Microbes in Human Welfare

This is the eighth chapter of NCERT's Class 12 Biology textbook (pages 149–160 of the 2026-27 reprint), the second and last chapter of Unit VIII, Biology in Human Welfare. It has six parts — microbes in household products (8.1), industrial products (8.2, with beverages, antibiotics, and chemicals, enzymes and bioactive molecules), sewage treatment (8.3), biogas (8.4), biocontrol agents (8.5) and biofertilisers (8.6) — with Figures 8.1–8.8, a summary and 15 exercises. It is a short, descriptive chapter; this page uses eight concept sections, splitting industrial products into two.

• Neighbouring chapters: Human Health and Disease (Chapter 7) gives the disease side of microbes and the immunosuppressants and vaccines this chapter makes; Evolution (Chapter 6) explains antibiotic resistance. Carbon and its Compounds (Class 10 Science) covers ethanol and methane as chemicals; Environment and Sustainable Development (Class 11 Economics) and Rural Development treat biogas and organic farming as policy; Water Resources and Geographical Perspective on Selected Issues and Problems (Class 12 Geography) take up river pollution and the Ganga.
• Prelims — science and environment: gobar gas and biogas are mostly methane (1979, 1980, 1982, 1987); yeast raises dough (1981); organic acids (1988); mycorrhiza (1990, 1999, 2013); biological pest control (1991); cellulose digestion by gut bacteria (1992); microbes in industry (1998); Bt genes from a bacterium (1999, 2000, 2001); blue-green algae as rice biofertilisers (2000, 2010); probiotics (2008, 2022); bio-pesticides (2012); BOD (2017); culturing microbes (2021).
• GS3 — environment, agriculture and energy: sewage treatment and river cleaning (Mains 2015 on Namami Gange), microorganisms against fuel shortage (Mains 2023), biofertilisers and bio-pesticides in sustainable farming, waste-to-energy.

• Microbes everywhere — even in thermal vents at up to 100°C, deep soil, under snow, in acid. Protozoa, bacteria, fungi, viruses, viroids and prions (proteinaceous infectious agents). Bacteria and fungi grow as visible colonies on nutritive media.
• Household: Lactobacillus and other LAB → curd (acid coagulates milk protein; more vitamin B₁₂; check gut pathogens); bacteria ferment idli/dosa batter (CO₂ puffs it); baker's yeast Saccharomyces cerevisiae → bread; toddy from palm sap; fermented fish, soyabean, bamboo shoots; Swiss cheese holes = CO₂ from Propionibacterium sharmanii; Roquefort ripened by a fungus.
• Industry (in fermentors): brewer's yeast S. cerevisiae → ethanol; wine, beer without distillation; whisky, brandy, rum with. Penicillin: Fleming, Staphylococcus plate, Penicillium notatum; Chain and Florey; Nobel 1945. Acids: Aspergillus niger → citric; Acetobacter aceti → acetic; Clostridium butylicum → butyric; Lactobacillus → lactic. Lipases (detergents), pectinases and proteases (clear juice).
• Bioactive molecules: streptokinase (Streptococcus, gene-modified) — clot buster after myocardial infarction; cyclosporin A (Trichoderma polysporum) — immunosuppressant for transplants; statins (Monascus purpureus) — lower blood cholesterol by competitively inhibiting an enzyme.
• Sewage: primary = physical (filtration, sedimentation → primary sludge); secondary = biological (aeration tanks, flocs of bacteria + fungal filaments eat organic matter, BOD falls) → activated sludge (part returned as inoculum) → anaerobic sludge digesters → biogas (CH₄, H₂S, CO₂). High BOD = high pollution. Ganga and Yamuna Action Plans: more STPs.
• Biogas (mostly methane): methanogens such as Methanobacterium, anaerobic on cellulose, in sludge and cattle rumen; gobar gas plant — 10–15 ft concrete tank, floating cover, slurry as fertiliser; IARI and KVIC. Biocontrol: ladybird (aphids), dragonfly (mosquitoes), Bacillus thuringiensis (caterpillars; Bt cotton), Trichoderma (plant pathogens), baculoviruses (Nucleopolyhedrovirus; IPM). Biofertilisers: Rhizobium; free-living Azospirillum, Azotobacter; mycorrhiza (Glomus, phosphorus); cyanobacteria (Anabaena, Nostoc, Oscillatoria) in paddy.

Besides plants and animals we can see, microbes are major parts of every biological system. They live in soil, water and air, inside our bodies and those of other animals and plants, and even where nothing else can — inside geysers or thermal vents at up to 100°C, deep in the soil, under snow several metres thick, and in highly acidic places. They are diverse: protozoa, bacteria, fungi, microscopic animal and plant viruses, viroids, and prions, which are proteinaceous infectious agents. Bacteria and many fungi can be grown on nutritive media as colonies visible to the naked eye — cultures useful for studying them. In Chapter 7 microbes caused disease; here they are shown to be useful in many ways.

Curd. Lactobacillus and other lactic acid bacteria (LAB) grow in milk and turn it into curd. As they grow, they produce acids that coagulate and partially digest milk proteins. A little curd added to fresh milk as an inoculum or starter carries millions of LAB, which multiply at suitable temperatures. Curd is more nutritious than milk because of more vitamin B₁₂. In our stomach, LAB also help by checking disease-causing microbes.

Dough and batter. The batter for dosa and idli is fermented by bacteria; its puffed-up look comes from CO₂. Bread dough is fermented by baker's yeast, Saccharomyces cerevisiae. Traditional foods and drinks: toddy, a drink in parts of southern India, is fermented palm sap; microbes also ferment fish, soyabean and bamboo shoots. Cheese is one of the oldest microbial foods; each variety's texture, flavour and taste come from its microbes. The large holes in Swiss cheese come from a large amount of CO₂ made by the bacterium Propionibacterium sharmanii; Roquefort cheese is ripened by growing a particular fungus on it, which gives its flavour.

Industry grows microbes on a large scale in very large vessels called fermentors. Beverages: yeasts have long been used for wine, beer, whisky, brandy and rum. The same Saccharomyces cerevisiae, here called brewer's yeast, ferments malted cereals and fruit juices to produce ethanol. The raw material and processing decide the drink: wine and beer are made without distillation; whisky, brandy and rum are made by distilling the fermented broth.

Antibiotics — among the most significant discoveries of the twentieth century — are chemical substances produced by some microbes that kill or retard the growth of other (disease-causing) microbes. ‘Anti’ (Greek, against) + ‘bio’ (life): against the life of pathogens, but ‘pro life’ for humans. Penicillin was the first antibiotic, found by chance. Alexander Fleming, working on Staphylococci bacteria, saw a mould growing in an unwashed culture plate, around which Staphylococci could not grow; he traced this to a chemical from the mould and named it penicillin after the mould Penicillium notatum. Its full potential was established later by Ernest Chain and Howard Florey; it was used widely to treat wounded American soldiers in World War II. Fleming, Chain and Florey shared the Nobel Prize in 1945. Antibiotics have greatly improved the treatment of plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog).

Microbes are used to produce organic acids, alcohols and enzymes commercially. Acid producers: Aspergillus niger (a fungus) — citric acid; Acetobacter aceti (a bacterium) — acetic acid; Clostridium butylicum (a bacterium) — butyric acid; Lactobacillus (a bacterium) — lactic acid. Yeast (S. cerevisiae) is used for commercial ethanol. Enzymes: lipases in detergents remove oily stains; bottled fruit juices are clearer than home-made ones because they are clarified with pectinases and proteases.

Bioactive molecules. Streptokinase, produced by the bacterium Streptococcus and modified by genetic engineering, is a ‘clot buster’ that removes clots from the blood vessels of patients who have had a myocardial infarction leading to heart attack. Cyclosporin A, an immunosuppressive agent for organ-transplant patients, is produced by the fungus Trichoderma polysporum. Statins, produced by Monascus purpureus (NCERT calls it a yeast), are commercial blood-cholesterol-lowering agents; they act by competitively inhibiting the enzyme responsible for synthesising cholesterol.

Cities produce huge volumes of waste water, mostly human excreta — sewage — rich in organic matter and microbes, many pathogenic. It cannot be released straight into rivers, so it is treated in sewage treatment plants (STPs) by the heterotrophic microbes naturally present in sewage, in two stages.

Primary treatment is physical removal of particles by filtration and sedimentation: floating debris is removed by sequential filtration; then grit (soil and small pebbles) settles out. All the solids that settle form the primary sludge; the supernatant is the effluent, which goes on to secondary treatment.

Secondary (biological) treatment. The primary effluent goes into large aeration tanks, constantly agitated with air pumped in. Useful aerobic microbes grow vigorously into flocs — masses of bacteria bound with fungal filaments into mesh-like structures — which consume most of the organic matter, sharply reducing the BOD. NCERT defines BOD (biochemical oxygen demand) as the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The BOD test measures the rate at which microbes in a sample take up oxygen, so BOD is an indirect measure of organic matter: the greater the BOD of waste water, the greater its polluting potential. Treatment continues until the BOD is reduced significantly. The effluent then passes to a settling tank, where the flocs sediment as activated sludge. A small part is pumped back into the aeration tank as inoculum; the major part goes to large anaerobic sludge digesters, where other, anaerobic bacteria digest the bacteria and fungi of the sludge and produce methane, hydrogen sulphide and carbon dioxide — biogas, an inflammable fuel. The secondary effluent is usually released into rivers and streams.

This microbial method has been practised for more than a century worldwide, and no man-made technology has yet rivalled it. With urbanisation, sewage has grown faster than STP capacity, so untreated sewage often reaches rivers, polluting them and spreading water-borne disease. NCERT notes that the Ministry of Environment and Forests started the Ganga Action Plan and Yamuna Action Plan, proposing many STPs so that only treated sewage reaches these rivers.

Biogas is a mixture of gases, predominantly methane, produced by microbial activity and used as fuel. The gas a microbe makes depends on the microbe and its substrate: in dough, cheese and beverages it was CO₂. But certain bacteria growing anaerobically on cellulosic material produce large amounts of methane, with CO₂ and H₂. They are called methanogens; a common one is Methanobacterium. They live in the anaerobic sludge of sewage treatment and in the rumen of cattle, where they help break down cellulose and contribute to the cattle's nutrition. So cattle dung (gobar) is rich in methanogens, and the biogas made from it is gobar gas.

A biogas plant has a concrete tank 10–15 feet deep where bio-wastes are collected and a slurry of dung is fed. A floating cover over the slurry rises as gas is produced. An outlet pipe carries the gas to nearby houses; spent slurry leaves through another outlet and can be used as fertiliser. Dung is plentiful in villages, so plants are mostly rural, and the gas is used for cooking and lighting. The technology was developed in India mainly through the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

Biocontrol is the use of biological methods to control plant diseases and pests. Chemical insecticides, pesticides and weedicides are toxic and harmful to humans and animals and pollute soil, groundwater, fruits, vegetables and crops. NCERT sets out the organic farmer's view: biodiversity furthers health, and the more varied a landscape, the more sustainable it is; pests are kept at manageable levels by a web of checks and balances rather than eradicated, unlike ‘conventional’ chemical farming that kills useful and harmful life forms alike. Eradicating pests is undesirable because the beneficial predatory and parasitic insects that feed on or live in them would then not survive. So a biological farmer studies the field's predators and pests, their life cycles, feeding and preferred habitats.

The table records positions, not a verdict; Chapter 10 of the book returns to Bt cotton.

Chemical fertilisers, used to meet rising food demand, add significantly to environmental pollution; their overuse has created pressure to switch to organic farming and biofertilisers — organisms that enrich the nutrient quality of the soil. Their main sources are bacteria, fungi and cyanobacteria. Rhizobium forms nodules on the roots of leguminous plants in a symbiotic association and fixes atmospheric nitrogen into organic forms the plant uses. Azospirillum and Azotobacter fix nitrogen while free-living in soil, enriching its nitrogen.

Mycorrhiza is a symbiotic association of fungi with plants; many members of the genus Glomus form it. The fungus absorbs phosphorus from the soil and passes it to the plant. Plants with mycorrhiza also gain resistance to root-borne pathogens, tolerance to salinity and drought, and better overall growth. Cyanobacteria are autotrophic microbes of water and land, many of which fix atmospheric nitrogen — Anabaena, Nostoc, Oscillatoria. In paddy fields they are an important biofertiliser; blue-green algae also add organic matter and raise fertility. Many biofertilisers are sold commercially in India, and farmers use them to replenish nutrients and reduce dependence on chemical fertilisers.

The chapter sets nine questions and two visits in its running text.

Numbers

One-liners

• Lactic acid bacteria turn milk into curd and raise vitamin B12.
• Idli and dosa batter is fermented by bacteria; bread by yeast.
• Saccharomyces cerevisiae is both baker's and brewer's yeast.
• Swiss cheese holes come from CO2 made by Propionibacterium.
• Wine and beer are not distilled; whisky, brandy and rum are.
• Penicillin came from Penicillium notatum.
• Aspergillus niger makes citric acid.
• Acetobacter aceti makes acetic acid.
• Streptokinase is a clot buster from Streptococcus.
• Cyclosporin A from Trichoderma polysporum suppresses immunity.
• Statins from Monascus purpureus lower blood cholesterol.
• Primary sewage treatment is physical; secondary is biological.
• Flocs are bacteria bound with fungal filaments.
• High BOD means high organic pollution.
• Methanogens such as Methanobacterium make biogas.
• Bacillus thuringiensis kills caterpillars.
• Trichoderma controls plant pathogens.
• Baculoviruses are species-specific insecticides.
• Mycorrhizal fungi such as Glomus supply phosphorus.
• Cyanobacteria are biofertilisers in paddy fields.

Traps to avoid

• Idli batter is fermented by bacteria, not yeast.
• Penicillin is from a fungus, discovered on a bacterial plate.
• Citric acid comes from a fungus; acetic, butyric and lactic acids from bacteria.
• Biogas is mainly methane, not carbon monoxide.
• Aeration tanks are aerobic; sludge digesters are anaerobic.
• Activated sludge forms after secondary, not primary, treatment.
• Mycorrhiza supplies phosphorus; Rhizobium supplies nitrogen.
• Rhizobium is symbiotic; Azotobacter is free-living.
• Blue-green algae, not Rhizobium, fertilise rice.
• Trichoderma fights plant diseases; Bt kills insect larvae.
• Monascus purpureus is a mould, though NCERT calls it a yeast.
• Methanogens are archaea, though NCERT calls them bacteria.

Prelims — what UPSC asks

• Biogas and fermentation: gobar gas is mainly methane (1979, 1980, 1982); biogas is methane and CO₂ (1987); yeast raises bread dough (1981); organic acids and where they occur (1988); cows digest cellulose through gut bacteria (1992); microorganisms used in industry (1998); probiotics (2008, 2022); bacteria and fungi can be cultured on artificial media (2021).
• Farming with microbes: mycorrhiza is fungus + roots (1990, 1999) and helps plants on degraded land (2013); biological pest control by wasps and birds (1991); sustainable farming with organic manure and nitrogen-fixing plants (1993); Bt gene from a bacterium (1999, 2000, 2001); blue-green algae for rice (2000, 2010); bacteria, fungi and flowering plants as bio-pesticides (2012). Sewage: BOD as a pollution assay (2017).
• Key notes: 1998 keyed (d) ‘bacteria, micro-algae and fungi’ — disputed: NCERT's industrial examples are bacteria and fungi (b); micro-algae are used (e.g. Spirulina) but less widely; (d) is the expected answer. 1988 asks where acids occur (lactic–milk, acetic–vinegar, citric–lemon, butyric–rancid butter; key d stands), not which microbe makes them — read carefully. 1993 (key c) is an agriculture-policy question that rests on this chapter's biofertiliser idea. 2013 keyed (d) including ‘tolerate extremes of pH’ — beyond NCERT's list (drought, salinity, pathogens) but accepted.
• Likely next: microbe–product pairs (statins, cyclosporin A, streptokinase, citric acid), primary vs secondary treatment, BOD vs DO, sludge digesters and biogas, methanogens in the rumen and rice fields, Trichoderma and baculoviruses as bio-agents, Azolla–Anabaena, Swiss cheese and Propionibacterium, and distilled vs undistilled beverages.

Mains — where it appears and how to answer

• GS3 2015 asked about Namami Gange and the National Mission for Clean Ganga, and the mixed results of earlier schemes. NCERT itself names the cause — sewage generation outpacing STP capacity under the Ganga and Yamuna Action Plans. Frame: biology of treatment (primary physical, secondary biological, BOD as the indicator) → gaps (capacity, operation and maintenance, sludge, industrial effluents) → reforms (treatment capacity linked to performance, reuse of treated water, sludge-to-biogas, monitoring of BOD at outfalls).
• GS3 2023 asked how microorganisms can help meet the current fuel shortage. From this chapter: biogas from methanogens (dung, crop residue, sewage sludge, municipal waste), ethanol by yeast fermentation for petrol blending, plus slurry as manure; add microbial fuel cells and algal biofuels only as Beyond-NCERT pointers.
• Biofertilisers, bio-pesticides and IPM supply GS3 answers on sustainable agriculture and soil health; present GM-crop debates as attributed positions, as in Section 7.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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