Fungal Strains | Fungus of the Month: Aspergillus
By Dr. Harriet Burge
What is a fungal strain, and why are fungal strains important for indoor contamination?
Each fungal species is characterized by a specific set of genetic patterns. These patterns control all aspects of fungal life, from conditions under which germination can occur to the speed with which radial growth proceeds when spores are produced, etc.
The key word in the previous paragraph is "patterns". This means that there is not a single pattern that defines a fungal species, rather a range of patterns. Each individual spore has a slightly different set of environmental requirements. For most spores the set of requirements are very similar however, for a few, some requirements may be far from the norm. If you plot, for example, the optimum water activity for the germination of an individual spore in a population, you can see this kind of (hypothetical) pattern.
On this graph you can observe from the blue line that a very few spores in a population can germinate at a "low" water activity (0.65). If you only have a few spores on the surface, the chance of one of them being able to germinate is relatively low. However, if you have a large spore population, one or more may germinate and grow. This is one of the reasons why one worries about leaving large spore populations present after water intrusion problems have been repaired. The pink and green lines on the graph represent water requirements for radial growth (pink) and sporulation (green). These curves indicate that individual strains (represented by spores) have a range of water activity requirements for each growth process. The few spores in the large population that may have germinated at low water activity may or may not be able to continue to grow and sporulate under the same conditions.
The next graph compares three different hypothetical strains, each with a different water requirement for spore germination. The vertical axis represents % spore germination. As you can see, although the peaks (optimum water activity) are clearly different, the distributions for all three strains overlap.
Similar graphs can be plotted for other environmental conditions that affect spore germination, growth and sporulation. In truth, you cannot separate one condition from another because they all interact. Thus, changing the temperature changes the water activity requirements for a given strain or, providing a particular wavelength of light may allow a particular spore strain to use a particular substrate.
The key theme here is genetic diversity, both within and between species. It is this genetic diversity that allows fungi to adapt to new environments and control responses to changes in the environment. Using this genetic diversity, the fungi have been able to colonize nearly every carbon-containing compound on earth, from simple sugars to complex man-made polycarbonates. This ability to facultatively colonize nearly everything emphasizes the importance of using moisture control to limit fungal growth because there is a threshold level of water activity below which fungal growth is extremely unlikely.
By Subu Thiagarajan and Dr. Payam Fallah
The genus Aspergillus is important economically, ecologically and medically. It is cosmopolitan and ubiquitous in nature with over 185 species. The fungus owes its name, from the Latin word aspergillum, to the resemblance of its conidiophores to a device used to sprinkle holy water. Members of this genus have been recovered from a variety of habitats but are especially common as saprobes on decaying vegetation, soil, stored food, feed products and a variety of building materials found in indoor environments (e.g. wallboard, carpet, ceiling tiles etc.).
Aspergilli colonies are downy to powdery in texture. The surface color varies depending on the species. Microscopically, the asexual fruiting structure of Aspergillus species includes a long stipe (conidiophore), a rounded head (vesicle), and flask shape structures (phialides) from which spores (conidia) are formed. The conidia (2-5 µm in diameter) may be spherical to elongate and form chains which may radiate (e.g., in Aspergillus versicolor) or form themselves into compact columns (e.g., Aspergillus fumigatus and Aspergillus nidulans). Some species may form masses of thick-walled cells called "hülle cells". These cells are often visible on tape lift samples and are especially useful as an additional character to confirm the presence of Aspergillus growth. The morphological characters of Aspergillus species are more distinguishable in culture, which makes identification much easier.
On spore trap samples, Aspergillus spores are very similar to Penicillium spores. This is why they are clumped together as "Penicillium/Aspergillus types". We do the same reference on tape lifts because, sometimes, we only see masses of spores. This may indicate that growth is old and the underlying structures have disintegrated, or that the tape lift has been collected with inadequate pressure or with tape that has lost its stickiness.
Approximately 20 species of Aspergillus have been reported as causative agents of opportunistic infections in humans. Aspergillus fumigatus is the most important opportunist and is commonly encountered in hospitals as well as other environments. Other species, such as A. flavus, A. terreus, A. niger, and A. nidulans, can also cause human infections. A. fumigatus prefers high temperatures and its optimum temperature for growth is about 37°C (normal human body temperature). However, it can grow at temperatures from 20 to 50°C. In a susceptible host, conidia germinate into hyphae, the invasive form of the disease. Invasive aspergillosis rarely occurs in people with competent immune systems.
Aspergillus can also grow in the mucous that accumulates in the lungs of asthmatics and children with cystic fibrosis. This disease is called allergic bronchopulmonary aspergillosis. People who have this condition produce antibodies against the fungus. In addition, people with allergies may produce antibodies (IgE) against proteins in airborne Aspergillus spores. A well-defined allergen, Asp f 1, has been purified from germinated spores of Aspergillus fumigatus.
Another noteworthy species is Aspergillus nidulans. Its rapid growth on defined media, compact colony morphology, uninucleate conidia, and several other complex genetic traits make this fungus a suitable experimental system for the genetic analysis of gene regulation.
Some mycotoxins produced by Aspergillus species:
Aflatoxin - Aflatoxins are produced by strains of A. flavus and A. parasiticus. Aflatoxins are potent carcinogens and consumption of aflatoxin contaminated feed has been linked, epidemiologically, to hepatocellular carcinoma. Aspergillus bombycis, Aspergillus ochraceoroseus, Aspergillus nomius, and Aspergillus pseudotamari are also aflatoxin producing species, but they are found less frequently.
Sterigmatocystin - Is another mycotoxin produced in the aflatoxin pathway. It is the final biosynthetic product in this pathway for a number of species, including A. versicolor and A. nidulans. Sterigmatocystin is both mutagenic and tumorigenic but is less potent than aflatoxin.
Ochratoxin A - This mycotoxin is produced by A. ochraceus and is recognized as a potent nephrotoxin. Members of the ochratoxin family have been found as metabolites of many different species of Aspergillus, including Aspergillus alliaceus, Aspergillus auricomus, Aspergillus carbonarius, Aspergillus glaucus, Aspergillus melleus, and Aspergillus niger.
None of these mycotoxins have been shown to cause disease in indoor air related exposures however, farmers may be exposed to Aspergillus toxins in levels that could increase their risk of some diseases.
This article was originally published on February 2005.