Bacteria are small single-celled organisms possessing a hard cell wall. They exist as single cells or as a chain or group of cells. They are non-photosynthetic but are grouped under plants as they have a cell wall.
Bacteria (singular: bacterium) are found almost everywhere in air, water, soil, food and in the bodies of plants and animals including human beings.
Size and Structure:
Bacteria are microscopic, their size varying from 0.2 to 100 microns. (1 micron = 1/1000 of millimeter)
Bacteria are single-called, non-green plants possessing a tough cell wall. A bacterium does not have well-defined nucleus. The nuclear material is present in the middle of the cell but is not enclosed by a nuclear membrane. Many bacteria possess cilia or flagella that help them in locomotion. They may exist as single cells or as chains and large groups. They form a cyst around themselves during unfavorable conditions.
Depending upon their shape, the bacteria are classified as under:
Bacilli (singular: bacillus): Rod-shaped bacteria with or without flagella are called bacilli.
Coocci (singular: coccus): spherical bacteria are called cocci.
Spirilla (singular: spirillum): These bacteria have a spiral shape.
Vibrio: Comma-shapped bacteria are called vibrio.
Most bacteria are heterotrophic since they cannot make their own food. They may be saprophytic, obtaining nutrition from dead, decaying matter or they may be parasitic, obtaining nutrition from other living organisms called hosts. Some bacteria are autotrophic and make their own food by a process called chemosynthesis.
Many bacteria possess cilia or flagella which help them in locomotion.
Most bacteria require oxygen for respiration (breaking down of food to release energy). Such bacteria are called aerobic bacteria. Some bacteria, however, do not require oxygen for respiration. Such bacteria are called anaerobic bacteria.
Bacteria reproduce by binary fission. Under favorable conditions, a bacterium divides into two daughter cells. The nuclear material divides into two equal halves, one of which is passed on to each daughter cell. The daughter cells grow in size and divide again. Most bacteria reproduce very fast. Escherichia coli (E. coli), a bacterium commonly used for scientific research, divide every twenty minutes. Pseudomonas, a fast dividing bacterium divides every 9.5 minutes. If the bacteria were to multiply at this rate, they would fill up the entire Earth in no time! Fortunately, such a situation does not arise because one or the other factor necessary for the growth of bacteria (like food, oxygen, etc) becomes a limiting factory after some time and checks their growth. A warm and humid environment is ideal for the growth of bacteria. During unfavorable conditions like extremes of temperature or dryness, the bacteria grow a cyst around themselves.
Bacteria are useful to us in several ways:
Antibiotics like streptomycin, Chloromycetin and Aureomycin are obtained from bacteria. Filamentous bacteria actinomycetes are employed to produce streptomycin.
2. Food and Industry:
Milk Products like curd and cheese are made by bacterial fermentation. Lactobacillus is the bacterium that promotes the formation of curd.
Beverages: Bacteria are used in the production of tea, coffee and cocoa. Bacteria help in the ripening of tea leaves which give them characteristic flavor and aroma.
Wines and Vinegar: Bacteria act on fruit juices to produce wines and vinegar.
Tenderizing meat: Bacteria help in making meat tender by breaking down tough muscle fibers.
Cleansing the environment:
Bacteria act as scavengers by acting on dead bodies of plants and animals and converting them into simpler elements. Bacteria are employed to decompose industrial wastes and thus help in sewage disposal and reduce water pollution.
Recycling of nutrients:
Bacteria break down complex molecules in the bodies of dead plants and animals into simpler molecules and release them into soil, water and air.
Bacteria bring about decay and decomposition of fallen leaves and other organic wastes to produce humus, the topmost fertile layer of soil.
Increasing soil fertility:
Proteins present in the bodies of plants and animals are converted into ammonia by bacteria. Ammonia is acted upon by other bacteria and converted into nitrites and nitrates which increase soil fertility.
Manufacture of biogas:
Bacteria decompose plant and animal wastes to produce biogas, and efficient fuel.
4. Nitrogen fixation:
Rhizobium bacterium lives in the root nodules of leguminous plants (pulses, gram, pea, bean, etc.) These bacteria convert free atmospheric nitrogen into nitrates. This process is called nitrogen fixation. Certain free-living bacteria like azotobacer fix atmospheric nitrogen.
Plants use these nitrates to synthesize proteins.
5. Digestion of food:
In ruminants (animals like cows, buffaloes, goats, etc. that possesses a four-chambered stomach): certain bacteria live in the intestines of ruminants and help them in digesting cellulose. In the absence of these bacteria, herbivores would not be able to derive nourishment in the digestion and absorption of food.
Bacteria cause disease:
Bacteria cause a large number of diseases in plants and animals. Micro-organisms that produce disease are called pathogens. Tuberculosis, typhoid, tetanus, whooping cough, cholera and leprosy are some of the diseases caused by bacteria. Blight of rice, caused by bacteria produces pale-green lesions on the paddy leaf which dries up soon after.
Spoilage of Food:
Bacteria spoil food. Curding of milk occurs due to the action of lactobacillus. Milk and fruit juice turn sour after some time due to bacterial action. Fruits, vegetables and fresh foods are also spoilt by bacterial action.