Mycotoxins | Fungus of the Month: Aureobasidium species
By Dr. Harriet Burge
We are often asked what types of mycotoxins are produced by fungi and what types of health effects they cause. Fungi produce hundreds of mycotoxins,and yes, some of these toxins are produced by some common indoor fungi. A very short list of some familiar fungi and their mycotoxins is presented in Table 1.
Table 1. Abbreviated list of some familiar fungi and some of their mycotoxins.
However, a few comments about the importance, or lack thereof, of these mycotoxins in indoor air-related illness are needed.
1. Toxigenic means that the fungus is capable of producing toxins. It doesn't mean that they always do, or that the presence of a specific fungus can be taken to mean that the toxins are being produced. Even though a fungal species is known to have strains that produce mycotoxins, not all strains have this capability, and even those with the capability do not make the toxins under all conditions. Unfortunately, little data is available in these areas for fungi other than Stachybotrys, Aspergillus, and a few others.
2. If you want to know whether or not toxins are present, you must sample for the toxin itself. There are an increasing number of toxin assays available. My caution about using these services is to be sure that any sample collection and analysis is hypothesis driven, and that you know how you are going to interpret the data once you have it.
3. Mycotoxins are not volatile, and inhalation exposure is probably primarily related to airborne spores.
4. Health effects for most of the mycotoxins are known only from either agricultural environments, or from laboratory experiments, and virtually all of the data involves ingestion of the mycotoxins.
5. Extrapolating from the amount of toxin necessary to cause health effects by ingestion, and, given that the toxin content of individual spores is quite low, inhalation exposure in non-agricultural environments (i.e. in normal homes and offices) to enough spores of even the most toxic strains to reach a dose likely to produce human health effects is unlikely and probably very rare.
6. Concentrating on the so-called toxigenic fungi in indoor environmental investigations is only appropriate if you are only concerned about specific symptoms that you are convinced could only be due to the mycotoxins (I personally don't know what those symptoms would be). To ignore the other fungi is to ignore potentially hazardous conditions that could lead to serious respiratory diseases (I know none of you would do that!).
One final reminder: Most field investigators should not be investigating health effects, but rather the conditions that lead to fungal growth and the means for correcting those conditions. Focusing on health effects (which is implicit when concentrating on mycotoxins) when you don't have the credentials to back up your opinions may eventually get you into very hot water!
By Karen P. Abella
Aureobasidium is a widely distributed fungal genus usually found in soil, fresh water, dead plant material, marine estuary sediments and wood. There are approximately 15 accepted species in this genus with the most commonly known being A. pullulans. This genus has also been observed to grow on textiles, foodstuffs, fruits and painted surfaces. In the indoor environment, Aureobasidium growth is commonly found in moist places such as bathrooms and kitchens, especially on shower curtains, tile grout and windowsills. The spores are usually disseminated by wind (when dry) and water.
Aureobasidium spores are difficult to identify on spore traps because of morphologic variation. Its most distinguishing feature is the production of primary blastospores (spores produced by a budding process) arising directly from pigmented, vegetative hyphae on short denticles (protuberances in the hyphae). The spores may be hyaline or pigmented, variable in size, one-celled, ellipsoid or ovoid, and completely encased in a slimy coat. These primary spores can give rise to secondary or tertiary spores through yeast-like budding. The conidia (spores) adhere together to form slimy heads. The brown hyphae can differentiate to form chlamydospores (resting spores) or arthroconidia (asexual spores) at maturity. Generally, we report irregular clumps of dark brown hyphae, dividing in more than one plane to form chlamydospores, as A. pullulans. However, vegetative hyphae from other unrelated dematiaceous (pigmented) fungi, such as Cladosporium, may be indistinguishable from Aureobasidium when blastospores are absent. When chlamydospore-like structures are indistinguishable, we report them in the "other brown" category. Because this fungus is sticky and slimy, spores do not readily become airborne and are not commonly found on spore traps. In direct microscopic examination, it is recognizable if enough diagnostic structures have been preserved on tape lifts or swabs.
In culture, Aureobasidium species grow rapidly on MEA and, at first, produce colonies that are yeast-like and cream or pink in color. As the colony ages, a slimy exudate appears and the coloration changes to dark brown or black on the surface. As seen from the reverse side of the agar plate the colony is a pale beige. The mycelium is characterized by irregular dichotomous (two part) branching, with cells sometimes rounding off and separating, and is variable in thickness. Aureobasidium colonies exhibit distinct radial, "fan-shaped" growth that makes them recognizable among other colonies.
Aureobadisium pullulans has been used to produce pullulan, a biodegradable polysaccharide which, when processed, becomes a shiny and strong fiber used to package food and drugs. It has also been used industrially to remove unwanted components of raw textile materials. One of its negative economic impacts is that it has been associated with the deterioration of pears and oranges in storage or in transit.
This article was originally published on July 2005.