitors of DNA, RNA, and protein synthesis.
Despite the far-reaching public health measures that have emerged as a result of recent publications, the health risks from environmental exposure to Stachybotrys remain poorly defined. Further research to clarify the association between Stachybotrys and disease should begin by focusing on the critical toxicologic distinctions between exposure and dose.
It is possible to find peer-reviewed literature from the scientific community about severe adverse health effects due to ingestion of Stachybotrys, or other toxin producing fungi. It is also possible to find peer reviewed publications of adverse effects due to airborne exposure in agricultural environments. However, it is important to note that current scientific evidence does not support the hypotheses that people have had significant adversetoxic effects due to inhalation of Stachybotrys in home, school, or office environments. Note the important distinction of the route of exposure (and dose) between inhalation of airborne spores and ingestion of contaminated food. Also note the distinction between “normal” indoor environments and agricultural exposure. Kuhn and Ghannoum’s article “Indoor Mold, Toxigenic Fungi, and Stachybotrys chartarum: Infections Disease Perspective” in Clinical Microbiology Reviews, January, 2003; and the ACOEM(American College of Occupational and Environmental Medicine) article “Adverse Human Health Effects Associated with Molds in the Indoor Environment”, October, 2002 provide thorough reviews of scientific literature on this topic and are good sources of additional reading.
Rationale for not speciating Stachybotrys on spore trap and direct microscopic examination laboratory report
When we think of the genus Stachybotrys, the immediate species that comes to mind is chartarum. There are more than 40 species of Stachybotrys described in the literature. It is widely accepted worldwide that S. chartarum is the primary species associated with water-damaged buildings. A recent paper in the journal Mycologia (vol. 95(6), 2003, pp.1227-1238) reports on a new species, S. chlorohalonata. The discovery of this new species was based on morphological, chemical, and molecular analyses, making this fungal species distinct from chartarum species as well as others in that genus.
S. chlorohalonata is reported from wet cellulose-containing material such as fabric, hay, seaweed, grain, paper and soil. Its distribution is world wide including, but not yet limited to, the United States, Denmark, Belgium, Finland, Iraq, New Guinea, and Spain.
Macroscopically, S. chlorohalonata resembles S. chartarum in having black powdery appearance on natural substrata; however, it differs from S. chartarum in having smooth spores (conidia), restricted colony growth, and producing a green extracellular pigment on the CYA medium (Czapek yeast autolysate agar). Spores in S. chartarum have distinct roughening and the fungus does not produce the pigmentation in CYA medium.
Fungal growth on environmental samples is often subjected to a variety of environmental factors or chemicals used as disinfectants (e.g. bleach or certain biocides). The use of such chemicals may distort the surface texture of Stachybotryschartarum spores which is rough to smooth, making the species recognition difficult, and; therefore, be mistaken with S. chlorohalonata species. Identification of Stachybotrys as chartarum on spore trap and as growth on direct microscopic examination can no longer be reliable; therefore, the EMLab reports will list the fungus as Stachybotrys species. For all culturable fungal analyses, we will continue to speciate Stachybotrys.
Adverse human health effects associated with molds in the indoor environment. 2002. Evidence-based Statement. American College of Occupational and Environmental Medicine (ACEOM).
Kuhn, D.M and Ghannoum, M.A. 2003. Indoor mold, toxigenic fungi, and Stachybotrys chartarum: Infectious diseases perspective
This article was originally published on July 2004.