Aspergillus

Aspergillus on a tomato in detail

Aspergillus is defined as a group of conidial fungi—that is, fungi in an asexual state. Some of them, however, are known to have a teleomorph (sexual state) in the Ascomycota. With DNA evidence, all members of the genus Aspergillus are members of the phylum Ascomycota.

Members of the genus possess the ability to grow where a high osmotic pressure exists (high concentration of sugar, salt, etc.). Aspergillus species are highly aerobic and are found in almost all oxygen-rich environments, where they commonly grow as molds on the surface of a substrate, as a result of the high oxygen tension. Commonly, fungi grow on carbon-rich substrates like monosaccharides (such as glucose) and polysaccharides (such as amylose). Aspergillus species are common contaminants of starchy foods (such as bread and potatoes), and grow in or on many plants and trees.

In addition to growth on carbon sources, many species of Aspergillus demonstrate oligotrophy where they are capable of growing in nutrient-depleted environments, or environments with a complete lack of key nutrients. Aspergillus niger is a prime example of this; it can be found growing on damp walls, as a major component of mildew.

Several species of Aspergillus, including A. niger and A. fumigatus, will readily colonise buildings,[6] favouring warm and damp or humid areas such as bathrooms and around window frames.[7]

Aspergillus are found in millions of pillows.[8]

Various Penicillium, Aspergillus spp. and other fungi growing in axenic culture

Historical model of Aspergillus, Botanical Museum Greifswald

Species of Aspergillus are important medically and commercially. Some species can cause infection in humans and other animals. Some infections found in animals have been studied for years, while other species found in animals have been described as new and specific to the investigated disease, and others have been known as names already in use for organisms such as saprophytes. More than 60 Aspergillus species are medically relevant pathogens.[9] For humans, a range of diseases such as infection to the external ear, skin lesions, and ulcers classed as mycetomas are found.

Other species are important in commercial microbial fermentations. For example, alcoholic beverages such as Japanese sake are often made from rice or other starchy ingredients (like manioc), rather than from grapes or malted barley. Typical microorganisms used to make alcohol, such as yeasts of the genus Saccharomyces, cannot ferment these starches. Therefore, koji mold such as Aspergillus oryzae is used to first break down the starches into simpler sugars.[10]

Members of the genus are also sources of natural products that can be used in the development of medications to treat human disease.[11] Aspergillus spp. are known to produce anthraquinone which has commercial importance due to its antibacterial and antifungal properties. [12]

Perhaps the largest application of Aspergillus niger is as the major source of citric acid; this organism accounts for over 99% of global citric acid production, or more than 1.4 million tonnes (>1.5 million US tons) per year. A. niger is also commonly used for the production of native and foreign enzymes, including glucose oxidase, lysozyme, and lactase.[13] In these instances, the culture is rarely grown on a solid substrate, although this is still common practice in Japan, but is more often grown as a submerged culture in a bioreactor. In this way, the most important parameters can be strictly controlled, and maximal productivity can be achieved. This process also makes it far easier to separate the chemical or enzyme of importance from the medium, and is therefore far more cost-effective.

Four three-day-old Aspergillus colonies. Clockwise from top-left: an A. nidulans laboratory strain; a similar strain with a mutation in the yA marker gene involved in green pigmentation; an A. oryzae strain used in soy fermentation; A. oryzae RIB40

A scan of Aspergillus taken at 235 magnifications under a scanning electron microscope

A. nidulans (Emericella nidulans) has been used as a research organism for many years and was used by Guido Pontecorvo to demonstrate parasexuality in fungi. Recently, A. nidulans was one of the pioneering organisms to have its genome sequenced by researchers at the Broad Institute. As of 2008, a further seven Aspergillus species have had their genomes sequenced: the industrially useful A. niger (two strains), A. oryzae, and A. terreus, and the pathogens A. clavatus, A. fischerianus (Neosartorya fischeri), A. flavus, and A. fumigatus (two strains).[14] A. fischerianus is hardly ever pathogenic, but is very closely related to the common pathogen A. fumigatus; it was sequenced in part to better understand A. fumigatus pathogenicity.[15]

Some Aspergillus species cause serious disease in humans and animals. The most common pathogenic species are A. fumigatus and A. flavus, which produces aflatoxin which is both a toxin and a carcinogen, and which can contaminate foods such as nuts. The most common species causing allergic disease are A. fumigatus and A. clavatus. Other species are important as agricultural pathogens. Aspergillus spp. cause disease on many grain crops, especially maize, and some variants synthesize mycotoxins, including aflatoxin. Aspergillus can cause neonatal infections.[29]

A. fumigatus (the most common species) infections are primary pulmonary infections and can potentially become a rapidly necrotizing pneumonia with a potential to disseminate. The organism can be differentiated from other common mold infections based on the fact that it takes on a mold form both in the environment and in the host (unlike Candida albicans which is a dimorphic mold in the environment and a yeast in the body).

Pulmonary aspergillosis

Aspergillosis is the group of diseases caused by Aspergillus. The most common species among paranasal sinus infections associated with aspergillosis is A. fumigatus.[30] The symptoms include fever, cough, chest pain, or breathlessness, which also occur in many other illnesses, so diagnosis can be difficult. Usually, only patients with already weakened immune systems or who suffer other lung conditions are susceptible.

In humans, the major forms of disease are:[31][32]

• Acute invasive aspergillosis, a form that grows into surrounding tissue, more common in those with weakened immune systems such as AIDS or chemotherapy patients
• Allergic bronchopulmonary aspergillosis, which affects patients with respiratory diseases such as asthma, cystic fibrosis, and sinusitis
• Aspergilloma, a "fungus ball" that can form within cavities such as the lung
• Disseminated invasive aspergillosis, an infection spread widely through the body

Fungal infections from Aspergillus spores remain one theory of sickness and untimely death of some early Egyptologists and tomb explorers. Ancient spores which grew on the remains of food offerings and mummies sealed in tombs and chambers may have been blown around and inhaled by the excavators, ultimately linked to the notion of the curse of the pharaohs.[33]

Aspergillosis of the air passages is also frequently reported in birds, and certain species of Aspergillus have been known to infect insects.[9]

Most people inhale Aspergillus into their lungs everyday,[34] but generally only the immuno-compromised become sick with aspergillosis.[34]

• Mold health issues
• Sick building syndrome

Wikispecies has information related to Aspergillus.

• Aspergillus Comparative Database Comparative genomic resource at the Broad Institute
• Aspergillus Genome Resources (NIH)
• Aspergillus surveillance project at a large tertiary-care hospital. (PDF).
• Central Aspergillus Data Repository
• FungiDB: An integrated functional genomics database for fungi and oomycetes
• Mold and Mildew
• The Aspergillus Genome Database Archived 2021-05-25 at the Wayback Machine
• The Aspergillus/Aspergillosis Website An encyclopedia of Aspergillus for patients, doctors and scientists
• The Fungal Genetics Stock Center Archived 2006-10-01 at the Wayback Machine