Among the best known of the medically important fungi are the Penicillium molds (Fig. 19.32), which secrete penicillin, the well-known and widely used antibiotic (a substance produced by a living organism that interferes with the normal metabolism of another living organism). In 1929, Sir Alexander Fleming of England noticed that certain bacteria would not grow in the vicinity of the mycelium of a Penicillium mold and gave the name penicillin to the element in the mold that prevented the bacterial growth.
Fleming apparently did not grasp the significance of his findings, and the findings did not particularly excite the medical profession until the outbreak of World War II some 10 years later. At that time, a team of British and American scientists at Northern Regional Laboratories in Peoria,
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Figure 1Q.32 A Penicillium colony.
Illinois, set out to see if they could coax Penicillium molds into producing more of this antibacterial substance, primarily because war casualties created a need for greater quantities of more effective medicines that could keep wounds from becoming infected and to prevent the spread of venereal diseases.
The scientists began with cultures from the original mold observed by Fleming, but the amounts of penicillin it produced were so small that it was very expensive to obtain significant quantities for human use. They appealed to the general public, asking them to send in any material they found with a greenish or bluish mold on it. They received whole trainloads of moldy trash from all over the United States.
The breakthrough in the research came, however, when a different species of Penicillium mold—one that yielded 25 times the penicillin produced by the original culture—was found on a moldy cantaloupe from a local market. The scientists set to work germinating individual spores of this new mold on culture media, and by careful selection, they eventually were able to isolate a strain that produced more than 80 times the original quantity of penicillin. Later, when this strain was subjected to X-radiation, still other forms were produced that upped the penicillin output to 225 times that of Fleming's mold. Today, most of the penicillin produced around the world comes from descendants of that cantaloupe mold. Literally hundreds of other antibiotics effective in combating human and animal diseases have been discovered since the close of World War II, and the production of these drugs is a vast worldwide industry.
Penicillium molds are also used in other ways. Some are introduced into the milk of cows, sheep, and goats at stages in the production of gourmet cheeses, such as blue (Fig. 19.33), Camembert, Roquefort, Gorgonzola, and Stilton. The molds produce enzymes that break down proteins and fats in the milk, giving the cheeses their characteristic flavors.
Since the early 1980s, organ transplants have been aided by the discovery and production of a "wonder drug" from an imperfect fungus found in soil. Called cyclosporine, the drug suppresses immune reactions that cause rejection of transplanted organs without risking the development of leukemia and other undesirable side effects associated with other drugs.
Aspergillus is a genus of imperfect fungi whose species produce dark brown to blackish or yellow spores (Fig. 19.34). It is closely related to the Penicillium molds and is extensively used in industry. One or more species is used commercially for the production from sugar of citric acid, a substance for flavoring foods and for the manufacture of effervescent salts that were originally obtained from oranges. Citric acid is also used in the manufacture of inks and in medicines, and it is even used as a chicle substitute in some chewing gums.
Aspergillus fungi also produce gallic acid used in photographic developers, dyes, and indelible black ink. Other species are used in the production of artificial flavoring and perfume substances, chlorine, alcohols, and several acids. Further uses are in the manufacture of plastics, toothpaste, and soap and in the silvering of mirrors.
One species of Aspergillus is used in the Orient and elsewhere to make soy sauce, or shoyu, by fermenting soybeans with the fungus. A Japanese food called miso is made by fermenting soybeans, salt, and rice with the same fungus. More than one-half million tons of miso are consumed annually.
A number of diseases of both human and animals are caused by Aspergillus species. The diseases, called aspergilloses, attack the respiratory tract after the spores have been inhaled. One type thrives on and in human ears. Other diseases caused by different genera of imperfect fungi include those responsible for the widespread problem of athlete's foot and ringworm, for white piedra (a mild disease of beards and mustaches), and for tropical diseases of the hands and feet that cause the limbs to swell in grotesque fashion.
Aspergillus flavus, which grows on moist seeds, secretes aflatoxin, the most potent natural carcinogen known. The toxin causes liver cancer, and no more than 50 parts per billion is allowed in human food. In humid climates, such as those of the southeastern United States and adjacent Mexico, improperly stored grain can become moist enough to support the fungus so that foods such as peanuts, peanut butter, and peanut-based dairy feeds become carcinogenic. Dairy cattle feed is even more strictly controlled because concentrations of aflatoxin can accumulate in milk.
One serious disease called valley fever (coccidiomyco-sis), found primarily in the drier regions of the southwestern United States, usually starts with the inhalation of dustborne spores of an imperfect fungus (Coccidioides immitis) that produces lesions in the upper respiratory tract and lungs. The disease may spread elsewhere in the body, with sometimes fatal results.
Two imperfect fungi show promise as biological controls of pest organisms. One has already been used with some success in controlling scale insects in Florida and other warm, humid regions. Another may be used to combat water hyacinths, which have caused serious clogging of waterways in areas of the world with mild to tropical climates.
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