There is no accurate figure for the total number of fungal species that exist. However, there have been estimates suggesting well over a million different species. Currently, there are only about 100,000 described fungal species. Some selected fungal groupings (i.e., genera) are summarized below. For each fungal genus summarized below, information pertaining to their common habitats, species information, known health effects, and characteristics (such as appearance and water activity) is described. However, most fungal genera have several species. Therefore, the species diversity is far more extensive than the genus diversity listed below.
Alternaria
Common Habitats: Alternaria is a very common fungus found on decaying wood, compost, plants, food, and in different types of soil. Indoors, it can be found in house dust, carpets, on damp spots around showers, around window frames, and around any area where condensation occurs.
Species Information and Health Effects: Due to its abundance and ubiquity, Alternaria is a common fungal allergen. Alternaria currently contains about 50 species. A. alternata is the most common species isolated from human infections. Other Alternaria spp. isolated from infections includes A. chartarum, A. dianthicola, A. geophilia, A. infectoria, A. stemphyloides, and A. teunissima. Alternaria spp. is also a common cause of asthma.
Characteristics and Features: Alternaria conidia have a distinctive ovoid to pear-like shape that forms in chains and has a large spore size. Colonies grow fast, are suede-like to floccose (i.e., soft woolly hairs), and black to olive-black or grayish in color. The required water activity (Aw) ranges from 0.85 to 0.89.
Aspergillus
Common Habitats: Species of Aspergillus are found in soil, compost piles, plant debris, stored grain, and water-damaged building materials. Spores can also be found in moist ventilation systems and house dust.
Species Information and Health Effects: There are over 160 different species of Aspergillus. Approximately 20 species have been reported to cause opportunistic infections, but rarely occur in individuals with normally functioning immune systems. A. fumigatus is the most commonly species isolated as a causative agent for opportunistic infections, followed by A. flavus and A. niger. Other less common isolated species include A. clavatus, A. glaucus, A. nidulans, A. oryzae, A. terreus, A. ustus, and A. versicolor. Many Aspergillus species produce mycotoxins. Toxin production is dependent on the species. Aspergillus is also a common cause for extrinsic asthma.
Characteristics and Features: Aspergillus colonies are soft to powdery in texture. The surface color varies depending on species. The required Aw ranges from 0.71 to 0.94.
Aureobasidium
Common Habitats: Aureobasidium is commonly found in plant debris, soil, wood, textiles, and indoor air environment. Common indoor sources of this fungus are kitchens, bathrooms, and poorly maintained ventilation systems. Aureobasidium facilitates the decomposing activity of other fungi, such as Cladosporium. Aureobasidium may also colonize in hair, skin, and nails.
Species Information and Health Effects: There are 14 species of Aureobasidium. A. pullulans is a well-known species. A. pullulans is often found as a black stain on damp materials such as painted wood. This species has been reported to cause chronic cutaneous infections of the skin in immunocompromised individuals.
Characteristics and Features: Aureobasidium grows moderately fast. This fungus produces abundant spores that are 5 – 7 microns in size. The colonies are flat, smooth, moist, yeast-like, pasty, shiny and leathery in appearance. The surface is white, pale pink or yellow at the beginning and becomes brown to black and velvety with a grayish fringe by aging. The reverse color is pale or black.
Bipolaris
Common Habitats: Bipolaris is found in plant debris, soil, and grasses.
Species Information and Health Effects: There are about 45 species of Bipolaris. B. australiensis, B. hawaiiensis, and B. specifera are well-documented human pathogens. Bipolaris spp. causes allergic sinusitis and may cause infections in the eyes and skin.
Characteristics and Features: Bipolaris grows rapidly. The spores are large and are typically deposited in the upper respiratory tract. The texture is velvety to woolly. The surface of the colony is initially white to grayish brown and becomes olive green to black with a raised grayish periphery as it matures. The reverse is also darkly pigmented and olive to black in color. This fungus can grow in semi-dry environments.
Chaetomium
Common Habitats: Chaetomium is found in soil, air, and plant debris. It readily digests cellulose and is frequently found in decomposing plant material, especially wood or straw-like materials. In water-damaged buildings, it is frequently found on gypsum board and paper products. These fungi are able to digest and dissolve the cellulose fibers in cotton and paper, and cause such materials to deteriorate or disintegrate. Chaetomium is often found indoors with Stachybotrys.
Species Information and Health Effects: There are about 80 species of Chaetomium. The most common ones are C. atrobrunneum, C. funicola, C. globosum, and C. strumarium. Chaetomium spp. may cause skin and nail infections and is considered to be an allergen. Chaetomium spp. is reported to produce mycotoxins but is not a common human pathogen.
Characteristics and Features: Chaetomium is easily identified by its large lemon-shaped spores with straight and/or wavy filamentous appendages.
Chrysosporium
Common Habitats: Chrysosporium is commonly found in soil, plant material, dung, and birds. Certain species are occasionally found in skin and nail scrapings.
Species Information and Health Effects: There are approximately 22 species of Chrysosporium. The most common ones are C. keratinophilum, C. tropicum, C. merdarium, C. inops, C. pannicola, C. queenslandicum, C. zonatum, and C. parvum. The species are differentiated from each other by the texture of the colony, morphology, location, and size of the conidia. Aside from being an allergen, this fungus may cause superficial infections. This fungus has also been isolated from systemic infections in immunocompromised individuals.
Characteristics and Features: Chrysosporium colonies grow moderately fast. They may be granular, woolly, or cottony and flat or raised and have a folded appearance. From the front, the color is white cream, yello,w or tan to pale brown. The reverse is white to brown. The required Aw ranges from 0.61 to 0.71.
Cladosporium
Common Habitats: Cladosporium is frequently found in outdoor air in temperate climates. This fungus may also be encountered in dirty refrigerators, on moist window frames, and on rotten organic material. Cladosporium often discolors interior paint, paper, and textiles stored under humid conditions.
Species Information and Health Effects: The genus Cladosporium includes over 30 species. The most common ones include C. elatum, C. herbarum, C. sphaerospermum, and C. cladosporioides. Cladosporium commonly causes asthma and hay fever. Some species are causative agents for skin lesions, keratitis (i.e., inflammation of the cornea), and pulmonary infections.
Characteristics and Features: The conidia are comprised of very fragile chains, easily become airborne, and are transported over long distances. Cladosporium may appear green to black from the front and black from the reverse. The required Aw ranges from 0.61 to 0.71.
Curvularia
Common Habitats: Curvularia is found in soil, plants, and cereals in tropical or subtropical areas.
Species Information and Health Effects: Curvularia includes about 35 species. C. lunata, C. pallescens, and C. geniculata have been isolated from human infections in immunocompromised individuals. However, Curvularia may also cause infections in individuals with intact immune systems. This fungus is also an allergen.
Characteristics and Features: Curvularia produces rapidly growing woolly colonies. From the front, the color of the colony is white to pinkish gray. It turns to olive brown or black as the colony matures.
Epicoccum
Common Habitats: Epicoccum is found in plants, soils, house dust, grains, textiles, and paper products where Cladosporium and Aureobasidium are present.
Species Information and Health Effects: The genus Epicoccum contains a single species, E. purpurascens. Epicoccum is a known allergen and can cause skin infections in humans.
Characteristics and Features: Epicoccum grows rapidly and produces woolly to cottony or felty colonies. From the front, the colonies are yellow to orange or orange to red or pink. It becomes greenish brown to black as it matures. From the reverse, the same color is observed but is usually more intense than the front view. Epicoccum is considered a dry weather spore and is very resistant to changes in water activity. The required Aw ranges from 0.86 to 0.90.
Fusarium
Common Habitats: Fusarium is widely distributed on plants and in the soil. This fungus requires extreme wet conditions and is often found in humidifiers.
Species Information and Health Effects: The genus Fusarium contains over 20 species. The most common of these are F. solani, F. oxysporum, and F. chloamydosporum. Fusarium spp. may cause opportunistic infections. Some species produce very harmful mycotoxins, especially if ingested.
Characteristics and Features: Colonies of Fusarium generally grow rapidly and produce woolly to cottony and flat-spreading colonies. From the front, the color of the colony may be white, cream, tan, salmon, cinnamon, yellow, red, violet, pink, or purple. From the reverse, it may be colorless, tan, red, dark purple, or brown. The required Aw ranges from 0.86 to 0.90.
Mucor
Common Habitats: Mucor is found in soil, plants, decaying fruits, vegetables, horse dung, animal hair, and stored grain. It is also frequently found in air samples from indoor environments. Accumulated dust and poorly maintained carpeting may contain high concentrations of Mucor.
Species Information and Health Effects: There are about 50 species of Mucor. The most common species include M. amphibiorum, M. circinelloides, M. hiemalis, M. indicus, M. racemosus, and M. ramosissimus. Asthmatic reactions have been associated with Mucor spp. Also, this fungus may cause opportunistic infections in immunocompromised individuals. The sites of infection are the lung, nasal sinus, brain, eye, and skin.
Characteristics and Features: Mucor is a rapidly growing fungus. Colonies are cottony to fluffy, white to yellow, and become grayish-brown as they mature. The reverse color is white. The required Aw ranges from 0.90 to 0.94.
Paecilomyces
Common Habitats: Paecilomyces is commonly found in warm, arid regions in dead plants and compost. It is also known to cause the decomposition of food items. Some species are parasitic on insects.
Species Information and Health Effects: Paecilomyces contains several species. The most common are P. lilacinus and P. variotii. Paecilomyces spp. is an opportunistic pathogen for immunocompromised individuals. Some species of this genus are reported to cause pneumonia and pulmonary infections, skin infections, and eye infections. Some species produce mycotoxins. This fungus may also produce arsine gas if growing on an arsenic substrate.
Characteristics and Features: Colonies of Paecilomyces grow rapidly. The colonies are flat, powdery or velvety in texture. The color is initially white and becomes yellow, yellow-green, yellow-brown, olive-brown, pink, or violet, depending on the species. The reverse color is dirty white, buff, or brown. The required Aw ranges from 0.74 to 0.84.
Penicillium
Common Habitats: Penicillium is commonly found in air samples, carpet, wallpaper, and interior fiberglass duct insulation. It is also common in soil, food, grains, and compost piles.
Species Information and Health Effects: There are approximately 200 species. The most common ones include P. chrysogenum, P. citrinum, P. janthinellum, P. marneffei, and P. purpurogenum. Species of Penicillium are occasional causes of infections in humans. Many species of Penicillium are also known to produce mycotoxins. This fungus can also cause extrinsic asthma.
Characteristics and Features: The colonies of Penicillium generally grow rapidly and are flat and filamentous. The texture consists of velvety, woolly, or cottony. The colonies are initially white and become blue green, gray green, olive-gray, yellow or pinkish as it matures. The reverse color is usually pale to yellow. The required Aw ranges from 0.78 to 0.86.
Stachybotrys
Common Habitats: Stachybotrys thrives on water-damaged, cellulose-rich materials such as gypsum board, paper, ceiling tiles, cellulose-containing insulation backing, and wallpaper.
Species Information and Health Effects: There are about 15 species of Stachybotrys, but one well-known species is S. chartarum. Stachybotrys produces potent mycotoxins. Symptoms include dermatitis, cough, rhinitis, nose bleeds, a burning sensation in the mouth and nasal passage, cold- and flu-like symptoms, headache, general malaise, and fever.
Characteristics and Features: Stachybotrys produces cottony, rapidly growing colonies that appear as a sooty black fungus occasionally accompanied by a thick mass of white mycelia. From both front and rear, the color of the colony is initially white and turns black as it matures. The required Aw ranges from 0.91 to more than 0.98.
Trichoderma
Common Habitats: Trichoderma colonizes on dead leaves, fallen timber, compost heaps, activated sludge, polluted waters, and dung. Trichoderma readily degrades cellulose, so it is commonly found in water or moisture-damaged buildings. Some species are considered to be parasitic on other fungi.
Species Information and Health Effects: There are about 20 species, which include T. harzianum, T. koningii, T. longibrachiatum, T. pseudokoningii, and T. viridae. This fungus is considered an allergen. Opportunistic infections may occur in immunocompromised individuals; however, very few human cases due to Trichoderma have been identified.
Characteristics and Features: Thrichoderma is fast-growing and quickly spreads white, green, or yellow colonies. As the conidia are formed, scattered blue-green or yellow-green patches become visible.
Ulocladium
Common Habitats: Ulocladium is commonly found as a saprophyte on plant materials, soil, dung, grass, compost, paper, textiles, and wood. Ulocladium frequently occurs in air and dust samples and is also found on water-damaged building materials.
Species Information and Health Effects: Ulocladium has two active species – U. chartarum, and U. botrytis. Ulocladium can be very allergenic but it may rarely cause human disease. This fungus has been reported to cause cutaneous and subcutaneous infections; however, the infection sites vary widely.
Characteristics and Features: Colonies of Ulocladium grow moderately fast. The colonies are wooly to cottony or velvety and are olive-brown to black or grayish in color. The required Aw is about 0.89.
Mold, Mildew, and Yeast
The terms – mold, mildew, and yeasts – are commonly used when describing fungi. “Mold” is a general term used to describe fungi and is considered a class of fungi; although, the term, “mold,” has no technical significance. “Mildew” is often used to refer to fungi growing on fabrics, windowsills, or bathroom tile. “Yeasts” refers to fungi that do not contain mycelia. In other words, yeasts are solitary, rounded forms that reproduce by making more rounded forms through budding or fission and form pasty colonies similar to those of bacteria.
Fungal Reproduction
Fungi generally reproduce sexually by means of tiny (generally 2 to 10 μm), single-celled bodies called spores. However, many fungi can also produce asexually by fragmentation of the mycelia. Asexual spores are called conidia if they are formed externally to the spore-producing cell. The number of spores produced by a single colony is quite high. Many of them can freely move around in the atmosphere and travel a long distance. However, the fungal types vary substantially in their capacity to produce and release spores. For example, Penicillium and Aspergillus typically produce large numbers of spores and are easily released into the air. Whereas, Stachybotrys and Chaetomium produce fewer spores and are not readily released into the air.
Fungi are generally composed of long filaments known as hyphae, which are long tubes divided by cross-walls. When hyphae are grouped together, they form what is called the mycelium. This makes up the vegetative (or asexual) state of the fungus. Unlike plants, fungi do not have chlorophyll, so they must absorb food from others. In doing so, the fungus forms a colony. A fungal colony is a visible mass of interwoven hyphae that form a mycelium. Colonies may appear cottony, velvety, granular, or leathery and may be white, gray, brown, black, yellow, greenish, or other colors.
Fungi release enzymes in order to digest complex organic compounds into glucose, which they then absorb. Fungi may be saprobic, parasitic, or both. The term, “saprophytic,” describes fungus that lives on dead organic material. Almost all fungi can grow as saprobes and can be cultured on non-living growth media. Most of the parasitic fungi can survive primarily as saprobes and then invade living tissues when suitable hosts are available. A few fungi are symbiotic, which means that they grow in close association with another living organism, often to the benefit of both.
Water Activity
Moisture is one of the most critical factors when controlling indoor mold growth. The term, “water activity” (Aw), is a measure of water within a substrate that an organism can use to support growth. For most fungi, the optimum Aw will be above 0.90. The lowest Aw at which the most tolerant (i.e., xerophilic) fungi may grow is 0.7, which corresponds to an equilibrium relative humidity of 70%. Some species such as Aspergillus versicolor may start growing at these conditions. Most fungi require conditions of at least an Aw of 0.85 to 0.90. The Aw range depends on temperature and the amounts and types of nutrients available. Classifications of fungi by water activity include the following:
Hydrophilic fungi (a minimum Aw that is greater than 0.90): Colonizes on continuously wet materials (e.g., soaked wallboard, water reservoirs for humidifiers, drip pans, etc.)
Mesophilic fungi (a minimum Aw between 0.80 and 0.9 with an optimum Aw that is greater than 0.9): Colonizes on continuously damp materials (e.g., damp wallboard, damp fabrics, etc.).
Xerotolerant fungi (a minimum Aw 0.80 with an optimum Aw that is greater than 0.8): Colonizes on relatively dry materials (e.g., house dust at high relative humidity)
Xerophilic fungi (a minimum Aw of 0.80): Colonizes on very dry materials (e.g., high-sugar foods, some building materials, etc.)
References
Carlile, M., Watkinson, S., and Gooday, G. (2001). The Fungi (2nd ed.). London: Academic Press.
Damp Indoor Spaces and Health. (2004). Washington, DC: The National Academies Press.
Doctor Fungus Corporation. (2005). <www.doctorfungus.org>.
Macher, J. (Ed.). (1999). Bioaerosols Assessment and Control. Cincinnati, OH: American Conference for Governmental Industrial Hygienists (ACGIH).
Watling, R. (2003). Fungi. London: The Natural History Museum.