Microbes in Human Welfare Class 12 Biology Notes And Questions

Notes Class 12

Please refer to Microbes in Human Welfare Class 12 Biology notes and questions with solutions below. These revision notes and important examination questions have been prepared based on the latest Science books for Class 12. You can go through the questions and solutions below which will help you to get better marks in your examinations. We have provided the latest Class 12 Biology Notes and Questions for all chapters in your NCERT Class 12 Biology Book.

Class 12 Biology Microbes in Human Welfare Notes and Questions

Microbes In Household Products

  • Micro-organisms such as Lactobacillus and others commonly called lactic acid bacteria (LAB) grow in milk and convert it to curd.
  • During growth, the LAB produces acids that coagulate and partially digest the milk proteins. A small amount of curd added to the fresh milk as inoculum or starter contain millions of LAB, which at suitable temperatures multiply, thus converting milk to curd, which also improves its nutritional quality by increasing vitamin B12.
  • In our stomach, the LAB play very beneficial role in checking disease-causing microbes.
  • The dough, which is used for making foods such as dosa and idlis, also fermented by bacteria.
  • The puffed-up appearance of dough is due to the production of CO2 gas.
  • The large holes in ‘Swiss cheese’ are due to production of a large amount of CO2 by a bacterium named Propionibacterium sharmanii.

Microbes in Industrial Products

Fermented Beverages

  • Saccharomyces cerevisiae is used for bread-making and commonly called brewer’s yeast.
  • Wine and beer are produced without distillation
  • Whisky, brandy and rum are produced by distillation of the fermented broth.


  • Alexander Fleming while working on Staphylococci bacteria, once observed a mould growing in one of his unwashed culture plates around which Staphylococci could not grow.
  • He found out that it was due to a chemical produced by the mould and he named it Penicillin after the mould Penicillium notatum.
  • However, its full potential as an effective antibiotic was established much later by Ernest Chain and Howard Florey.
  • This antibiotic was extensively used to treat American soldiers wounded in World War II.
  • Fleming, Chain and Florey were awarded the Nobel Prize in 1945, for this discovery.

Chemicals, Enzymes And Other Bioactive Molecules

  • Aspergillus niger (a fungus): Citric acid
  • Acetobacter aceti (a bacterium): Acetic acid
  • Clostridium butylicum (a bacterium): Butyric acid
  • Lactobacillus (a bacterium): Lactic acid
  • Saccharomyces cerevisiae (Yeast) is used for commercial production of ethanol
  • Lipases are used in detergent formulations and are helpful in removing oil stains from the laundry.
  • The bottled juices are clarified by the use of pectinases and proteases.
  • Streptokinase produced by the bacterium Streptococcus and modified by genetic engineering is used as a ‘clot buster’ for removing clots from the blood vessels of patients who have undergone myocardial infarction leading to heart attack.
  • Bioactive molecule, Cyclosporin – A, an immunosuppressive agent in organ-transplant patients, is produced by the fungus Trichoderma polysporum.
  • Statins produced by the yeast Monascus purpureus have been commercialized as blood-cholesterol lowering agents.
  • It acts by competitively inhibiting the enzyme responsible for synthesis of cholesterol.
  • A biological control being developed for use in the treatment of plant disease is the fungus Trichoderma.
  • Trichoderma species are free-living fungi that are very common in the root ecosystems. They are effective biocontrol agents of several plant pathogens. Baculoviruses are pathogens that attack insects and other arthropods.

Microbes in Sewage Treatment

Primary Treatment:

  • These treatment steps basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation.
  • These are removed in stages; initially, floating debris is removed by sequential filtration.
  • Then the grit (soil and small pebbles) are removed by sedimentation.
  • All solids that settle form the primary sludge, and the supernatant forms the effluent.
  • The effluent from the primary settling tank is taken for secondary treatment.

Secondary Or Biological Treatment:

  • The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it.
  • This allows vigorous growth of useful aerobic microbes into flocs (masses bacteria associated with fungal filaments to form mesh like structures).
  • While growing, these microbes consume the major part of the organic matter in the effluent.
  • This significantly reduces the BOD (biochemical oxygen demand) of the effluent.
  • BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria.
  • The sewage water is treated till the BOD is reduced.
  • The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water.
  • The greater the BOD of waste water more is its polluting potential. Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment.
  • This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.
  • The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge.
  • During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide.
  • These gases form biogas and can be used as source of energy as it is inflammable. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

Microbes in the Production of Biogas

  • Certain bacteria, which grow anaerobically on cellulosic material, produce large amount of methane along with CO2 and H2.
  • These bacteria are collectively called methanogens, and one such common bacterium is Methanobacterium. These bacteria are commonly found in the anaerobic sludge during sewage treatment.
  • In rumen, these bacteria help in the breakdown of cellulose and play an important role in the nutrition of cattle.
  • The excreta (dung) of cattle, commonly called Gobar, is rich in these bacteria. Dung can be used for generation of biogas, commonly called Gobar gas.
  • The technology of biogas production was developed in India mainly due to the efforts of Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC).
  • Biogas: It is an ideal gas made up of 50-70% methane (CH4), 30-40% CO2, 1-5% H2, traces of O2 and N2 and H2S.

Microbes As Biocontrol Agents

  • The use of biocontrol measures will greatly reduce our dependence on toxic chemicals and pesticides.
  • The Ladybird and Dragonflies are useful to get rid of aphids and mosquitoes, respectively.
  • An example of microbial biocontrol agents that can be introduced in order to control butterfly caterpillars is the bacteria Bacillus thuringiensis (often written as Bt).
  • These are available in sachets as dried spores which are mixed with water and sprayed onto vulnerable plants such as Brassica and fruit trees, where these are eaten by the insect larvae.
  • In the gut of the larvae, the toxin is released and the larvae get killed.

Microbes As Biofertilizers

  • The main sources of biofertilizer are bacteria, fungi and Cyanobacteria.
  • Bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.
  • Fungi are also known to form symbiotic associations with plants (Mycorrhiza).
  • Many members of the genus Glomus form Mycorrhiza.
  • Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc.
  • In paddy fields, Cyanobacteria serve as an important biofertilizer.
Microbes in Human Welfare Class 12 Biology