Bacteria And Man (How It Is Affecting Crop And Plants)



Bacteria are very small organisms consisting of single cells, rarely more than 0-01 mm in length and visible only under the higher powers of the microscope. Unlike plant cells they do not have cellulose walls or chloroplasts. Their cell walls are of proteinaceous and fatty substances, and the single chromosome is not enclosed in a nuclear membrane. Granules of glycogen, fat and other food reserves may be present. Bacteria reproduce rapidly by cell division, as often as once in 20 minutes, and may form chains of individuals, clumps or films over the surface of static water. The individuals may be spherical, rod-shaped, or spira! and some have filaments protruding from them called flagella, the lashing action of which moves the bacterium about. Some bacteria need Oxygen to respire while others can respire anaerobically, that is, obtain their energy by breaking down compounds without using oxygen.

Spores. Some bacteria can form spores, a resting stage in which the protoplasm is concentrated in one part of the cell, and surrounded by a thick wall. Bacterial spores are very widespread, being easily distributed by air currents, and occur on the surface of most objects, in soil, in dust and in the air.

These spores are very resistant to extremes of temperature, and some can withstand the temperature of boiling water for long periods. Most normal bacteria can, however, be killed by temperatures above 50° C although they can live at very low temperatures. In favourable conditions, the spores break open and the bacteria reproduce and grow in the normal way.

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Viruses are disease-producing agents far smaller than bacteria; they seem to consist of nuclear chemicals enclosed in a protein coat. They can pass through filters which trap bacteria and they can be crystallized. They can live and reproduce only in living animals or plants and cannot be cultured except in living far smaller living tissues. In the right environment they can multiply rapidly, and in man cause diseases such as poliomyelitis, influenza and the common cold. They can be transmitted in ways similar to those described for bacteria.

  Poliomyelitis is an intestinal disease and can be transmitted in food and water like cholera. Influenza and the common cold are transmitted by droplet infection; smallpox is spread by contagion; yellow fever is carried by mosquitoes.

The part played in nature by bacteria

Most bacteria live freely in the soil, in water and in decaying organic matter where they obtain their food saprophytically, bringing about the decay of dead material. Enzymes made in the bacterial cells are secreted into the food, dissolving it to form soluble organic compounds such as amino acids which can be absorbed through the cell wall. Some bacteria obtain their energy by converting these organic compounds to inorganic substances, such as ammonia, which can then be absorbed by different bacteria and oxidized further to nitrates, releasing energy for their metabolism.

Thus the dead remains of plants and animals are converted once more to simpler substances which can be taken in by plants and used to synthesize food. In this way the resources of the Earth are re-cycled and made available for re-use by living organisms.

The activities of decay bacteria are harnessed in the sewage Works. Anaerobic bacteria (see below) act on the organic solids in the settling tanks and release soluble organic compounds from them. By spraying this liquid from the tanks over filter beds or by agitating it with paddles, it is oxygenated sufficiently for aerobic bacteria to flourish and convert the soluble organic compounds to inorganic salts such as nitrates and phosphates which are released with the effluent into rivers. Any industrial wastes which harm the bacteria in the sewage plant severely interfere with the effective treatment of sewage. Similarly, substances which cannot be used as a source of food by the bacteria will pass unchanged through the sewage works and contaminate the effluent. Hence the urgent need to use, for example, bio-degradable detergents which can be metabolized by the bacteria.

Some types of bacteria live in the alimentary canals of animals. A few of these cause diseases such as cholera and typhoid but the majority are harmless or even beneficial. For example, in the intestine of man there are bacteria which appear to play a part in the synthesis of vitamin K and the vitamins of the B, complex. In the alimentary canals of many herbivorous animals there is a dense population of bacteria which play a part in digestion. Few animals can produce an enzyme which digests cellulose, yet many herbivores feed exclusively on vegetation, the most nourishing part of which is enclosed in cellulose cell walls. These walls are broken open partly by efficient chewing and partly by the enzymes of the bacteria and single-celled animals in the gut; in the caecum and appendix of the rabbit, for example, and the rumen (or paunch) of cows and sheep.

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Certain bacteria are able to produce food by photosynthesis and some of these can use H,S (hydrogen sulphide) instead of H,O as a source of hydrogen atoms.

Anaerobic bacteria can live in situations where no free oxygen is present. They obtain their energy either by processes similar to fermentation or by using inorganic chemicals. The denitrifying bacteria in the soil can use the oxygen from the nitrate (-NO,) present in the soil to oxidize carbohydrates.

Aerobic bacteria are those which normally use free oxygen for their respiration.

Parasitic bacteria live in or on living animals or plants and are often harmful to the organism.

Harmful bacteria. Some bacteria which live parasitically in plants and animals do a great deal of harm on account of the damage to tissues and highly poisonous proteins, toxins, that they produce, e.g. diphtheria and tetanus. For example, five millionths of a gram of dried tetanus toxin will kill a mouse and 0-00023g is fatal for a man. Certain bacteria are the causes o major diseases such as typhoid, Cholera, plague and tuber culosis. 1he diseases are caught as a result or the bacteria being passed directly or indirectly from one person to another.

Bacteria can reproduce so rapidly, a Single bacterium may oi nise to a million oflspring in a few hours.

For a person to catch a bacterial disease, bacteria must e the body either through the natural openings such as t mouth, nose or vagina, or through the skin via a wound caused by accident or by a blood-sucking insect. The body has natural defences to bacteria but if a sufficient number oof bacteria penetrate these defences and multiply in the body, they may become numerous enough to cause the symptoms of a disease.

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