Volume 2 | Issue 6
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Insight into Mold Growth and Sporulation in Buildings
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
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.
Fungus of the
month III: Stachybotrys chartarum, with additional note regarding speciation of the fungus in
By Subu Thiagarajan,
and Payam Fallah, and Dave Gallup
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
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
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
Rationale for not
speciating Stachybotrys on spore trap and direct microscopic examination laboratory
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.
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
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
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