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An Overview of the Genus Aspergillus 1 Joan W caister.com/aspergillus2 An Overview of the Genus Aspergillus 1 Joan W. Bennett Abstract processes. Evolutionary biologists are focusing on Aspergillus is the name used for a genus of moulds the forces that drive variation within and among that reproduce only by asexual means. Te mor- population. Economically important species are phology of the conidiophore, the structure that being re-tested for new capabilities using new bears asexual spores, is the most important taxo- screens developed with the aid of post genomic nomic character used in Aspergillus taxonomy. technologies. Aspergillus species are common and widespread. Tey are among the most successful groups of Introduction moulds with important roles in natural ecosys- One of the oldest named genera of fungi, Aspergil- tems and the human economy. lus received its name from Micheli in 1729. In Among scientists working on Aspergillus, viewing the microscopic spore-bearing structure, there is a continuing fascination with their bio- Micheli was reminded of a device used by the technological potential. In addition to producing Roman Catholic clergy to sprinkle holy water numerous useful extracellular enzymes and or- during a part of the liturgy called the asperges ganic acids, these moulds also produce secondary (Ainsworth, 1976). Te asperges was described as metabolites of importance in biotechnology. follows in the 11th edition of Te Encyclopaedia Some Aspergillus species function as plant Britannica: and/or animal pathogens. Aspergillosis is the name given to all animal diseases caused by ASPERGES (‘thou wilt sprinkle,’ fom the growth of any member of the genus on a liv- Latin verb aspergere), the ceremony of the ing host. Immunosuppression is generally a people with holy water before High Mass in the prerequisite for systemic Aspergillus infections in Roman Catholic Church … with which the humans. Te incidence of systemic aspergillosis, priest begins the ceremony. Te brush used for the most serious form, is on the rise and imposes sprinkling is an aspergill (aspergillum)…’ an increasing medical burden upon hospitals and (Anonymous, 1910) physicians. Beter antifungal drugs and diagnostic methods are needed. By the time Tom and Church published Advances in Aspergillus genomics are giving the frst major monograph on the genus in 1926, us new tools for understanding this extremely di- Aspergillus had become one of the best-known verse genus. Hitherto undiscovered sexual stages and most studied mould groups. Teir prevalence have been discovered based on fndings from in the natural environment, their ease of cultiva- genomics. Molecular biologists are trying to un- tion on laboratory media and the economic derstand the mechanisms by which pathogenic- importance of several of its species ensured that ity and sexuality work and to deconstruct the many mycologists and industrial microbiologists physiological pathways that are central to these were atracted to their study. Aspergilli grow 2 | Bennett caister.com/aspergillus2 abundantly as saprophytes on decaying veg- their food substrates. Species of Aspergillus are etation where have been found in large numbers typical examples of the fungal life style. Tey are from mouldy hay, organic compost piles, leaf liter most ofen found in terrestrial habitats and are and the like. Most species are adapted for the deg- commonly isolated from soil and associated plant radation of complex plant polymers, but they can liter. Te decomposition process carried out also dine on substrates as diverse as dung, human by these moulds is important in driving natural tissues, and antique parchment (Polacheck et al., cycling of chemical elements, particularly in the 1989). Tere is even a report of an unidentifed carbon cycle where they contribute to replenish- Aspergillus species being capable of the solubiliza- ment of the supply of carbon dioxide and other tion of low rank coal (Torzilli and Isbister, 1994). inorganic compounds (Carroll and Wicklow, Furthermore, this common mould is in- 1992). volved in many industrial processes including In the ecosystem, diferent substrates are at- enzymes (e.g. amylases), commodity chemicals tacked at diferent rates by consortia of organisms (e.g. citric acid) and food stufs (e.g. soy sauce). from diferent kingdoms. Aspergillus and other In classical fungal genetics, one species Aspergillus moulds play an important role in these consortia (Emericella) nidulans has been used to elucidate because they are adept at recycling starches, the parasexual cycle, as well as to understand basic hemicelluloses, celluloses, pectins and other sugar concepts in regulation of metabolic pathways, polymers. Some aspergilli are capable of degrad- the cell cycle, intron splicing and hyphal polar- ing more refractory compounds such as fats, oils, ity. Several species contaminate grains and other chitin, and keratin. Maximum decomposition oc- foods with toxic metabolites that are a threat to curs when there is sufcient nitrogen, phosphorus the health of humans and other animals. Certain and other essential inorganic nutrients. Fungi also Aspergillus species also can infect humans directly provide food for many soil organisms (Carroll causing both localized and systemic infections, and Wicklow, 1992). especially in immunocompromised individuals. For Aspergillus the process of degradation is the means of obtaining nutrients. When these Physiology and ecology moulds degrade human-made substrates, the Aspergillus spores are common components of process usually is called biodeterioration. Both aerosols where they drif on air currents, dispers- paper and textiles (coton, jute, and linen) are ing themselves both short and long distances particularly vulnerable to Aspergillus degradation. depending on environmental conditions. When Our artistic heritage is also subject to Aspergillus the spores come in contact with a solid or liquid assault. To give but one example, afer Florence surface, they are deposited and if conditions of in Italy fooded in 1969, 74% of the isolates from moisture are right, they germinate (Kanaani et a damaged Ghirlandaio fresco in the Ognissanti al., 2008). Te ability to disperse globally in air church were Aspergillus versicolor (Ciferri, 1999). currents and to grow almost anywhere when ap- Similarly, foods utilized by humans and our propriate food and water are available means that domestic animals are good nutritional sources for ‘ubiquitous’ is among the most common adjec- Aspergillus. Words like ‘decay,’ ‘rot’ and ‘spoilage’ tives used to describe these moulds. are used to describe such fungal utilization of One of the defning characteristics of the our foodstufs, which can occur in the feld prior entire fungal kingdom is its distinctive nutritional to harvest, during storage, and afer commercial strategy. Tese organisms secrete acids and en- processing or cooking in the home. Although zymes into the surrounding environment, break- foods with an acidic pH, dried foods, and those ing down polymeric molecules down into simpler with a high concentration of sugars such as jams ones that are then absorbed back into the fungal and jellies normally do not support microbial cell. Fungi, like animals, are heterotrophic. While growth many members of the A. glaucus group animals eat their food and then digest it, fungi (Eurotium) are able to grow at low water activity. frst digest their food and then ‘eat’ it. Gaining Tese species even have been isolated from salted, access to nutrients is aided by mechanical forces dry fsh (Hocking, 1993). In the same way, grains, whereby fungal hyphal tips grow into and through nuts and spices, all of which have relatively low caister.com/aspergillus2 An Overview of the Genus Aspergillus | 3 amounts of water, regularly are atacked by mod- the foot cell is the conidiophore, which enlarges erately xerophilic species of Aspergillus (Lacey, at its apex to form a rounded, elliptical or club 1994). shaped vesicle. Te fertile area of the vesicle gives rise to a layer of cells called phialides (or steigmata Taxonomy in the older literature) that produce long chains of Te great English bacteriologist, Samuel Cowan, mitotic spores called conidia or conidiospores. once wrote of ‘Te Trinity that is Taxonomy’: Te size and arrangement of the conidial heads as identifcation, nomenclature, and classifcation. well as the colour of the spores they bear are im- Te genus Aspergillus poses taxonomic problems portant identifying characteristics. For example, in all three realms. Important manuals and mono- species in the A. niger group bear black spores, graphs devoted to Aspergillus taxonomy are listed the A. ochraceus group is yellow to brown, while in Table 1.1. A. fumigatus, A. nidulans, and A. favus are green. Te major cultural features used in species Identifcation and intrageneric identifcation are the colour of the colony, the categories growth rate and thermotolerance. Aspergilli have Te defning characteristic of the genus Aspergil- varying morphological and growth response to lus is the aspergillum-like spore-bearing structure. diferent nutrients so it is important to standard- It is the most important microscopic character ize conditions. Species identifcation depends used in Aspergillus taxonomy. During mycelial dif- upon pure cultures grown on known media. Te ferentiation certain cells enlarge, develop a heavy early taxonomic micrographs used a defned me- cell wall and form ‘T’ or ‘L’ shaped ‘foot cells’ dium adapted from Czapek by Dox, ofen called (which are not separate cells) that produce
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