Microbes in Human Welfare
Welcome to HSLC Guru. This page provides complete English-medium notes, summary, question answers, MCQs, fill-in-the-blanks, true/false statements and a glossary table for Class 12 Biology Chapter 10 — Microbes in Human Welfare based on the ASSEB syllabus. The content is prepared in simple language to help you understand how microbes are useful in our daily life, in industry, agriculture and in the management of waste, and to help you score well in the HS final examination.
Summary
Microbes are everywhere — in soil, water, air, in our food and even inside our body. Although a few microbes cause diseases, the majority of them are extremely useful to mankind. In our household, the bacterium Lactobacillus converts milk into curd by producing lactic acid; the same bacterium also gives curd its nutritional value because it produces vitamin B12. The yeast Saccharomyces cerevisiae, popularly known as baker’s yeast, is used in the preparation of bread and in the fermentation of batter for dosa and idli. The fungus Aspergillus is associated with the fermentation of sap of palms to produce toddy. In cheese production, Penicillium roquefortii ripens the famous Roquefort cheese, while Propionibacterium shermanii produces the large holes in Swiss cheese due to the release of CO2.
Microbes are extensively used in industry to produce fermented beverages such as wine, beer, whisky, brandy and rum. Saccharomyces cerevisiae (brewer’s yeast) is the chief organism used in fermentation of fruit juices and malted cereals. Antibiotics are perhaps the greatest gift of microbes to mankind. Penicillin was the first antibiotic to be discovered by Alexander Fleming in 1928 from the mould Penicillium notatum. Later, Ernest Chain and Howard Florey developed it as a drug. Streptomycin, used in the treatment of tuberculosis, is obtained from the actinomycete Streptomyces. Microbes also produce useful chemicals such as citric acid (Aspergillus niger), acetic acid (Acetobacter aceti), butyric acid (Clostridium butylicum) and lactic acid (Lactobacillus). Industrially important enzymes such as lipase, pectinase and protease are obtained from microbes; streptokinase from Streptococcus is used as a clot-buster, cyclosporin A from Trichoderma polysporum is used as an immuno-suppressive agent in organ transplant patients, and statins from Monascus purpureus are used as cholesterol-lowering agents.
Microbes play an important role in the treatment of sewage, the municipal waste-water that contains a large amount of organic matter and pathogens. Sewage treatment is carried out in three stages — primary treatment (physical removal of small and large particles by filtration and sedimentation), secondary treatment or biological treatment (the primary effluent is passed into aeration tanks where vigorous growth of useful aerobic microbes forms flocs that consume the major part of organic matter and reduce the BOD), and finally the activated sludge is passed into anaerobic sludge digesters where methanogenic bacteria like Methanobacterium produce a mixture of methane, hydrogen sulphide and carbon dioxide called biogas. Tertiary treatment removes remaining nutrients and pathogens before releasing the effluent into water bodies. Biogas is used as fuel for cooking and lighting; the technology of biogas production was developed in India by IARI and KVIC.
Microbes are also used as biocontrol agents against plant pathogens and pests as a safer alternative to chemical pesticides. Trichoderma species are free-living fungi that act as biocontrol agents in several plant diseases. The bacterium Bacillus thuringiensis (Bt) produces crystals containing proteins that are toxic to insect larvae, and is used to control butterfly caterpillars. Baculoviruses, particularly the genus Nucleopolyhedrovirus (NPV), are used as species-specific narrow-spectrum insecticides. As biofertilisers, Rhizobium in symbiosis with leguminous plants fixes atmospheric nitrogen in root nodules; Azotobacter is a free-living nitrogen-fixing bacterium; cyanobacteria like Anabaena and Nostoc add organic matter and fix nitrogen in paddy fields; and Mycorrhiza (VAM) — a symbiotic association of fungus with roots — absorbs phosphorus from soil and protects plants from root pathogens. Together, these microbes increase soil fertility without polluting the environment.
1-Mark Question Answers
Q1. Name the bacterium that converts milk into curd.
Answer: Lactobacillus (lactic acid bacteria, LAB).
Q2. Which yeast is used in the preparation of bread?
Answer: Saccharomyces cerevisiae, commonly known as baker’s yeast.
Q3. Who discovered penicillin and from which fungus?
Answer: Penicillin was discovered by Alexander Fleming in 1928 from the fungus Penicillium notatum.
Q4. Name the microbe used in the production of citric acid.
Answer: Aspergillus niger, a fungus.
Q5. What is biogas?
Answer: Biogas is a mixture of gases (mainly methane, with CO2 and H2S) produced by the microbial activity of methanogens like Methanobacterium on organic waste, used as fuel.
Q6. Name two free-living nitrogen-fixing bacteria used as biofertilisers.
Answer: Azotobacter and Azospirillum.
Q7. Which microbe is used as a clot-buster in heart-attack patients?
Answer: Streptokinase, an enzyme obtained from the bacterium Streptococcus.
Q8. Name the source organism of cyclosporin A.
Answer: The fungus Trichoderma polysporum.
Q9. Which fungus produces statins?
Answer: The yeast Monascus purpureus.
Q10. What causes the large holes in Swiss cheese?
Answer: The release of large amounts of CO2 by the bacterium Propionibacterium shermanii.
Q11. Name the symbiotic association of fungus with the roots of higher plants.
Answer: Mycorrhiza; the most common type is VAM (Vesicular Arbuscular Mycorrhiza).
Q12. What is the role of Acetobacter aceti?
Answer: It is a bacterium used in the commercial production of acetic acid (vinegar).
2-3 Mark Question Answers
Q1. How does Lactobacillus help in the formation of curd? Mention any two advantages.
Answer: When a small amount of curd is added to warm milk, the lactic acid bacteria Lactobacillus multiply and convert the milk sugar lactose into lactic acid. The acid coagulates the milk protein casein, producing the semi-solid curd. Advantages: (i) curd is more digestible than milk and is rich in vitamin B12 produced by the bacteria; (ii) lactic acid checks the growth of harmful disease-causing bacteria in the gut and improves intestinal health.
Q2. Differentiate between Roquefort cheese and Swiss cheese.
Answer: Roquefort cheese is ripened by the fungus Penicillium roquefortii, which gives it a strong flavour and characteristic blue-green veins. Swiss cheese is produced with the help of the bacterium Propionibacterium shermanii, which liberates large amounts of CO2 — these gas bubbles form the large holes seen in Swiss cheese. Thus the two differ in the microbe used, the type of texture and the flavour developed.
Q3. What are antibiotics? Why is penicillin called a “wonder drug”?
Answer: Antibiotics are chemical substances produced by some microbes which can kill or inhibit the growth of other disease-causing microbes (the term means “against life”). Penicillin is called a wonder drug because it was the first antibiotic discovered (by Fleming in 1928 from Penicillium notatum) and saved the lives of countless soldiers wounded in the Second World War. It is effective against a wide range of bacterial infections such as pneumonia, meningitis and diphtheria.
Q4. What is BOD? Why does the BOD of sewage decrease after secondary treatment?
Answer: BOD (Biochemical Oxygen Demand) is the amount of oxygen required by microorganisms to oxidise the organic matter present in one litre of water. The greater the organic load, the higher the BOD. During secondary treatment of sewage, vigorous growth of aerobic microbes forms flocs in the aeration tank, and these microbes consume the major part of the organic matter, thereby drastically reducing the BOD of the effluent and making it suitable for discharge.
Q5. Name two industrial chemicals produced by microbes along with the producing organism.
Answer: (i) Citric acid is produced by the fungus Aspergillus niger. (ii) Acetic acid is produced by the bacterium Acetobacter aceti. In addition, Lactobacillus produces lactic acid and Clostridium butylicum produces butyric acid. These chemicals are used in food, beverages, leather and pharmaceutical industries.
Q6. Mention any two roles of Bacillus thuringiensis in agriculture.
Answer: (i) Spores of Bacillus thuringiensis (Bt) contain protein crystals that, on entering the gut of insect larvae, are released as toxic proteins which kill the larvae — thus Bt is sprayed as a biopesticide against butterfly caterpillars. (ii) The Bt toxin gene (cry gene) has been transferred into crop plants like Bt cotton and Bt brinjal to develop pest-resistant transgenic crops, reducing dependence on chemical pesticides.
5-7 Mark Question Answers
Q1. Describe with examples the role of microbes in household products.
Answer: Microbes are used in our households for the production of many fermented food products. (1) Curd: The bacterium Lactobacillus (LAB) is added to warm milk where it multiplies and converts lactose into lactic acid; the acid coagulates milk protein to give curd. The curd is rich in vitamin B12 and improves digestion. (2) Bread: Baker’s yeast Saccharomyces cerevisiae ferments the sugar in the dough liberating CO2 which makes the dough rise; on baking, the gas pockets are trapped giving a soft spongy bread. (3) Dosa and Idli: A batter of rice and black gram is fermented overnight by bacteria and yeasts (Saccharomyces and LAB) which produce CO2 and make the batter puffy. (4) Toddy: A traditional drink of southern India prepared by fermenting the sap of palm trees by the action of Aspergillus and yeasts. (5) Cheese: Bacteria like LAB ferment milk; Penicillium roquefortii ripens Roquefort cheese while Propionibacterium shermanii develops the holes in Swiss cheese. Other examples include fermented fish (Ngari), soya sauce and pickles.
Q2. Describe the role of microbes in industrial products with respect to (a) fermented beverages, (b) antibiotics, (c) chemicals and (d) bioactive molecules.
Answer: (a) Fermented beverages: The yeast Saccharomyces cerevisiae (brewer’s yeast) is used to ferment fruit juices and malted cereals to produce wine, beer, whisky, brandy and rum. Wine and beer are produced without distillation while whisky, brandy and rum are produced after distillation of the fermented broth. (b) Antibiotics: Penicillin was the first antibiotic discovered by Alexander Fleming in 1928 from Penicillium notatum; it was developed as a drug by Chain and Florey. Streptomyces gives streptomycin (TB), Bacillus brevis gives tyrothricin and Streptomyces venezuelae gives chloromycetin. (c) Chemicals: Citric acid (Aspergillus niger), acetic acid (Acetobacter aceti), butyric acid (Clostridium butylicum) and lactic acid (Lactobacillus) are produced industrially by microbes. Microbes also produce ethanol, vitamins (B2, B12) and amino acids. (d) Bioactive molecules: Lipase is used in detergents and to remove oil stains; pectinase and protease are used to clarify bottled juices; streptokinase from Streptococcus is used as a “clot buster” to remove clots in heart patients; cyclosporin A from Trichoderma polysporum is used as an immunosuppressive agent in organ transplant patients; and statins from Monascus purpureus lower blood cholesterol by inhibiting the enzyme that synthesises cholesterol.
Q3. Explain the various stages of sewage treatment and the role of microbes in it.
Answer: Sewage is the municipal waste-water containing a large amount of human faeces and other organic matter together with pathogenic microbes. It cannot be released directly into rivers; it has to be treated in Sewage Treatment Plants (STPs) in the following stages. (1) Primary treatment: This is a physical process. The sewage is passed through grit chambers and then through sedimentation tanks where the floating debris is removed by sequential filtration and the grit (soil and pebbles) settles down. The sediment forms the primary sludge, while the supernatant forms the primary effluent. (2) Secondary or Biological treatment: The primary effluent is passed into large aeration tanks where it is constantly agitated and air is pumped in. This allows vigorous growth of useful aerobic microbes in the form of flocs (mesh-like masses of bacteria with fungal filaments). These microbes consume the major part of the organic matter, thereby reducing the BOD of the effluent significantly. The effluent is then passed into a settling tank where the bacterial flocs sediment as activated sludge; a small part of it is used as inoculum for the aeration tank, and the rest is pumped into anaerobic sludge digesters. (3) Inside the digesters, anaerobic bacteria including methanogens like Methanobacterium digest the bacteria and the sludge to produce a mixture of methane, hydrogen sulphide and carbon dioxide called biogas, which is used as a source of energy. (4) Tertiary treatment: The remaining effluent is treated to remove additional nitrogen, phosphorus and pathogens, and is finally released into water bodies. Thus, microbes play a central role in cleaning sewage and producing useful biogas in the bargain.
Q4. What are biofertilisers? Discuss the various types with examples.
Answer: Biofertilisers are living organisms that enrich the nutrient quality of the soil. Unlike chemical fertilisers, they do not pollute the soil and water. The main types are: (i) Bacterial biofertilisers: Rhizobium lives in symbiotic association with the root nodules of leguminous plants such as pea, gram and pulses, where it fixes atmospheric nitrogen into ammonia. Azotobacter and Azospirillum are free-living soil bacteria that fix nitrogen non-symbiotically. (ii) Cyanobacteria (Blue-Green Algae): Anabaena, Nostoc and Oscillatoria add organic matter and fix atmospheric nitrogen in paddy fields; Anabaena azollae lives symbiotically with the water fern Azolla and is used in rice fields. (iii) Fungal biofertilisers — Mycorrhiza: A symbiotic association of fungus with the roots of higher plants. The most common is VAM (Vesicular Arbuscular Mycorrhiza) formed by the fungal genus Glomus. The fungus absorbs phosphorus from the soil and passes it to the plant, while it also imparts resistance to root pathogens, tolerance to drought and overall increase in plant growth. The use of biofertilisers is an important component of organic farming.
Q5. What are biocontrol agents? Discuss the role of Trichoderma, Bacillus thuringiensis and Baculovirus as biocontrol agents.
Answer: Biocontrol agents are living organisms used to control plant pests and pathogens, instead of using chemical insecticides and pesticides which pollute the environment and are toxic to humans and animals. (1) Trichoderma: Free-living fungi very common in the root ecosystems. Several species of Trichoderma are effective biocontrol agents against soil-borne plant pathogens such as Pythium and Fusarium that cause damping off and wilt diseases. (2) Bacillus thuringiensis (Bt): A bacterium whose spores contain protein crystals (Cry proteins). When the dry spores are sprayed on crops, insect larvae eat them; in the alkaline gut of the larvae the crystals dissolve releasing the toxin which kills the larvae. Bt is highly specific to butterfly caterpillars and is harmless to other animals. The Bt toxin gene has also been incorporated into crops like Bt cotton to make them pest-resistant. (3) Baculoviruses: Pathogens that attack insects and other arthropods. The genus Nucleopolyhedrovirus (NPV) is the most useful — it is highly species-specific, has a narrow spectrum of insecticidal action and has no negative effect on plants, mammals, birds and non-target insects. Hence it is desirable in IPM (Integrated Pest Management) programmes and in organic farming.
Multiple Choice Questions (MCQs)
Q1. The bacterium responsible for converting milk into curd is —
(a) Streptococcus
(b) Lactobacillus
(c) Bacillus
(d) Clostridium
Answer: (b) Lactobacillus.
Q2. Saccharomyces cerevisiae is used in the preparation of —
(a) Curd
(b) Cheese
(c) Bread
(d) Antibiotic
Answer: (c) Bread.
Q3. The large holes in Swiss cheese are due to —
(a) Penicillium roquefortii
(b) Propionibacterium shermanii
(c) Lactobacillus
(d) Aspergillus niger
Answer: (b) Propionibacterium shermanii.
Q4. Penicillin was discovered by —
(a) Robert Koch
(b) Louis Pasteur
(c) Alexander Fleming
(d) Edward Jenner
Answer: (c) Alexander Fleming.
Q5. Citric acid is commercially produced by —
(a) Acetobacter aceti
(b) Aspergillus niger
(c) Lactobacillus
(d) Saccharomyces
Answer: (b) Aspergillus niger.
Q6. Cyclosporin A, an immunosuppressive drug, is obtained from —
(a) Monascus purpureus
(b) Trichoderma polysporum
(c) Penicillium notatum
(d) Streptococcus
Answer: (b) Trichoderma polysporum.
Q7. The “clot-buster” enzyme used in heart-attack patients is —
(a) Lipase
(b) Pectinase
(c) Streptokinase
(d) Protease
Answer: (c) Streptokinase.
Q8. The methanogen that produces biogas from sludge is —
(a) Methanobacterium
(b) Rhizobium
(c) Azotobacter
(d) Anabaena
Answer: (a) Methanobacterium.
Q9. The biofertiliser association of fungus with plant roots is called —
(a) Lichen
(b) Mycorrhiza
(c) Nodule
(d) Mycelium
Answer: (b) Mycorrhiza.
Q10. Bacillus thuringiensis is used to control —
(a) Plants
(b) Butterfly caterpillars
(c) Mammals
(d) Birds
Answer: (b) Butterfly caterpillars.
Fill in the Blanks
Q1. The fungus _________ is used in the production of toddy from palm sap.
Answer: Aspergillus.
Q2. _________ is the bacterium used in symbiosis with leguminous plants for nitrogen fixation.
Answer: Rhizobium.
Q3. Statins, used as cholesterol-lowering agents, are obtained from _________.
Answer: Monascus purpureus.
Q4. The full form of BOD is _________.
Answer: Biochemical Oxygen Demand.
Q5. The Baculovirus genus used as a narrow-spectrum biocontrol agent is _________.
Answer: Nucleopolyhedrovirus (NPV).
True / False
Q1. Lactobacillus produces vitamin B12 in curd.
Answer: True.
Q2. Penicillin was discovered by Louis Pasteur.
Answer: False. (Penicillin was discovered by Alexander Fleming in 1928.)
Q3. Acetic acid is produced commercially by Acetobacter aceti.
Answer: True.
Q4. Activated sludge is rich in aerobic microbes.
Answer: True.
Q5. Anabaena and Nostoc are fungal biofertilisers.
Answer: False. (They are cyanobacteria, i.e., blue-green algae.)
Glossary
| Term | Meaning |
|---|---|
| Lactobacillus | Lactic acid bacterium that converts milk into curd and produces vitamin B12. |
| Saccharomyces cerevisiae | Baker’s/brewer’s yeast used in bread, dosa, idli and fermented beverages. |
| Aspergillus | Fungus involved in fermentation of palm sap to produce toddy; A. niger produces citric acid. |
| Penicillium roquefortii | Fungus that ripens Roquefort cheese giving its blue veins and flavour. |
| Propionibacterium shermanii | Bacterium that produces large holes in Swiss cheese by releasing CO2. |
| Antibiotic | Chemical produced by a microbe that kills or inhibits another microbe. |
| Penicillin | First antibiotic, discovered by Fleming (1928) from Penicillium notatum. |
| Streptomycin | Antibiotic from Streptomyces, used in the treatment of tuberculosis. |
| Citric acid | Organic acid produced commercially by Aspergillus niger. |
| Acetic acid | Vinegar acid produced by Acetobacter aceti. |
| Streptokinase | Clot-busting enzyme obtained from Streptococcus. |
| Cyclosporin A | Immunosuppressive drug from Trichoderma polysporum used in organ transplants. |
| Statins | Cholesterol-lowering compounds from Monascus purpureus. |
| BOD | Biochemical Oxygen Demand — oxygen needed by microbes to oxidise organic matter. |
| Flocs | Mesh-like masses of bacteria with fungal filaments formed in aeration tanks. |
| Activated sludge | Sediment of microbe-rich material in the settling tank of an STP. |
| Biogas | Mixture of CH4, CO2 and H2S produced by methanogens like Methanobacterium. |
| Methanobacterium | Anaerobic methanogen producing biogas in sludge digesters. |
| Trichoderma | Free-living fungus used as biocontrol agent against plant pathogens. |
| Bacillus thuringiensis (Bt) | Bacterium with Cry proteins toxic to insect larvae; used as biopesticide. |
| Baculovirus / NPV | Insect-pathogenic virus used as a narrow-spectrum biocontrol agent. |
| Rhizobium | Symbiotic nitrogen-fixing bacterium of legume root nodules. |
| Azotobacter | Free-living aerobic nitrogen-fixing soil bacterium. |
| Anabaena / Nostoc | Cyanobacteria (BGA) that fix nitrogen in paddy fields. |
| Mycorrhiza (VAM) | Symbiotic fungus-root association that absorbs phosphorus and protects roots. |