Under the term Aspergillus, about 350 species of molds are summarized, which are characterized by spore carrier reminiscent of an aspergillus. Molds of this type often form so-called fungal lawns with different colorations from milky-white to greenish-gray, red, brown and yellowish to black. A few of the worldwide distributed and almost ubiquitous Aspergillus species produce mycotoxins that are very toxic to humans or they are triggers of so-called aspergilloses.
What is Aspergillus?
Aspergillus, also called watering can mold, includes about 350 different species of molds. Its name derives from aspergillus, the lithurgical device used to sprinkle holy water in the Catholic Church. Likewise, the name watering-can mold derives from the shape of the spout of a watering can, because under the light microscope, the conidia carriers (conidiophore) typical of Aspergillus species resemble the aspergillus and the watering-can spout in their appearance. Although the conidia are formed by vegetative means, there is now recognition that many Aspergillus species also know a mode of sexual reproduction, and that sexual and asexual reproduction may alternate. Like most other fungal species, watering can molds live as saprobionts on dead organic matter. Their special characteristic is to secrete acids and enzymes that can break down macromolecules of the dead organic material. Only then do they absorb the already broken down substances, e.g. peptides, amino acids and lipids, with their hyphae. Thus, practically before uptake by the hyphae, splitting of long-chain molecules already takes place. While a few species produce very toxic mycotoxins and other species can produce the clinical picture of aspergillosis as pathogenic germs, there are also some species that make positive and important contributions to food production as noble molds.
Occurrence, distribution and characteristics
The vast majority of the 350 known Aspergillus species live completely inconspicuously as saprobionts and make an important contribution to the utilization of dead organic material, which they metabolize. The distribution range of watering can fungi is practically ubiquitous. A problem with respect to human health exists with a few species that are virtually food competitors and can colonize, spoil, and poison human (organic) food. Warm, humid habitats are particularly at risk. For example, the species Aspergillus flavus and Aspergillus niger, also known as black mold, produce highly toxic aflatoxins and can cause aspergillosis if immune deficiency is present. In the case of Aspergillus niger, various organs can be affected, as well as the skin and nails. There is always a higher risk of infection in organs that have small, scarred cavities, for example from previous diseases, in which Aspergillus can settle. Aspergillus fumigatus, which is frequently involved in causing aspergillosis, has also been shown to be infectious. Immunocompromised and HIV-infected individuals are particularly at risk, as they have little defense against fungal infection. Invasive aspergillosis, which can also affect the central nervous system, then progresses with usually a very poor prognosis. Those Aspergillus species that are associated with the synthesis of mycotoxins are usually also known as possible triggers for allergic reactions.
Significance and function
Not all pathogenic Aspergillus species are exclusively harmful to humans. Aspergillus niger, which is recognizable in appearance by its black spores and can even infect hair and nails in the form of aspergillosis, can also be used positively. The chemical and pharmaceutical industries use the metabolic capacity of black mold in large-scale plants to produce certain enzymes and organic acids such as citric acid and tartaric acid. Aspergillus melleus is “bred” by the pharmaceutical industry for the large-scale production of a wide variety of enzymes such as proteinases, acylases and hydrolases. Another group of watering can fungi and some penicillin species are not pathogenic, but are valued and needed as food refiners. Well known are the various blue cheeses such as Roquefort, Gorgonzola and Stilton.Precious mushroom cultures are also required for the production of various types of sausage and ham. The noble fungi give the food the desired flavor and keep “foreign” molds away that would spoil the food. The beneficial molds are thus not only needed to improve flavor through enzymes and other substance conversions, but also serve to preserve food. For example, Aspergillus oryzae is used in the production of soy sauce.
Diseases and ailments
It is easy to protect oneself from the toxins of some typical Aspergillus species, since affected foodstuffs can already be recognized externally by mold growth, rotting or an unpleasant odor. If mold is already visible, it can be assumed that large parts of the moldy food are already covered with fungal hyphae, since the process of spreading takes place before the visible formation of the fruiting bodies and their conidia or spores. For example, jam that is in a jar and shows visible mold on the surface is no longer suitable for consumption for safety reasons. While one can largely protect oneself from the toxins and allergenic substances of watering can fungi by paying attention and following a few precautionary measures, effective protection from inhaling spores or conidia of the fungi is virtually impossible because the tiny spores are almost ubiquitous in the normal air we breathe. Normally, this does not pose a problem for an intact human immune system because sufficient defense mechanisms are in place to keep the pathogenic germs at bay. However, the risk of contracting aspergillosis or having allergic reactions from inhaled Aspergillus spores increases significantly when the immune system is lowered by an underlying disease such as AIDS, by artificial immunosuppression, or by influenza, for example.
