Nutrients. Fungi are achlorophyllous and nonphotosynthetic. Their survival and growth rely heavily on their ability to obtain nutrients, such as sugars, amino acids, vitamins, and macro- and micronutrients, from the substrates. Different species of fungi have different abilities to access and utilize simple or complex forms of carbohydrate, organics, and mineral nutrients.16 Some fungi, such as species of Aspergillus and Penicillium, are called "sugar fungi" because they are fast-growing and prefer simple sugars. On the other hand, some fungi can break down complex carbohydrates (such as cellulose and lignin) or complex organics (such as wood) to obtain nutrients, and are usually late colonizers of the substrates.17 Decomposition and degradation of a substrate is due to enzyme activities. The types of enzymes required depend on the substrates.16
The primary food source for indoor fungi is cellulosic matter. Cellulose is made of glucose units in a linear linkup with b-(1 -4) bonds. Cellulose chains are further crosslinked by hydrogen bonds to form microfibrils and then cellulose fibers.18 Cellulose fibers may come from several different plant sources: wood, herbaceous stem (i.e., flax and jute), leaf (i.e., manila), or seed (i.e., cotton). The majority of cellulose fibers used in building materials are processed, delignified wood fibers, which become much more susceptible to fungal attack, due to chemical or physical means used in the pulping process, than in their native form. The susceptibility of cellulose fibers to fungal attack is associated with their chemicophysical form. Cellulose in its native state consists mostly of crystalline form with a few amorphous sites. The number of amorphous sites increases along native cellulose during the pulping process, which increases the susceptibility to fungal attack.18 This may also help explain the observation made by Chang et al. that used ceiling tiles were more susceptible to fungal growth than new ones in a moist-chamber study.19 Aging of the ceiling tiles may also increase the number of amorphous sites. The widespread use of cellulosic materials in building materials as well as in building construction increases the availability of food sources for fungal growth. Karunasena et al. studied colonization and growth of Cladosporium, Penicillium, and Stachybotrys on ceiling tiles made of inorganics or containing organics (i.e., cellulose).20 The results showed that inorganic ceiling tiles did not support the growth of fungi, while cellulose-containing ceiling tiles did. The authors suggested that inorganic ceiling tiles could serve as a replacement for cellulose-containing ceiling tiles to avoid fungal growth.
In wood, cellulose fibers are chemically bound to lignin. Lignin consists of large molecule units of complex polyphenolic polymers made of carbon, hydrogen, and oxygen elements.18,21 Lignin helps protect cellulose from microbial attacks by reducing the accessibility of susceptible amorphous sites.18 Stachybotrys chartarum, a known cellulolytic fungus22 that grows mostly on paper products rather than wood products, is a good example.
In addition to wood and cellulose products, chemically modified cellulose derivatives are used in building finishing products. Emulsion paints may contain carbox-ymethyl cellulose. These derivatives are as susceptible to fungal attack as cellulose fibers.18
Although cellulose substrates are the primary carbon source for many cellulo-lytic fungi growing indoors, a study by Murtoneimi et al. demonstrated that other chemical components of building materials were found to influence growth and sporulation of Stachybotrys chartarum as well as cytotoxicity of the spores on 13 modified gypsum wallboards.23 The spores produced on the wallboards were also tested for the inflammatory potential. It was reported that, in comparison to the reference board (nonmodified), growth and sporulation of the fungus decreased on wallboards in which (1) the paper liner was treated with a fungicide, (2) starch was removed from the plasterboard, or (3) desulfurization gypsum was used in the core. Spores collected from all wallboards exhibited cytotoxicity to macrophages. Biocide application did not reduce fungal growth; however, spores collected from biocide-treated board exhibited the highest degree of cytotoxicity. The conventional additives (a foam, an accelerator, and a water-reducing agent) used in the core were found to have some inhibitory effects on growth. Recycled wallboard and the board without the starch triggered spore-induced tumor necrosis factor alpha (TNF-a) production in macrophages. The study concluded that growth of a Stachybotrys chartarum strain on wallboard and the subsequent cytotoxicity of spores were affected by minor changes in the composition of the core or paper liners.
Most fungi identified as capable of growing indoors or on building materials7,24-26 are also fungi known to cause food spoilage and biodeterioration.3 With the existence of a wide range of organic matter in an indoor environment, one should not be surprised to find unusual fungi growing indoors.27
Temperature. Fungi, again like most organisms, usually grow in a wide range temperature. In this context, range, including minimum, optimum, and maximum temperatures, can be defined as a temperature profile. Each species has its own profile. Some are narrow and some are wide. Fungi that can grow in a wider temperature range may also have a competitive edge, for example, A. fumigatus. Most fungi, known as mesophiles, grow best at a temperature range near room temperature. Some fungi are classified as psychrophilic, literally cold-loving, or thermophilic, literally heat-loving. Psychrophilic fungi are defined as not growing above 20°C and having a minimum temperature at or below 0°C and an optimum temperature in the range of 0-17°C.28 Thermophilic fungi, such as Aspergillus fumigatus and Absidia corybifera, have minima at or above 20°C, maxima at or above 50°C, and optima somewhere in the higher half of that range. Mesophiles are defined as those having minima above 0°C, maxima below 50°C, and optima between 10°C and 40°C. Unfortunately, fungi seldom follow this rigid artificial classification. Aspergillus fumigatus has a wide temperature range of 12-52°C and does not fit into a classification neatly. However, most fungi, particularly those commonly found growing indoors, are mesophilic.
Moisture. Fungi can utilize organics by secreting enzymes to break down organics into small molecules. Enzymes require water as a solvent for their activities as well as for the endproducts, such as simple sugars and amino acids. The most commonly utilized organics are sugars of various kinds, from simple sugars, such as glucose, to sugar polymers, such as cellulose. Some fungi, such as species of Penicillium and Aspergillus, grow quickly in the presence of simple sugars and are often called "sugar fungi."17 Stachybotrys chartarum, Trichoderma species, Chaetomium globo-sum, and Chrysosporium pannorum (syn. Geomyces pannorum), to name only a few, are cellulolytic and capable of secreting cellulases to break down cellulose into simple sugars. Wood decay fungi are capable of producing cellulases, ligninases, or both to utilize complex organics, namely, wood.21
In the indoor environment food is usually abundant and temperature is usually moderate. Moisture is usually the critical factor limiting the germination of fungal spores and subsequent fungal growth.26,29 It is biologically important for some fungi to regulate their spore release, such as some members of basidiomycetes and ascomycetes. Fungi, like all organisms, require water for various physiological as well as metabolic activities. Water serves as a solvent for carrying solutes in and out of hyphae and for enzymatic and other metabolic reactions.28 It is also used to regulate and maintain the turgor pressure inside the cell so that it does not collapse as a result of high external pressure. Various terms, including osmotic pressure, osmotic potential, water potential, and water activity, have been used to define or describe the role that water plays in a biological system. Water activity (aw), probably the most commonly used term, describes and defines the available free (not chemically or covalently bound) water in a substrate available for biological growth. It is measured when equilibrium is reached between atmospheric relative humidity and the water content in a substrate or a solution, or it expresses the available water in a substrate as a decimal fraction of the amount present when the substrate is in equilibrium with a saturated atmosphere (an equilibrium relative humidity of 70% around the substrate means that the substrate has a water activity of 0.70).30 Water activity can be calculated as aw = p/p0, where p is the partial pressure of water pressure in the substrate and p0 is the saturation vapor pressure of pure water under the same conditions.5 Water activity is numerically equal to equilibrium relative humidity (ERH) expressed as a decimal. If a sample of substrate is held at constant temperature in a sealed enclosure until the water in the sample reaches equilibrium with the water vapor in the enclosure, then aw = ERH/100. Another commonly used measurement of moisture in a substrate is moisture content in percentage of dry weight. It is often used in measuring moisture content of wood or wood products.
The use of ERH in calculation of aw often gives the false impression that relative humidity (RH) is critically important and an overriding factor in indoor fungal growth. Relative humidity is the measurement of water molecules and water droplets in a given airspace. Since fungi do not and cannot grow in air, RH has only secondary effects on fungal growth, condensation, and hygroscopicity of materials. In fact, most indoor fungal growth occurs as a result of incoming water but not just high RH and condensation on indoor surfaces.9'31'32 Scott suggested that Stachybotrys chartarum is a principal colonizer of paper products following water damage, but is rarely involved in materials subjected to condensation.7 He also referenced that Cladosporium cladosporioides, C. spheraospermum, Alternaria, Ulocladium, Aspergillus versicolor, Penicillium chrysogenum, P. griseofulvum, and P. spinulo-sum were reported to cause disfiguration on interior surfaces of buildings due to con-densation.33 However, these observations may not be applied to every situation, since prolonged or large-scale condensation is no different from water damage. It is more likely that the difference in the observations was due to the duration of wetness and progession in fungal succession. Persistently high RH in a poorly ventilated condition does allow hygroscopic materials to increase aw to a level that favors growth of xerophilic fungi.
The measurement of water activity indicates the minimal water requirement for a fungus to grow, which is seldom the optimal condition for fungi to compete and grow in the environment. In practice, it is also important to also look at the optimal aw for each fungus. Clearly, fungi have competitive advantages when they are at their optimal growth conditions. Aspergillus versicolor has a reported aw of 0.79 at 25°C32 but an optimal aw of 0.98 at 27°C.34 It is, in fact, common to observe growth of Aspergillus versicolor on water-damaged materials, presumably at higher aw. However, the range of aw that a fungus can grow can be a competitive edge in finding an ecological niche. Fungi that can germinate and grow at a wider aw range clearly have a competitive advantage. Water activities of specific fungi are available from many reference sources.24'26'31'35'36
Grant et al. found that minimal aw for spore germination and growth of indoor fungi on building materials was much higher than on the fungal growth medium MEA.32 Minimal aw required was different for spore germination and growth of indoor fungi, and aw for fungal growth is usually higher than for spore germination.32 On building materials fungal growth and production of significant quantities of mycotoxins required significantly different water activity, 0.8 versus 0.95.31 Moderately xerophilic fungi, such as Penicillium spp. and Aspergillus spp., will begin to grow at aw between 0.78 and 0.90 depending on the composition of substrates of construction materials.31
Clarke et al., on the basis of an extensive review of mycological literature and laboratory validations with mold samples' defined six groups of fungi ranging from highly hydrophilic to highly xerophilic.37 They used a growth limit curve, which is defined by the minimum combination of local surface temperature and humidity for which growth will occur on building surfaces, to define the six groups. The laboratory validation tests showed the minimum time for growth at the lowest RH to be 75 days. Highly xerophilic fungi have growth limited to >98% RH, while highly xerophilic fungi have growth limited to >75% RH. Aspergillus repens is considered highly xerophilic; A. versicolor, xerophilic; Penicillium chrysogenum, moderately xerophilic; Cladosporium sphaerospermum, moderately hydrophilic; Ulocladium consortiale, hydrophilic; and Stachybotrys chartarum, highly hydrophilic. The study did not determine the optimal RH for their growth. The authors suggested using such information to provide a design tool that can predict the likelihood and extent of mold growth.
In a similar but controlled laboratory study, Chang et al. found that new ceiling tiles supported the growth of Penicillium chrysogenum and P. glabrum at aw 0.85 and a corresponding moisture content >2.2% and of Aspergillus niger at aw 0.94 and a corresponding moisture content >4.3% on used ceiling tiles.19 Penicillium chrysogenum is known to have a germination aw at 0.78-0.85 and the minimum aw for growth at 0.79,26 while Aspergillus niger is reported to have a minimum germination aw at 0.77.5
In a moist chamber study of new gypsum boards just off the production line, Doll and Burge showed that 11 fungal genera were present on new gypsum board without artificial inoculation. Penicillium spp. and Aspergillus spp. were found at 95% RH in moist chambers only on the paper sides, and the number of fungi found on the new gypsum board increased with increasing moisture content.38 On one occasion Stachybotrys sp. was present on the gypsum boards. This study showed that new gypsum boards were not free from naturally occurring fungal spores, which will readily germinate and grow when a suitable moist condition was met.
Another common measurement of moisture content in a substrate or building material is percentage (%) of moisture on an oven-dry-weight basis.21,39 Although moisture level on a percentage basis does not indicate the availability and amount of free water in a substrate and has no direct bearing on mold growth, the expression is still commonly used in the literature of certain industries. In the wood industry, the precent of moisture content is frequently used in practice as well as in the literature. Wood decay seldom occurs when the moisture of the wood products is below fiber saturation point, on average 30%.21'40 However, the golden rule that is still in use today by many wood users to prevent the growth of microbes and the development of wood decay is not to allow moisture levels in wood to exceed 20%.21
To fungi, water is essential for growth whether it is clean, gray, or black.1 Although contaminated water may have a higher content of organics to serve as nutrients, the requirement of water for fungal growth is otherwise the same.
Light. The effects of light on fungal development and ecology may be underestimated. According to Tan,41 many fungi required light for the successful completion of morphogenesis or sporulation. He also opined that light may be one of the most crucial external factors controlling fungal development. Light elicits a number of fungal reactions from light-induced uptake of glucose in Blastocladiella britannica and the light-stimulated synthesis of protein and polychaccharides by B. emerso-nii,42'43 through the influence on orientation and growth rate of hyphae and fruiting bodies, to the morphogenetic initiation of fruiting bodies.44 Trinci and Banbury45 noted that photoinduced changes in pigmentation and conidiophore growth occurred only in the part of a mycelium of Aspergillus giganteus exposed to light directly, but not the adjacent area kept in the dark. Light may affect the release of conidia of Botrytis squamosa4 A number of the perithecial ascomycetes need light to initiate ascospore discharge.47,48 It was observed that light triggers spore release in several fungi.49 A number of fungi have evolved phototropic mechanisms for their spore release so as to increase the efficiencies for spore dispersal.30 In the Ascomycetes class, radiation, minimum humidity, changes in humidity, and minimum wind velocity were all directly correlated with levels of airborne ascospores.48
However, light in indoor environments is different from the light outdoors in spectra and intensity. It should be noted that there is literally no report on the responses of fungi to artificial light (fluorescent and incandescent light) and the light passing through window glass in indoor environments. Light normally is not considered a limiting factor for the development and reproduction of most indoor fungi. Further research on the effect of light on indoor fungi is necessary.
Oxygen and CO2. Most fungi are obligate aerobes. Moore50 noted that development of fungal fruit bodies requires oxidative metabolism [amplified glycolysis and the tricarboxylic acid (TCA) cycle activity] and, subsequently, oxygen supplies. Oxygen is not a limiting factor for indoor fungi in most situations. It was observed that fungi did not develop on the building materials continuously submerged in water. CO2 is also very important to fungal development.50 Buston et al.51 found that elevated CO2 concentration was necessary for perithecium formation of Chae-tomium globosum, and the fungus developed the maximal number of perithecia in 10% of CO2. It was found that high CO2 concentration promoted mushroom stem elongation of agaricus whereas cap expanding was enhanced by lowered CO2 concentration.52'53 Poorly maintained or malfunctioning HVAC systems may lead to elevated CO2 concentration and deteriorating indoor air quality. However, the effects of oxygen and CO2 concentration on indoor fungi are not well understood because of lack of research.
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