What Is The Causitive Agent Of Bread Mold

310 Chapter 12 The Eukaryotic Members of the Microbial World

310 Chapter 12 The Eukaryotic Members of the Microbial World

Black Spores Mycelium Cakes

Figure 12.16 Examples of a Mycelium on Various Foods (a)The cottony white mass inside the potato is an example of a mycelium. (b) Magnified hyphae of Rhizopus stolonifer, black bread mold, showing the hyphae and the spores.





Figure 12.16 Examples of a Mycelium on Various Foods (a)The cottony white mass inside the potato is an example of a mycelium. (b) Magnified hyphae of Rhizopus stolonifer, black bread mold, showing the hyphae and the spores.

surface of the bread; the rest is buried deep within. Some mycelia appear above the surface of the substrate as a mushroom, orpuffball (figure 12.17). These macroscopic structures produce reproductive spores. Some large mushrooms are edible.

Hyphae are well adapted to absorb food. They are narrow and threadlike. With their high surface-to-volume ratio, they can absorb large amounts of nutrients. Hyphae release enzymes that break down the material into readily absorbed smaller organic compounds. In addition, these enzymes act to repel the growth of other hyphae near it. As a result, hyphae spread throughout the food source, ensuring that each hypha will have access to adequate nutrients.

Parasitic fungi have specialized hyphae called haustoria, which can penetrate animal or plant cell walls to gain nutrients. Saprophytic fungi sometimes have specialized hyphae called rhi-zoids, which anchor them to the substrate.

Dimorphic Fungi

Dimorphic fungi are capable of growing either as yeastlike cells or as mycelia, depending on the environmental conditions. Many of the fungi that cause disease in humans are dimorphic. Certain fungi such as Coccidioides immitis grow in the soil as molds. When their spores, which are readily carried in the air, are inhaled into the warm, moist environment of the lungs, they develop into the yeast form of the organism and cause disease.

Fungal Habitats

Fungi are found in virtually every habitat on the earth where organic materials exist. Whereas algae and protozoa grow primarily in aquatic environments, the fungi are mainly terrestrial organisms. Some species occur only on a particular strain of one genus of plants, whereas others are extremely versatile in what they can attack and use as a source of carbon and energy. Materials such as leather, cork, hair, wax, ink, jet fuel, and even some synthetic plastics like the polyvinyls can be attacked by fungi. Some species can grow in concentrations of salts, sugars, or acids strong enough to kill most bacteria. Thus, fungi are often responsible for spoiling pickles, fruit preserves, and other foods. Some fungi are resistant to pasteurization and others can grow at temperatures below the freezing point of water, rotting bulbs and destroying grass in frozen ground. Fungi are found in the thermal pools at Yellowstone National Park, in volcanic craters, and in lakes with very high salt content, such as the Great Salt Lake and the Dead Sea. ■ food spoilage, p. 811

Fungal reproductive cells, or spores, are found throughout the earth. They also occur in tremendous numbers in the air near the earth's surface as well as at altitudes of more than 7 miles. Although not as resistant as bacterial endospores, fungal spores are generally resistant to the ultraviolet rays of sunlight. Sunlight will, however, sometimes kill fungal vegetative cells. Fungal spores are a major cause of asthma. ■ asthma, p. 443

Growth Requirements of Fungi

Most fungi prefer a slightly moist environment with a relative humidity of 70% or more, and various species can grow at temperatures ranging from -6°C to 50°C. The optimal temperature for the majority of fungi is in the range of 20°C to 35°C.

The pH at which different fungi can grow varies widely, ranging from as low as 2.2 to as high as 9.6, but fungi usually grow well at an acid pH of 5.0 or lower. This explains why fungi grow well on fruits and many vegetables that tend to be acidic.

As heterotrophs, fungi secrete a wide variety of enzymes that degrade organic materials, especially complex carbohydrates, into small molecules that can be readily absorbed. Most fungi are

Figure 12.17 A Meadow Mushroom, Agaricus campestri (a) A drawing shows the extensive underground mycelium with a fruiting body emerging as a small button. (b) A photograph of Lepiota rachodes, a similar mushroom, showing the fruiting bodies and gills. (c)The underside of the cap is composed of radiating gills. (d) Magnified view of the surface of the gill showing a mass of basidia, bearing spores.

aerobic, but some of the yeasts are facultative anaerobes and carry out alcoholic fermentation. Facultatively anaerobic fungi live in the intestines of certain species of fish and help degrade algae. Some fungi living in the rumen of cows and sheep are known to be obligately anaerobic. They are important in the digestion of the plant material that these animals ingest.

Fungal Disease in Humans

Fungi cause disease in humans in one of four ways. First, a person may develop an allergic reaction to fungal spores or vegetative cells. Second, a person may react to the toxins produced by fungi. Third, the fungi may actually grow on or in the human body, causing disease or mycoses. Fourth, fungi can destroy the human food supply, causing starvation and death.

Allergic Reactions in Humans

Medical mycologists study fungi that affect humans, including fungi that cause allergic reactions. Allergic diseases such as hay fever and asthma can result from inhaling fungi or their spores if exposed humans have become sensitized. Sometimes, severe, long-term allergic lung disease results from these allergic reactions. ■ hay fever, p. 443 ■ asthma, p. 443

Effect of Fungal Toxins

For their hallucinogenic properties, certain mushrooms have long been used as part of religious ceremonies in some cultures. The lethal effects of many mushrooms have also been known for centuries. The poisonous effects of a rye smut called ergot were known during the Middle Ages, but only recently has the active chemical been purified from this fungus to yield the drug ergot-amine, which is now used to control uterine bleeding, relieve migraine headaches, and assist in childbirth.

Some fungi produce toxins that are carcinogenic. The most thoroughly studied of these carcinogenic toxins, produced by species of Aspergillus, are called aflatoxins. Ingestion of afla-toxins in moldy foods, such as grains and peanuts, has been implicated in the development of liver cancer (hepatoma) and thyroid cancer in hatchery fish. Governmental agencies monitor levels of aflatoxins in foods such as peanuts, and if a certain level is exceeded, the food cannot be sold. ■ aflatoxin, p. 811





(b) Mycelium

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  • asphodel
    What is the causitive agent of bread mold?
    1 year ago
    What is the caustive agent of bread mold?
    1 year ago
    Is bread mould dimorphic?
    7 months ago

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