True Bacteria Useful to Humans

For many years, we controlled insect pests of food plants mostly through the use of toxic sprays. Residues of the sprays remaining on the fruits and vegetables have accumulated in human tissues, often with adverse effects, while at the same time, many organisms have become immune or resistant to the toxins. In addition, the sprays kill useful organisms, and precipitation runoff washes the toxins into streams, lakes, and oceans, harming or killing aquatic organisms.

As we have become aware of the undesirable effects of the use of toxic sprays, we have looked for alternative means of controlling crop pests. Today, many harmful pests

Text and even weeds can be significantly limited through the use of biological controls, which are discussed in Appendix 2, and with transgenic plants, discussed in Chapter 14.

Bacillus Thuringiensis and Bacillus Popilliae

Three biological control bacteria have been registered for use by the U. S. Department of Agriculture. One, Bacillus thuringiensis (often referred to as Bt), has been remarkably effective against a wide range of caterpillars and worms, including peach tree borers, European corn borers, bollworms, cabbage worms and loopers, tomato and fruit hornworms (Fig. 17.9), tent caterpillars, fall webworms, leaf miners, alfalfa caterpillars, leaf rollers, gypsy moth larvae, and cankerworms.

The bacteria, which are easily mass-produced by commercial companies, are sold in the form of a stable wet-table dust containing millions of spores. When the spores are sprayed on food plants, they are harmless to humans, birds, animals, earthworms, or any living creatures other than moth or butterfly larvae. When a caterpillar ingests any tissue with Bt spores on it, the bacteria quickly become active and multiply within the digestive tract, soon

Bacillis Thuringienis DustBacillus Thuringiensis Spray

Figure 17.9 A tomato hornworm (A) before and (B) 3 days after spraying with Bacillus thuringiensis.

paralyzing the gut. The caterpillar stops feeding within 2 or 3 hours and slowly turns black, dropping off the plant in 2 to 4 days.

The toxin-producing gene from Bacillus thuringiensis was introduced into another bacterium, Pseudomonas fluo-rescens, which is used on corn to control black cutworms.

In 1983, a variety of Bacillus thuringiensis (var. israe-lensis) was introduced into the commercial market for the control of mosquitoes. Called BtI, the bacterium attacks only mosquito larvae ("wigglers") and one or two other pests. Six years of experiments performed on more than 70 species of fish, snails, shrimp, and insects demonstrated no adverse effects on either plants or animals other than mosquitoes (and a couple of lesser pests) even at dosages 100 times more powerful than those needed to kill mosquito larvae. Use of this bacterium to control mosquitoes in the future may constitute a significant step in lessening the ecological damage and disruption that so frequently accompanies the use of toxic chemicals for pest control.

Another bacterium, Bacillus popilliae, is also marketed in a powder form. When it is applied to soil where grubs of the highly destructive Japanese beetle are present, it causes what is known as "milky spore disease" in the grubs, which die in a few days. The spores are carried throughout the top-soil by rain water, foraging grubs, and by organisms that feed on the grubs.

Bioremediation

Naturally occurring bacteria have shown much potential in the developing science of bioremediation, which involves the study of the use of living organisms in the cleanup of toxic wastes and pollution. One bacterium produces enzymes that break down nitroglycerin and trinitrotoluene, which are contaminants in the soil around some munitions factories and explosive sites.

Preliminary tests indicate that the bacteria can decompose such waste and contaminant materials into harmless residues within 6 months. Other bacteria, such as Pseudomonas cepa-cia, can perform similar feats in oil spills and chemical dumps containing degreasers, such as trichloroethylene (TCE), creosote, and even 2,4,5-T, the Agent Orange defoliant of Vietnam fame. The pollution-fighting bacteria may, however, need a few nutrients added to the contaminants to stimulate their activities. Some scientists believe bacteria may also eventually be used to break down nuclear wastes. With the probability of the development of bacteria specifically engineered (see "Gene Splicing and Transgenic Plants" in Chapter 14) to deal with a host of pollution problems, their use for this purpose may become widespread in the future.

Oth er Useful Bacteria

Figure 17.9 A tomato hornworm (A) before and (B) 3 days after spraying with Bacillus thuringiensis.

Human eyes contain rhodopsin, a protein that is so sensitive it reacts to light in less than one millionth of a second. Certain bacteria that contain a form of rhodopsin have

Chapter 17

proved to be invaluable in research on the chemistry of vision and have led to our understanding of how eyes convert light energy into vision.

Bacteria play a major role in the dairy industry. Milk, which is composed of proteins, carbohydrates, fats, minerals, vitamins, and about 87% water, has exceptional nutritive value for animals and is also an excellent medium for the growth of many kinds of bacteria. Milk is sterile when secreted within the udder of the cow, but it picks up bacteria as it leaves the cow's body. It spoils rapidly if it is not obtained and stored under strictly sanitary conditions. Even after it has been pasteurized and refrigerated, the numbers of bacteria in it will increase the longer it stands. If milk is left in open containers in household refrigerators, for example, bacterial growth will not be kept in check for much longer than 24 hours.

Except for milk itself, either alone or in mixtures (e.g., ice cream), all dairy products are manufactured from raw or pasteurized milk by the controlled introduction of various bacteria. Such products include buttermilk, acidophilus milk, yogurt, sour cream, kefir, and cheese. Whey, the watery part of the milk separated from curd during cheese-making, is used, along with starches and molasses, for the commercial production of lactic acid. Lactic acid is used extensively in the manufacture of textile and laundry products, in the leather tanning industry, as a solvent in lacquers, and in the treatment of calcium and iron deficiencies in humans.

Beneficial bacteria, such as Lactobacillus acidophilus (a common organism in healthy digestive tracts), aid in digestion, reduce the risk of cancer, and may even reduce cholesterol levels. Acidophilus bacteria also have been widely used, along with antibiotics, to control or eliminate human female yeast infections. Researchers at the University of Minnesota are working on developing strains of intestinal bacteria that have an external layer of sticky polysaccharides that make them resistant to being eliminated by less desirable bacteria in the digestive tract.

In their metabolism of various sugars, proteins, and other organic substances, bacteria also produce waste products that have important industrial uses. Such products are often produced in large quantities by culturing bacteria in huge vats. These products include acetone, used in the manufacture of photographic film; explosives; solvents (e.g., nail polish remover); butyl alcohol, used in the manufacture of synthetic lacquers; dextran, used as a food stabilizer and as a blood plasma substitute; sorbose, used in the manufacture of ascorbic acid (vitamin C); and citric acid, which is the principal citrus-like tart flavoring of soft drinks, candies, and other foods. Some vitamin and medicinal preparations also involve bacterial synthesis.

Bacteria are used in the curing of vanilla pods, cocoa beans, coffee, and black tea and in the production of vinegar, sauerkraut, and dill pickles. Fibers for linen cloth are separated from flax stems by bacteria, and green plant materials are fermented in silos to produce ensilage for cattle feed. In recent years, the production of several important amino acids by bacteria has been exploited commercially. More than 6,800 metric tons (7,500 tons) of one amino acid, glutamic acid, are produced in North America each year. This is in demand as a flavor-enhancing agent in the form of monosodium glutamate.

In 1989, Patricia Mertz of the University of Miami discovered that Brevibacterium, the genus of bacteria responsible for the odor of Limburger and Brie cheeses, also is the source of foot odor in certain people. Bactericides are being tested to improve foot pads that merely absorb odor rather than kill the bacteria.

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  • benito marchesi
    How are true bacteria helpful to humans?
    8 years ago

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