Importance Of Bacteria

Biological Classification of Class 11

Harmful activities

(i) Spoilage of Food : Saprotrophic bacteria cause rotting of vegetables, fruits, meat, bread, souring of milk, cheese, butter and spoilage of jams, jellies and pickles.

(ii) Food Poisoning : Botulism is caused by an anaerobic bacterium Clostridium botulinum (=C. perfringens). The bacterium infects canned food. Common food poisoning is caused by Staphylococcus aureus. The poisoning is accompanied by diarrhoea and vomiting. Another is salmonellosis which is generally produced on eating contaminated meat. Bacterium causing this type of poisoning is Salmonella enteridis and S. typhimurium.

(iii) Deterioration of Domestic Articles : Spirochaete cytophaga deteriorates cotton fibres, leather and wooden articles.

(iv) Destruction of Penicillin : Bacillus brevis destroys penicillin.

(v) Denitrification of Soils : Thiobacillus denitrificans and Micrococcus denitrificans convert nitrates of the soil into gaseous nitrogen.

(vi) Desulphurification of Soils : Desulfovibrio desulfuricans changes soil sulphates into hydrogen sulphide.

(vii) Disease : Over 90% of human and animal diseases are caused by bacteria and over 40% of plant diseases are due to them.

Beneficial Activites

Role in Agriculture

(a) Nature’s Scavengers : Alongwith saprotrophic fungi, saprotrophic bacteria cause decay and decomposition of dead bodies of plants and animals. In the process they cleanse the earth and release raw materials for new generations.

(b) Sewage Disposal : Organic content of sewage is broken down by the bacteria.

(c) Ammonifying Bacteria : Bacillus vulgaris releases ammonia from amino acids.

(d) Nitrifying Bacteria : Nitrosomonas and Nitrosococcus oxidise ammonium salts to nitrites. The nitrites are further changed into nitrates by Nitrobacter and Nitrocystis.

(e) Nitrogen Fixing Bacteria : A few free living bacteria are able to pick up dinitrogen from the soil atmosphere and convert it into organic nitrogenous materials like amino acids, e.g., Azotobacter, Beijerinckia, Clostridium pasteurianum.

Symbiotic nitrogen fixing bacteria of the genus Rhizobium occur in the root nodules of a number of legumes. These legumes are used in crop rotation and green manuring. The plants as well as their seeds are rich in proteins. Root nodules containing symbiotic nitrogen bacteria also occur in Casuarina and Alnus. Leaf nodules containing such bacteria are found in Ardisia.

(f) Manure : The saprotrophic bacteria convert farm refuse, dung and other organic wastes into manure.

(g) Gobar Gas Plants : They employ bacteria for converting animal dung and other organic wastes into manure alongwith production of fuel gas.

(h) Sulphur Bacteria : Beggiatoa and other sulphur bacteria pick up H2S released during putrefaction of proteins to produce sulphates.

(i) Ensilage : Ensilage is preserved cattle feed or fodder. It is formed by packing fresh chopped fodder in silos sprinkled with molasses. Fermentation activity of bacteria produces lactic acid which has a preservative action.

Role in Industry

(j) Dairy Industry : Lactic acid bacteria (e.g., Streptococcus lactis) convert milk sugar lactose into lactic acid.

Importance of Bacteria

Lactic acid is able to coagulate milk protein casein. This converts milk into yoghurt, curd and cheese.

(k) Lactic Acid : Lactic acid is commercially obtained from ammoniated sugar solution through fermentation caused by Lactobacillus delbreuckii. Acid is used in food preservation, tanning and preparation of a number of drugs.

(l) Vinegar : Acetic acid bacteria (Acetobacter aceti) oxidise ethyl alcohol into acetic acid. Ethyl alcohol is obtained from molasses.

(m) Butyl Alcohol and Acetone : Clostridium acetabutylicum is able to produce butyl alcohol, methyl alcohol and acetone from molasses.

(n) Retting of Fibres : Stem and leaf fibres are separated from softer tissues by bacterial action of two types - dew retting (e.g., Pseudomonas fluorescence) and anaerobic retting (e.g., Clostridium spp).

(o) Curing : Leaves of tea and tobacco are cured off their bitterness with the help of certain bacteria, e.g., Bacillus megatherium. Beans of coffee and cocoa are similarly cured.

(p) Cleaning of Hides : Hides are cleaned of their fat, hair and other attached tissues by bacterial action. Commercial sponges are cleaned similarly.

(q) Antibiotics : A number of antibiotics are obtained from mycelial bacterium.

(r) Vitamins : Riboflavin was formely prepared from Clostridium butylicum. Cobalamine (B12) is obtained from bacteria like Bacillus megatherium. Acetic acid bacteria are used in some steps during the preparation of vitamin C. Escherichia coli present in the human intestine produces large quantities of vitamin K and B complex vitamins.

Animal Diseases Plant Diseases

Disease Causal bacterial species Disease Causal bacterial species

Cholera Vibrio cholerae Angular leaf spot Xanthomonas malvacearum

Typhoid Salmonella typhi of cotton

Tetanus Clostridium tetani Blight of beans Pseudomonas phaseolicola

Diphtheria Corynebacterium diptheriae Blight of paddy Xanthomonas oryzae

Whooping cough Bordetella pertussis Canker of citrus Xanthomonas citri

Pneumonia Diplococcus pneumoniae Tundu of wheat Corynebacterium tritici

Tuberculosis Mycobacterium tuberculosis Wilt of maize Xanthomonas stewartii

Leprosy Mycobacterium leprae Crown gall of apple Agrobacterium tumaefaciens

Bubonic plague Pasteurella pestis Leaf spot of cucumber Xanthomonas cucurbitae                


Cyanobacteria (Gk. Cyano = blue, bact = rod) or blue green algae are Gram negative photosynthetic prokaryotes, being the most primitive organisms to have oxygenic photosynthesis. They are the most successful and self dependent organisms on the earth and survived successfully for more than three billion years. They added oxygen to the atmosphere, which is indispensible for the existence of aerobic forms of living organisms.


They are mainly fresh water forms, though few are marine. A few thermophilic species grow in hot water springs having a temperature range 70°C - 75°C (e.g., Phormidium) and others grow at very low temperature in the polar regions (e.g., Nostoc, Schizothrix, etc.,). Red sea is named because of abundant occurrence of a cyanobacterium, Trichodesmium erythraeum, which imparts red coloration to water. They occur in symbiotic association with almost every group of eukaryotes.

Structural organization

They may be unicellular and multicellular. The latter may be colonial or filamentous. Unicellular forms may be spherical or oval in shape.

Filamentous form consists of one or more cellular strands, called trichomes, surrounded by mucilaginous sheath. A filament may contain one (e.g., Oscillatoria) to many trichomes (e.g., Schizothrix).

Cyanobacteria are characterized by the absence of flagellum even in motile forms, although most of these are motile.

Cell structure

The cell structure in cyanobacteria is basically similar to that in bacteria. The cell lacks a well defined nucleus and the chromatin material is centrally located resembling the bacterial chromosome. Like bacteria, small circular DNA segments may also occur in addition to nucleoid. They are known as plasmids or transposers.

The cell wall is invariably covered by mucilaginous sheath, composed largely of mucopeptide.

Cell protoplast is often differentiated into outer pigmented chromoplasm and inner colourless centroplasm.

A coiled membranous outgrowth of plasmalemma, called lamellosome is present.

The protoplast lacks membrane bound organelles and contains 70S ribosomes.

The sap vacuoles, found in bacterial cell, are absent. Instead, the cell may contain gas filled vacuoles that help to regulate the buoyancy of the organism in water.

The characteristic feature of a cyanobacterial cell is the presence of a system of photosynthetic lamellae called thylakoids, which make the structure more elaborate in comparison to that in bacteria. The characteristic photosynthetic pigments present in the thylakoids are chlorophyll a and phycobilins i.e., phycocyanin (blue coloured) allophycocyanin (blue) and phycoerythrin (red coloured).

The cell contains reserve food material in the form of special starch called cyanophycean starch. Other granules present in a cyanobacterial cell are volutin granules and polyhedral bodies.


They are most self dependent organisms, because most of these are capable of converting atmospheric nitrogen into ammonium compounds besides utilizing atmospheric CO2 for synthesis of organic food during photosynthesis. Nitrogen fixation under anaerobic conditions mainly occurs in specialized cells called heterocysts. Heterocysts are large sized pale coloured mucilage free thick walled cells which are impermeable to oxygen.


Cyanobacteria reproduce vegetatively and asexually. Typically sexual reproduction is absent. Gene recombination is, however, reported to occur. Cyanobacteria multiply by  

  • Binary fission: It occurs in unicellular forms. The daughter cells formed by amitotic division, separate immediately after the division.
  • Fragmentation: It occurs in filamentous forms. The filament breaks up into short pieces or fragments that grow to form new filaments.
  • Hormogones: They are small trichome segments which separate from the parent due to death of intervening cells (necridia).

Importance of cyanobacteria

They play significant role in evolution of aerobic forms of life.

They convert atmospheric nitrogen into ammonium compounds and excess of these compounds is excreted out enriching the soil.

Cyanobacteria like Nostoc and Anabaena have been used for reclaiming usar soils. They benefit the partner by providing nitrogenous compounds becasue of their capability of nitrogen fixation.

Some cyanobacteria like Nostoc serve as food.

Extract of Lyngbia, a cyanobacterium is used for manufacture of antibiotic.

Some cyanobacteria like Microcystis aeruginosa, Anabaena flosaquae, Aphanizomenon flos-aquae cause algal blooms as they secrete toxins into the surroundings, which are harmful to aquatic animals and even to human beings, this leads to eutrophication of water bodies.


They are ancient, and the most primitive prokaryotes. They are living under extremely adverse conditions like very high temperature (hot water sprigns) and high salt concentration (salt marshes). Very few other organisms can survive under such environmental conditions. They are termed living fossils as they have survived the geological changes successfully.

The cell wall in archaebacteria contains proteins and non cellulosic polysaccharides. It lacks peptidoglycan, the characteristic cell wall material in bacteria and cyanobacteria. The cell membrane contains branched chain of lipids. This chemical composition of the cell membrane enables these organisms to withstand extremes of temperature and pH.

They are grouped into obligate anaerobes and facultative anaerobes.

Obligate or strict anaerobes

They live under anaerobic conditions only and get killed in presence of oxygen.

These are further divided into two sub groups, the methanogens and halophiles.

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