Microbes in Human Welfare Class 12 Notes Biology Chapter 10


  • Microbes are various – bacteria, fungi, protozoa, bacteria, fungi and minute plants viruses, viroids furthermore prions (proteinaceous agents of infection).
  • Microbes are present everywhere that is in soil, water, air, inside the body of plants and animals, inside thermal vents, deep in the soil, under the layers of snow several meters thick and in highly acidic environments.
  • Microbes can be grown on culture nutritive media to form colonies which are used for mass production of certain desirable products and for scientific study of metabolism, genetics and structure of micro-organism.


  • LAB is lactic acid bacteria which produce acids that coagulates and partially digest the milk proteins. Milk is changed into curd, yoghurt & cheese etc.
  • Louis Pasteur discovered fermentation in 1857.
  • The dough, which is used for making foods such as dosa and idli is fermented by bacteria. The puffed-up appearance of dough is due to the production of CO2 gas which is released by the microbes.
  • Similarly, the dough, which is used for making bread, is fermented usingbaker’s yeast (Saccharomyces cerevisiae).
  • A number of traditional drinks and foods are also made by fermentation by the microbes. ‘Toddy’, a traditional drink of some parts of southern India is made by fermenting sap from palms.
  • Ethanol, also called ethyl alcohol, is the alcohol found in alcoholic beverages. It is produced by fermenting sugars with yeast (Saccharomyces cerevisiae).
  • Antibiotics are chemical substances, which are produced by some microbes and can kill or retard growth of other disease causing microbes.
  • Penicillin was the first antibiotic to be discovered. Alexander Fleming while working on Staphylococci plates observed that in one of unwashed culture plate, it could not grow because of chemical produced by mould Penicilliun notatum grown on it which produces chemical penicillin.
  • Antibiotics have been extensively used for treatment of certain epidemic diseases like cough, diphtheria, leprosy, plague, etc.
  • Examples of acid producers are Aspergillus niger (a fungus) of citric acid, Acetobacter aceti (a bacterium) of acetic acid, Clostridium butylicum (a bacterium) of butyric acid, Lactobacillus (a bacterium) of lactic acid, Saccharomyces cerevisiae oflactic acid etc.
  • Streptokinase, produced by bacterium Streptococcus, is used as clot buster for removing blood clot in patients suffering from myocardial infractions.
  • Cyclosporin A is a bioactive immunosuppressive agent and used in organ transplant patient. It is produced by Trichoderma polysporum.
  • Statin is blood cholesterol lowering agent produced by yeast Monascus and it acts bycompetitively inhibiting enzyme responsible for cholesterol synthesis. Thiobacillus thiooxidans and thiobacillus ferroxidans, when grown in presence of copper ore produce acid and affect oxidation process known as leaching and improves recovery of metal from ore and is non-polluting to the atmosphere.
  • Wastewater treatment process are carried out by municipal treatment plant as a community process. These are primary treatment & secondary treatment. Primary (1°) Treatment : This is a physical process that involves physical removal of particles from the sewage through filtration and sedimentation. So, it remove coarse solids and settleable solids in sedimentation units (tanks, basin, or mechanical devices) by which particulate material is concentrated as sludge. The sludge and liquid effluent are processed separately. The effluent from the primary settling tank is taken to secondary treatment.
  • Secondary (2°) treatment: This treatment involves the oxidation of organic material in effluent by microbial method. These are –
    • Activated sludge process.
    • Filteration by intermittent sand filters and trickling filter.
  • In activated sludge process, aeration of sewage in large aeration and agitation tank, lead to the formation of floes (masses of bacteria associated with fungal filaments to form mesh like structure) and floccules. When they are vigorously aerated, flocculation occurs and this contain large number of actively metabolising bacteria, yeast, mould and protozoa. This growth is effective in oxidation of organic compound. This significantly reduces BOD (Biochemical oxygen demand).
  • BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria.
  • The sewage water is treated till the BOD is reduced. BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, more is its polluting potential.
  • Anaerobic sludge digester : Solid which accumulates during sedimentation are pumped into sludge digester. This sludge is digested with the help of anaerobic and facultative bacteria. These micro-organisms bring about a decrease in organic solid by degrading them to soluble substances and gaseous products which has 60-70% methane and 20-30% CO2. This gas mixture is used as biogas for heating purpose and for operating power.
  • Composting is decomposition of dewatered sludge by thermophilic bacteria under forced aeration. Effective period of time is 21 days after which it transforms into humus type material and is suitable for use as a soil conditioner.


  • Biogas is a gaseous fuel, produced by methanogens, which are anaerobic bacteria growing on cellulosic material and produce large amount of methane along with CO2 and H2, e.g., Methanococcus, Methanobacterium.
  • Biogas plant works based on the process called anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste or any other biodegradable feedstock under anaerobic conditions.


  • Use of predators, animal composite, micro-organisms such as bacteria/virus to control insect pest, pathogen and weed is called biological control and biological agents are called biocontrol agents.
  • Bt cotton is genetically modified cotton having Bacillus thuringiensis genes. Bacterium Bacillus thuringiensis (Bt) is used as microbial control agent in order to control butterfly caterpillars. It is sprayed on plants like Brassica and fruit trees which are attacked by insect larvae. Et release toxin and insect larvae are killed. It is killing only caterpillars, leaving all other insects unharmed.
  • Biofertilisers are organisms that enrich nutrient quality of soil. The main source is bacteria, fungi and cyanobacterium.
  • Examples:
    • Rhizobium species in root nodules of leguminous plant.
    • Azospirillium and Azotobacter are free living species in soil.
    • Fungi : Glomus in symbiotic association with plant where fungal symbiont absorb phosphorus from soil and passes to plants. Plants inturn show better resistance to root borne pathogens, tolerance to salinity and draught, overall growth of plant.
    • Cyanobacteria such asAnabaena, Nostoc and Oscillatoria (autotrophic microbes) in paddy field.
  • In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.
  • Organic farming is use of biofertilisers, biocontrol agents and biologically produced compost.

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