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Mycotoxins: Continuing Review of Literature | Basidiospores

Mycotoxins: Continuing Review of the Literature

By Dr. Harriet Burge

Mycotoxins continue to be a concern in association with indoor fungal growth. Because of the high impact of this issue, we continually monitor the literature for any new information that either documents or provides evidence against an association between inhalation mycotoxin exposure and human illness.

The mycotoxin literature can be loosely divided into animal studies, experimental studies in which some human factor is included, human epidemiological studies, and case studies. Animal studies generally rely on relatively high doses of toxin to elicit an immediate effect, and somewhat lower doses to approximate chronic doses. These doses are then extrapolated to humans. Because animals may be more or less sensitive to the toxin than humans, correction factors are used. Usually, these correction factors consider that the animals are less sensitive, and doses are reduced by factors of 100 or 1,000 to approximate an appropriate human dose. Human studies rely on human cell culture, blood, urine, or other parts. Cell cultures may be exposed to mycotoxins directly, or all of these parts can be studied for specific factors after presumed mycotoxin exposure. Epidemiological studies evaluate natural environmental exposure and relate such exposure to specific health outcomes. Specific matched control groups are used, or models are developed documenting increasing risk of a specific health outcome with increasing dose of the toxin.

So far, the animal studies reported in the literature verify that mycotoxins produced by some fungi that grow in indoor environments can produce changes in some physiological parameters in the animals. Thus, very high doses of appropriate strains of Stachybotrys chartarum spores produce indicators of lung damage (Rosenblum et al., 2006) and nasal irritation (Islam et al., 2006). The models that extrapolate these doses to human health effects indicate that the no effect level is much higher than any exposures that have been recorded in indoor environments. (Kelman et al., 2004) This review suggests that levels of Stachybotrys spores, provided that they contain sufficient quantities of Satratoxin G and H could result in irritation if present in concentrations in excess of 2x10-5/m3 (20,000 spores/m3). This concentration has not been reported for undisturbed Stachybotrys spores, but could be experienced by professional remediators.

A very interesting study looking at serum from several people living in homes with mold contamination has been reported (Yike et al., 2006). These investigators are looking for biomarkers of exposure to mycotoxins and examined serum albumin for attached satratoxins. They had developed this procedure in highly dosed mice, but were able to verify these adducts in the human serum they used. The human work was preliminary, with only three "exposed" and one "control." It will be interesting to see follow up work by this group. Note, however, that a biomarker is only an indicator of exposure, not disease. Once good biomarkers have been established, then epidemiological studies are possible that could relate exposure to disease.

Trout et al., 2001 report an interesting and well-conducted case study in which a hotel manager is diagnosed with probably hypersensitivity pneumonitis in connection with exposure to fungi. These investigators tested serum from six "exposed" workers and two "control" workers from the hotel for antibodies to roridin, a mycotoxin produced by Stachybotrys. One exposed and one control worker tested positive to this test. It is not clear why the focus was on mycotoxins for this patient, since hypersensitivity pneumonitis is known to be associated with fungal and actinomycete antigens. In fact, the patient did have elevated IgG antibodies to Thermoactinomyces (actinomycete) antigens, although exposure assessment did not include sampling for this organism. When hypersensitivity pneumonitis is suspected, sampling for thermophilic actinomycetes is important. Cultural sampling is required, using bacterial medium (TSA or nutrient agar) with incubation at 56°C.

1. Trout D, Bernstein J, Martinez K, Biagini R, Wallingford K. 2001. Bioaerosol lung damage in a worker with repeated exposure to fungi in a water-damaged building. Environmental Health Perspectives 109:641-644.

2. Yike I, Distler AM, Ziady AG, Dearborn DG. 2006. Mycotoxin adducts on human serum albumin: biomarkers of exposure to Stachybotrys chartarum. Environmental Health Perspectives Aug 114(8):1221-6.

3. Islam Z, Harkema JR, Pestka JJ. 2006. Satratoxin G from the black mold Stachybotrys chartarum evokes olfactory sensory neuron loss and inflammation in the murine nose and brain. Environmental Health Perspectives 114(7):1099-107.

4. Rosenblum Lichtenstein JH, Molina RM, Donaghey TC, Brain JD. 2006. Strain differences influence murine pulmonary responses to Stachybotrys chartarum. Am J Respir Cell Mol Biol. (Epub ahead of print)

5. Kelman BJ, Robbins CA, Swenson LJ, Hardin BD. 2004 Risk from inhaled mycotoxins in indoor office and residential environments. International Journal of Toxicology 23:3-10.


Fungus of the Month: Basidiospores

By Michelle Seidl

Basidiospores are spores produced by members of the Kingdom Fungi in the division Basidiomycota. Three groups (classes) belong here and are known as Basidiomycetes (i.e. mushrooms), Teliomycetes (i.e. rusts) and Ustomycetes (i.e. smuts). The Basidiomycota contains about 30,000 described species, which is 37% of the described species of fungi (Kirk et al. 2001).

The spores are borne externally on cells called basidia, generally four to a basidium. Illustrated above is a basidium with two spores. Most Basidiomycetes have macroscopic fruiting bodies, which produce the microscopic spores. Examples in this group include: mushrooms, puffballs, polypores, coral fungi, boletes, teeth fungi, jelly fungi, crusts and parchment fungi. These fungi produce large quantities of basidiospores in the spring, late summer, or primarily fall. Some fruiting bodies produce impressive numbers of spores. A giant puffball specimen 30 centimeters in diameter contains approximately 7 trillion spores (Arora 1986). The fungus, which causes the disease known as "corn smut," produces about 25 billion spores per average-sized ear of infected corn. The fungus causing stem rust of wheat generates about 10 billion spores from an acre of moderately diseased plants (Hudler 1998). The wood decay fungus Ganoderma applanatum, has been estimated to produce basidiospores at the rate of 350,000 per second. It does this for up to six months a year (5.4 trillion spores) and may continue for ten years or more (Christensen 1965).

Most basidiospores are dispersed by wind and are released either passively (i.e. puffballs) or forcibly (i.e. mushrooms). With mushrooms, the force that ejects the basidiospores comes about from built up internal pressure in the spore-producing cell (basidium). When the basidiospores are mature, the pressure in the basidium shoots the spores between the gills of the mushroom. Although the actual distance that the basidiospores are ejected is very short, it is enough to allow them to drop between the gills without getting trapped. Once free of the gills, most drop directly beneath the cap of the mushroom, but some manage to stay afloat and be carried away by air currents. Once in the air, spores are able to stay afloat with ease, enabling long distance travel. So what are the distances spores that are wind dispersed can travel? One example comes from an extensive study of the economically important plant pathogen Puccinia graminis (Wheat Rust). This basidiomycete is responsible for billions of dollars in losses annually. The study traced the path of wheat rust epidemics for 30 years. In the spring, spores from infected wheat plants are carried northward from Mexico to Texas, over the Great Plains and on up into Canada. During the fall, spores are carried southward back down into the wheat-growing region where the young winter wheat is beginning to grow (Source: Spore Dispersal in Fungi). Although wind dispersal of basidiospores is the most common method, other means utilized include water, insects and animals.

Microscopic photo of Basidiospore

Copyright © 2006 Environmental Microbiology Laboratory, Inc.

Basidiospores occur in a variety of shapes and sizes. They range from perfectly globose to strongly elongated, round to nodulose, stellate, cross-shaped or angular in circumference. When viewed from one end they are roundish, laterally compressed or angular (Singer 1986). The walls can be thin or thick and smooth or ornamented to varying degrees. Most basidiospores have a distinctive asymmetrical attachment point called the apiculus appearing as a short projection on one end.

Basidiomycota are found in virtually all terrestrial ecosystems, as well as freshwater and marine habitats (Kohlmeyer and Kohlmeyer 1979; Hibbett and Binder 2001). The fungi grow on a variety of substrates and their modes of nutrition are also variable. Nutrients are obtained from living, decaying or non-living substrates. A large number of mushrooms grow symbiotically with plants. Most wood decay or wood rot fungi are also in this group and they break down complex carbohydrates that very few organisms can utilize. These are the basidiomycetes that can cause destructive dry rot and structural damage. Despite the problems these fungi cause, some have shown great potential in solving some environmental problems such as degradation of otherwise persistent hazardous chemicals (Lamar, et al. 1992).

Although basidiospores usually grow outdoors on any organic matter, they are occasionally found indoors and are infrequently detected growing in potted plants, bathrooms, carpeting, textiles, walls and anything made of wood.

In the outside air, basidiospores are usually recovered in spore trap samples with the highest quantities recovered in the fall. Recovery rates are generally 90 to 95% throughout the year. When recovered, the median spore density is about 370 spores per cubic meter but in some cases, the 97.5% value reaches almost 19,000 in November. It should be noted that the airborne concentrations of basidiospores varies tremendously by region of the country with the 50th percentile value for South Carolina being about 2,300 vs. 107 in New Mexico.

Basidiospore frequency of detection and spore density by month

Figure 1: Basidiospore frequency of detection and spore density by month.
The gray bars represent the frequency of detection, from 0 to 1 (1=100%), graphed against the left axis. The red, green, and purple lines represent the 2.5, 50, and 97.5 percentile airborne spore densities, when recovered, graphed against the right hand axis. (Source: EMLab™ MoldRANGE™ data. Total sample size for this graph: 39,878.)

Recovery rates for basidiospores remain high in most weather conditions, ranging from 75% in the snow to 98% in light rain. The average concentration is about 1,100 in the rain and only 50 in moderate snow.

Basidiospore frequency of detection and spore density vs. weather

Figure 2: Basidiospore frequency of detection and spore density vs. weather.
The gray bars represent the frequency of detection, from 0 to 1 (1=100%), graphed against the left axis. The red, green, and purple lines represent the 2.5, 50, and 97.5 percentile airborne spore densities, when recovered, graphed against the right hand axis. (Source: EMLab™ MoldRANGE™ data. Total sample size for this graph: 20,191.)

Basidiospores can be identified on spore traps, occasionally on tape lifts, swabs and bulk samples. In culture, most Basidiomycetes will not fruit or form spores on laboratory media. A predominant culture characteristic for Basidiomycota is that most have unique hyphal structures called clamp connections (illustrated below). The clamp connection aids in nuclear migration at the time of cell division.

Basidiomycota clamp connections

Basidiomycota clamp connections
Copyright © 2006 Environmental Microbiology Laboratory, Inc.

Fungi are known to produce secondary metabolites or byproducts, many of which have been used commercially in the production of drugs, antibiotics and enzymes. Mushroom poisons are also secondary compounds. Examples include cyclopeptides (amatoxins), ibotenic acid/muscimol, monomethylhydrazine, coprine and orellanine (Benjamin 1995). Mushrooms generally have to be ingested to cause serious and even lethal effects. Not much evidence exists for people being allergic to mushrooms. Very little evidence exists for people having truly allergic reactions to mushrooms (Koivikko et al. 1988). Compelling evidence exists for the effects of inhaled spores on the respiratory tract causing hay fever, asthma, hypersensitivity pneumonitis, lycoperdonosis, and mushroom culture sensitivity. The known disease conditions for basidiospores are keratitis and pneumonia in immune compromised patients, as well as allergens.

A microscopic member of division Basidiomycota is a human pathogen causing the respiratory disease known since the 1800s as cryptococcosis. Cryptococcus neoformans is the causative agent that can develop into skin and bone lesions or rarely the often lethal cryptococcal meningitis (Hudler 1998). As with many other human diseases caused by fungi, cryptococcosis is most severe in people predisposed by other factors such as leukemia, diabetes and steroid treatment.

1. Environmental Microbiology Laboratory, Inc.

2. Tree of Life: Basidiomycota

3. Arora, D. 1986. Mushrooms Demystified. Ten Speed Press, Berkeley, CA, USA. 959p.

4. Benjamin, D. R. 1995. Mushrooms: poisons and panaceas. W. H. Freeman & Co., New York. 422p.

5. Christensen, C. M. 1965. The molds and man. University of Minnesota Press, Minneapolis, MN.

6. Hibbett, D. S. and M. Binder. 2001. Evolution of marine mushrooms. Biol. Bull. 201:319-322.

7. G. W. Hudler. 1998. Magical mushrooms, mischievous molds. Princeton University Press, Princeton, NJ, USA. 248p.

8. Kirk, P.M., P. F. Cannon, J. C. David and J. Stalpers. 2001. Ainsworth and Bisby’s Dictionary of the Fungi. 9th ed. CAB International, Wallingford, UK.

9. Kohlmeyer, J. and E. Kohlmeyer. 1979. Marine Mycology—The Higher Fungi. Academic Press, New York.

10. Koivikko, A. and J. Savolainen. 1988. Mushroom allergy. Allergy 43(1):1-10.

11. R. T. Lamar, J. A. Glaser and T. K. Kirk. 1992. White-rot fungi in the treatment of hazardous waste. Pp. 127-143 in G. F. Leatham (ed.) Frontiers in Industrial Mycology. New York, Chapman & Hall.

12. R. Singer. 1986. The Agaricales in Modern Taxonomy. Koeltz Scientific Books, Koenigstein, Germany. 981p.


This article was originally published on August 2006.