11th Class Biology Biological Classification And System Of Classification / जैविक वर्गीकरण और वर्गीकरण की प्रणाली Importance of bacteria

Bacteria are our ‘friends and foes’ as they have both useful and harmful activities.

Useful activities

(1) In agriculture or In soil fertility : Some bacteria increase soil fertility. Nitrogen is essential for all plants. Nitrogen occupies 80% of the atmosphere. The plants take nitrogen in the form of nitrates. In soil, nitrates are formed by three processes :

By nitrogen fixing bacteria : Bacteria are found in soil either free e.g., Azotobacter and Clostridium or in root nodules of leguminous plants e.g., Rhizobium leguminosarum. These bacteria are capable of converting atmospheric free nitrogen into nitrogenous compounds.

Nitrifying bacteria : These bacteria convert nitrogen of ammonia into nitrite (NO₂) e.g., nitrosomonas and convert nitrite compounds into nitrates e.g., nitrobacter.

Decay of dead plants and animals : Some bacteria attack on dead bodies of plants and animals and convert their complex compounds into simpler substances e.g., carbon dioxide (CO₂), water (H₂O), nitrate (NO₃), sulphate (SO₄) etc.

(2) In dairy : Bacterium lactici acidi and B. acidi lactici are found in milk. These bacteria ferment lactose sugar found in milk to form lactic acid by which milk becomes sour.

Lactic acid bacteria bring together droplets of casein a protein found in milk and help in the formation of curd. On freezing of casein of milk protein it is fermented by bacteria with the result that foamy and soft substance, different in taste is formed.

(3) In industries : From industrial point of view bacteria are most important. Some of the uses of bacteria in industries are as follows :

Vinegar industry : Vinegar is manufactured from sugar solution in the presence of Acetobacter aceti.

Alcohol and acetone : Clostridium acetobutylicum takes part in the manufacture of butyl alcohol and acetone.

Fibre retting : By this process fibres of jute, hemp and flax are prepared. In the preparation of flax, hemp and jute the retting of stems of Linum usitatissimum (Flax = Hindi Sunn), Cannabis sativa (Hemp = Hindi Patson) and Corchorus capsularis (Jute) respectively is done.

Tobacco industry : Bacillus megatherium is used for its fermentative capacity for developing flavour and taste in tobacco leaves.

Tea industry : By fermentative action of Mycococcus condisans curing of tea leaves is done. By this process special taste is developed in the tea leaves.

Tanning of leather : Some bacteria decompose fats which are found in skin of animal with the result that skin and hairs are separated from each other and this leather becomes ready for use.

Disposal of sewage : Some bacteria convert organic faecal substances e.g., cow dung, decaying leaves of plants, etc. into manure and humus.

Human symbionts : Escherichia coli inhabitats the large intestine of man and other animals and it synthesizes vitamins.

(4) In medicines : Some of the antibiotics are manufactured by bacterial actions e.g., Bacillus brevis – antibiotic thyrothricin and B. subtilis – antibiotic subtelin. Vitamin B₂ is manufactured by fermentative action of Clostridium acetobutylicum.

Antibiotics : These are the chemical substances produces by living microorganisms capable of inhibiting or destroying other microbes. These are the products of secondary and minor metabolic pathways, mostly secreted extracellularly by the microorganisms. These are used in controlling various infectious diseases.

Harmful activities

(1) Food poisoning : Some saprotrophic bacteria cause decay of our food, i.e., they alter their normal form and induce unpleasant aroma, taste and appearance. Some bacteria produce powerful toxins in food to cause "food poisoning". Consumption of such food may cause serious illness or even death.

Botulism : It is caused by Clostridium botulinum. The main symptoms are vomitting followed by paralysis and death.

(2) Spoilage of food : Some examples of bacterial food spoilage are :

Greening on meat surface is caused by Lactobacillus and Leuconostoc. Souring of milk is caused by Lactobacillus and Streptococcus. Explosion of curd (gas production) is caused by Clostridium and Coliform bacteria. Ropiness (i.e., slimy milk) is caused by Klebsiella and Enterobacter sp.

(3) Pollution of water : There are reports of epidemics of cholera, typhoid, jaundice and other infectious diseases, which were caused by polluted water. Many pathogenic bacteria such as, Vibrio cholerae, Salmonella typhi, Leptospira cetero-haemorrhagiae, etc. pollute water and make it unfit for drinking. These are eliminated by chlorination.

(4) Deterioration of textiles : Some bacteria (e.g., Cytophaga, Vibrio and Cellulomonas) damage cellulose of textiles.

(5) Abortion : Bacteria like Salmonella induce abortion in goats, horses, sheep etc.

(6) Biological warfare : Some bacteria which cause diseases like anthrax, black-leg, tuberculosis etc, are employed as secret war agents.

(7) Denitrification : Denitrification bacteria like Bacillus licheniformis, Pseudomonas aeruginosa convert nitrates and nitrites into free nitrogen, thus responsible for the process of denitrification. Thus soil is depleted of essential nutrient like usable form of nitrogen.

(8) Putrefaction : It is the spoilage of protein in the absence of O₂ by the putrefying bacteria e.g., Proteus, Mycoides.

(9) Retting of fibres : It is the hydrolysis of pectic substances that bind the cells together. e.g., Clostridium sp., Pseudomonas fluorescence.

(10) Diseases : They are listed below :

Mycoplasmas were discovered by E. Nocard and E. R Roux (1898). They were first isolated from bovine sheep suffering from pleuropneumonia. They are often designated as pleuropneumonia–like organisms (PPLO). These organisms were later put under the generic name mycoplasma by Nowak (1929). In 1966 international commitee of Nomanclature of bacteria, placed mycoplasmas under the class mollicutes, which consists of two genera Mycoplasma and Acholeplasma. These are the simplest and smallest unicellular non-motile known aerobic prokaryotes without cell wall. So that they can change their shape therefore called Jockers of microbiological park.

Structure : They are one of simplest prokaryotic and gram negative organisms. Their size varies from 0.1 – 0.15 mm. They lack the cell wall. Due to the absence of the cell wall, these organisms are highly elastic and readily change their shape; hence the mycoplasmas are irregular and quite variable in shape. That is called pleomorphism. They may be coccoid, granular, pear–shaped, cluster–like or filamentous. Mycoplasma cells are covered with three layered plasma membrane.

Unit membrane is made up of lipoprotein. They lack the well organised nucleus, endoplasmic reticulum, mitochondria, plastids, golgi bodies, centrioles, flagella, etc. The genetic material is a single, linear, double–stranded molecule of DNA, without a nuclear envelope. Unlike other prokaryotes, it is coiled throughout the cytoplasm. The cytoplasm contains the ribosomes which are 70S. It also contains RNA, proteins, lipids and many kinds of enzymes used in biosynthetic reactions. Lipids include cholestrol and cholestrol esters which are characteristic of animal cells and are not found in true bacteria and cyanobacteria. The amount of DNA and RNA in the cells is usually less than half of that which occurs in other prokaryotes. There is 4% DNA and 8% RNA. Mycoplasmas are gram-negative.

Physiology and reproduction : Mycoplasma are usually non–motile. They are sensitive to tetracycline and resistant to penicillin. These are destroyed usually by treatment of heat at 50^o C for 6 hours mycoplasma are osmotically inactive. Mycoplasmas are heterotrophic in their mode of nutrition. Some of them are saprotrophs, but most of them are parasitic on plants and animals including man. They reproduce by budding or binary fission. Fragmentation specially in filamentous forms. Besides this, Mycoplasma reproduces by elementary cell bodies also. It is also called bleb particle. It is a kind of vegetative reproduction.