Mold

Mold is the general term given to a coating or discoloration found on the surface of certain materials; it is produced by the growth of a fungus. Mold also refers to the causative organism itself.

A mold is a microfungus (as opposed to the macrofungi, such as mushrooms and toadstools) that feeds on dead organic materials. Taxonomically, the molds belong to a group of true fungi known as the Ascomycotina. The characteristics of the Ascomycotina are that their spores, that is their reproductive propagules (the fungal equivalent of seeds), are produced inside a structure called an ascus (plural asci). The spores are usually developed eight per ascus, but there are many asci per fruiting body (structures used by the fungus to produce and disperse the spores). A fruiting body of the Ascomycotina is properly referred to as an ascomata. Another characteristic of molds is their rapid growth once suitable conditions are encountered. They can easily produce a patch visible to the naked eye within one day.

The visible appearance of the mold can be of a soft, velvety pad or cottony mass of fungal tissue. If closely observed, the mass can be seen to be made up of a dense aggregation of thread-like mycelia (singular, mycelium ) of the fungus. Molds can be commonly found on dead and decaying organic material, including improperly stored food stuffs.

The type of mold can be identified by its color and the nature of the substrate on which it is growing. One common example is white bread mold, caused by various species of the genera Mucor and Rhizobium. Citrus fruits often have quite distinctive blue and green molds of Penicillium. Because of the damages this group can cause, they are an economically important group.

In common with the other fungi, the molds reproduce by means of microscopic spores. These tiny spores are easily spread by even weak air currents, and consequently very few places are free of spores due to the astronomical number of spores a single ascomata can produce. Once a spore has landed on a suitable food supply, it requires the correct atmospheric conditions, i.e., a damp atmosphere, to germinate and grow.

Some molds such as Mucor and its close relatives have a particularly effective method of a sexual reproduction. A stalked structure is produced, which is topped by a clear, spherical ball with a black disc, within which the spores are developed and held. The whole structure is known as a sporangium (plural, sporangia). Upon maturity, the disc cracks open and releases the spores, which are spread far and wide by the wind. Some other molds, such as Pilobolus, fire their spores off like a gun and they land as a sticky mass up to 3 ft (1 m) away. Most of these never grow at all, but due to the vast number produced, up to 100,000 in some cases, this is not a problem for the fungus. As has already been mentioned, these fungi will grow on organic materials, including organic matter found within soil, so many types of molds are present in most places.

When sexual reproduction is carried out, each of the molds require a partner, as they are not capable of self-fertilization. This sexual process is carried out when two different breeding types grow together, and then swap haploid nuclei (containing only half the normal number of chromosomes ), which then fuse to produce diploid zygospores (a thick-walled cell with a full number of chromosomes). These then germinate and grow into new colonies.

The Mucor mold, when grown within a closed environment, has mycelia that are thickly covered with small droplets of water. These are, in fact, diluted solutions of secondary metabolites. Some of the products of mold metabolism have great importance.

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. 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.

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—a massive financial loss 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 food-stuffs 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 A. flavus growing on peanuts. However, A. flavus will grow on cereal grains (such as wheat, corn, barley, etc.), but the mycotoxin is not produced on these growth media. 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 .

The molds do not only present humans with problems. 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 P. roqueforti.

The Pencillium mold have another important use—the production of antibiotics. Two species have been used for the production of penicillin , the first antibiotic to be discovered: Penicillium notatum and P. 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 characteristic that this mold does not share with many other species is its capacity to survive at low temperatures. Its growth rate 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 now recognized as penicillin.

The anti-bacterial qualities of penicillin were originally discovered by Sanford Fleming in 1929. By careful selection of the Penicillium cultures used, the yield of antibiotic has been increased many hundred fold since the first attempts of commercial scale production during the 1930s.

Other molds are used in various industrial processes. Aspergillus terreus is used to manufacture icatonic acid, which is used in plastics production. Other molds are used in the production of alcohol, a process that utilizes Rhizopus, which can metabolize starch into glucose. The Rhizopus species can then directly ferment the glucose to give alcohol, but they are not efficient in this process, and at this point brewers yeast (Saccharomyces cerevisiae) is usually added to ferment the glucose much quicker. Other molds are used in the manufacture of flavorings and chemical additives for food stuffs.

Cheese production has already been mentioned. It is interesting to note that in previous times cheese was merely 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.

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 eczema in sheep, and slobber syndrome in various grazing animals. Some of the highly toxic chemicals are easy to identify and detect; 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.

See also Fermentation; Food preservation; Food safety; Mycology; Yeast genetics; Yeast, infectious

Mold

History of beneficial molds

Resources

Molds are microscopic fungi. 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, molds that grow indoors can be of particular concern because they 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 can be cause for concern. 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. 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.

The ability of particular mold can produce a mycotoxin does not mean that the toxin will be produced. 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.

History of beneficial molds

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 Alexander Fleming.

Since Fleming’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 simply 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 unpleasant flavors.

See also Infection; Poisons and toxins.

KEY TERMS

Mold spores —Packets that contain the genetic material necessary for the formation of a new mold.

Mycotoxin —A poisonous substance produced by a fungus.

Resources

BOOKS

Campbell, N.A. and J.B. Reece. Biology. 7th ed. Upper Saddle River, NJ: Benjamin Cummings, 2004.

Money, N.P. Carpet Monsters and Killer Spores: A Natural History of Toxic Mold. Oxford, United Kingdom: Oxford University Press, 2004.

Brian Hoyle

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 .

Beneficial molds history

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.

See also Infection; Poisons and toxins.

Resources

periodicals

Kirkland, T.N., and J. Fierer. "Coccidiodomycosis: A Reemerging Infectious Disease." Emerging Infectious Diseases 2 (July-September 1996): 191–199.

organizations

Centers for Disease Control and Prevention, National Center for Environmental Health, 1600 Clifton Road, Atlanta, GA 30333. (404) 639–3311 [cited October 20, 2002]. <http://www.cdc.gov/nceh/airpollution/mold/>.

Brian Hoyle

KEY TERMS

Mold spores

—Packets that contain the genetic material necessary for the formation of a new mold.

Mycotoxin

—A poisonous substance produced by a fungus.

mold¹ / mōld/ (Brit. mould) • n. a hollow container used to give shape to molten or hot liquid material (such as wax or metal) when it cools and hardens. ∎  something made in this way, esp. a gelatin dessert or a mousse: lobster mold with a sauce of carrots and port. ∎  [in sing.] fig. a distinctive and typical style, form, or character: he planned to conquer the world as a roving reporter in the mold of his hero the latest policy document is still stuck in the old mold. ∎  a frame or template for producing moldings. ∎ archaic the form or shape of something, esp. the features or physique of a person or the build of an animal.• v. [tr.] form (an object with a particular shape) out of easily manipulated material: a Connecticut inventor molded a catamaran out of polystyrene foam. ∎  give a shape to (a malleable substance): take the marzipan and mold it into a cone shape. ∎  influence the formation or development of: the professionals who were helping to mold US policy. ∎  shape (clothing) to fit a particular part of the body: [as adj.] (molded) a shoe with molded insole. ∎  [often as adj.] (molded) shape (a column, ceiling, or other part of a building) to a particular design, esp. a decorative molding: a corridor with a molded cornice.PHRASES: break the mold put an end to a restrictive pattern of events or behavior by doing things in a markedly different way: his work did much to break the mold of the old urban sociology.DERIVATIVES: mold·a·ble adj.mold·er n.mold² (Brit. mould) • n. a furry growth of minute fungal hyphae (subdivision Deuteromycotina, or Ascomycotina) occurring typically in moist conditions, esp. on food or other organic matter.mold³ (Brit. mould) • n. soft loose earth.See also leaf mold. ∎  the upper soil of cultivated land, esp. when rich in organic matter.

mold See mould.