Mold

Molds are fungi that are microscopic in size. Even though they can approximate bacteria in size, molds are eukaryotic organisms. That is, their genetic material is enclosed within a specialized membrane that lies in the interior of the organism.

Molds are present in virtually every environment that has been examined. Molds grow indoors and outdoors and, depending on the species, can grow year-round, even in winter. In the natural environment, molds are important and desirable because they hasten the decomposition of organic material such as fallen leaves and dead trees. Indoors, however, mold growth is undesirable. For humans, the molds that grow indoors can be of particular concern. This is because these can cause allergic reactions in those people who are sensitive to the compounds produced by the molds. The most common indoor molds are: Cladosporium, Penicillium, Alternaria, Aspergillus, and Mucor.

Molds reproduce by releasing spores (essentially packets that contain the genetic material necessary for the formation of a new mold). These spores can float through the air and, if landing in a hospitable environment, can germinate to form a new mold. One of the essential components of a hospitable environment is moisture. The many types of mold all require a moist surface for growth.

The Mucor mold, when grown within a closed environment, has mycelia that are thickly covered with small droplets of what appears to be water. The droplets are, in fact, dilute solutions of secondary metabolites (compounds produced by the mold during the breakdown and use of nutrients).

Some of the products of mold metabolism have great importance. For example, a mold called Rhizopus produces fumaric acid, which can be used in the production of the drug cortisone. Other molds can produce alcohol, citric acid, oxalic acid, or a wide range of other chemicals. Some molds can cause fatal neural diseases in humans and other animals. Moldy bread is nonpoisonous. Nevertheless, approximately one hundred million loaves of moldy bread are discarded annually in the United States. The molds typically cause spoilage rather than rendering the bread poisonous.

The presence of other molds is more than just inconvenient. Indeed, some molds growing on food are believed to cause cancer, particularly of the liver. Another curious effect of mold is related to old, green wallpaper. In the nineteenth century, wallpaper of this color was prepared using compounds of arsenic, and when molds grow on this substrate they have been known to release arsenic gas.

Some molds are important crop parasites of species such as corn and millet. A number of toxic molds grow on straw and are responsible for diseases of livestock, including facial excema in sheep, and slobber syndrome in various grazing animals. Some of the highly toxic chemicals are easy to identify and detect, while others are not. Appropriate and sensible storage conditions (i.e., those not favoring the growth of fungi) are an adequate control measure in most cases. If mold is suspected, then the use of anti fungal agents (fungicides) or destruction of the infected straw are the best options.

The first poison to be isolated from a mold is aflatoxin. This and other poisonous substances produced by molds and other fungi are referred to as mycotoxins. Some mycotoxins are deadly to humans in tiny doses, others will only affect certain animals. Aflatoxin was first isolated in 1960 in Great Britain. It was produced by Aspergillus flavus that had been growing on peanuts. In that year, aflatoxin had been responsible for the death of 100,000 turkeys. In fact, it was the massive financial loss from these deaths that led to the research that discovered aflatoxin.

From the beginning of the twentieth century, scientists had tentatively linked a number of diseases with molds, but had not been able to isolate the compounds responsible. With the discovery of aflatoxin, scientists were able to provide proof of the undesirable effects of a mold.

Just because a particular mold can produce a mycotoxin does not mean it always will. For example, Aspergillus flavus has been safely used for many centuries in China in the production of various cheeses and soy sauce. Aspergillus flavus and related species are relatively common, and will grow on a wide variety of substrates, including various foodstuffs and animal feeds. However, the optimum conditions for vegetative growth are different from those required for the production of aflatoxin. The mycotoxin in this species is produced in largest quantities at high moisture levels and moderate temperatures on certain substrates. For a damaging amount of the toxin to accumulate, about ten days at these conditions may be required. Aflatoxin can be produced by Aspergillus flavus growing on peanuts. However, the aflatoxin is not produced when the mold grows on cereal grains such as wheat, corn, and barley.

Aflatoxin production is best prevented by using appropriate storage techniques.

Other molds can produce other mycotoxins, which can be just as problematical as aflatoxin. The term mycotoxin can also include substances responsible for the death of bacteria, although these compounds are normally referred to as antibiotics.

Certain types of cheeses are ripened by mold fungi. Indeed, the molds responsible for this action have taken their names from the cheeses they affect. Camembert is ripened by Penicillium camemberti, and Roquefort is by Penicillium roquefortii.

The Penicillium mold has another important use, namely the production of antibiotics. Two species have been used for the production of penicillin, the first antibiotic to be discovered: Penicillium notatum and Penicillium chrysogenum. The Penicillium species can grow on different substrates, such as plants, cloth, leather, paper, wood, tree bark, cork, animal dung, carcasses, ink, syrup, seeds, and virtually any other item that is organic.

A unique characteristic of Penicillium species is their capacity to survive at low temperatures. The growth rate of Penicillium is greatly reduced, but not to the extent of its competition, so as the temperature rises the Penicillium is able to rapidly grow over new areas. However, this period of initial growth can be slowed by the presence of other, competing microorganisms. Most molds will have been killed by the cold, but various bacteria may still be present. By releasing a chemical into the environment capable of destroying these bacteria, the competition is removed and growth of the Penicillium can carry on. This bacteria killing chemical is what we now recognize as penicillin. The anti-bacterial qualities of penicillin were originally discovered in 1929 by Sir Sanford Fleming.

Since Flemming's pioneering observations, the careful selection of the Penicillium cultures has increased the yield of antibiotic many hundred fold since the first attempts of commercial scale production during the 1930s.

Other molds are used in various industrial processes. For example, Aspergillus terreus is used to manufacture icatonic acid, which is used in plastics production. Other molds are used in the production of alcohol. For example, Rhizopus, which can metabolize starch into glucose, directly ferments the glucose to give alcohol. Other molds are used in the manufacture of cheeses, flavorings and chemical additives for foods.

In times past, the involvement of mold in cheese making was more happenstance than by design. Often, a cheese was just left in a place where mold production was likely to occur. However, in modern production cheeses are inoculated with a pure culture of the mold (some past techniques involved adding a previously infected bit of cheese). Some of the mold varieties used in cheese production are domesticated, and are not found in the wild. In cheese production, the cultures are frequently checked to ensure that no mutants have arisen, which could produce unpalatable flavors.