Microorganisms — bacteria, fungi, viruses, algae, and protozoa — are invisible to the naked eye yet play enormous roles in human life. While some cause disease, a vast majority are beneficial and are harnessed in food production, medicine, industry, and environmental management. NCERT Class 12 Biology Chapter 8 focuses specifically on these beneficial roles.
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Microbes in Household and Industrial Processes
Fermentation is the key process: microbes break down organic molecules (sugars) in the absence of oxygen to produce useful products.
- Curd: Lactobacillus converts lactose in milk to lactic acid, causing curd to form. The acid denatures milk proteins and gives curd its texture and taste.
- Bread: Saccharomyces cerevisiae (baker's yeast) ferments sugars in dough, producing CO2 that causes dough to rise.
- Toddy / Alcoholic beverages: Fermentation of sugars by yeast produces ethanol. Beer and wine use natural fermentation; whisky and brandy require distillation.
- Cheese: Different microbes give different flavours. Large holes in Swiss cheese are made by Propionibacterium freudenreichii, which produces CO2. Roquefort cheese uses specific fungi for ripening.
- Idli and Dosa: Fermentation by Leuconostoc and Streptococcus bacteria increases palatability and nutritional value.
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Microbes in Industrial Production
- Beverages and Chemicals
- Fermented drinks: yeast-based ethanol production
- Organic acids: Aspergillus niger produces citric acid; Acetobacter aceti produces acetic acid (vinegar)
- Enzymes: Aspergillus produces amylases and proteases used in food and detergent industries. Lipases from microbes are used in detergents.
- Antibiotics — the greatest medical revolution:
- Penicillin: Discovered by Alexander Fleming in 1928 from the mould Penicillium notatum. He noticed that the mould killed surrounding bacteria (Staphylococcus). Ernest Chain and Howard Florey developed it into a drug (Nobel Prize 1945).
- Other antibiotics: Streptomycin (from Streptomyces griseus), chloramphenicol, tetracyclines — all from soil bacteria.
- Chemicals, Enzymes, and Bioactive molecules:
- Statins: Monascus purpureus (yeast) produces lovastatin — a cholesterol-lowering drug (statin). Statins competitively inhibit the enzyme for cholesterol synthesis.
- Cyclosporin A: produced by Trichoderma polysporum — used as an immunosuppressant in organ transplants.
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Microbes in Sewage Treatment
Sewage contains organic matter, human pathogens, and chemicals. Treatment uses microbes to break down waste in two stages:
Primary Treatment (Physical): Removal of large debris via screens, then settling in settling tanks where suspended particles form primary sludge while the liquid (supernatant) is called primary effluent.
- Secondary Treatment (Biological):
- Primary effluent is aerated in large aeration tanks — aerobic microbes multiply and break down organic matter.
- The microbial mass (flocs — bacteria + fungi in mesh of fungal filaments) settles in secondary settling tanks as activated sludge.
- A small part of the sludge is reused; the rest undergoes anaerobic digestion in anaerobic sludge digesters.
- Anaerobic bacteria decompose the sludge and produce biogas (methane + CO2 + H2S).
- BOD (Biochemical Oxygen Demand) — the amount of oxygen required by bacteria to break down organic matter in 1 litre of water. Higher BOD = more polluted water. After secondary treatment, BOD is significantly reduced before water is released.
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Microbes in Biogas Production
- Biogas: mixture of gases produced by microbial activity on organic waste — mainly methane.
- Microbes responsible: methanogens (e.g., Methanobacterium) — strictly anaerobic archaea found in marshy areas and the rumen of cattle.
- Biogas plants use cattle dung (gobar) in a slurry fed into a digester tank. Biogas is collected for cooking/lighting; the leftover slurry is nitrogen-rich manure.
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Microbes as Biocontrol Agents
Biocontrol = using living organisms to suppress pest populations — an alternative to chemical pesticides.
- Bacillus thuringiensis (Bt): produces Bt toxin (crystal proteins / Cry proteins) that kill insect larvae when ingested. Commercially available as dried spore formulations sprayed on crops. The Bt gene has also been inserted into crops (Bt cotton, Bt brinjal) to make them pest-resistant.
- Trichoderma (fungus) is used against several plant pathogens (fungal diseases of roots).
- Baculoviruses (e.g., Nucleopolyhedrovirus) infect and kill insects but are harmless to non-target organisms, making them excellent biocontrol agents.
- Ladybirds and dragonflies are classical biological control examples (predator-prey approach).
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Microbes as Biofertilisers
Biofertilisers add nutrients to the soil through biological activity — cheaper and eco-friendly compared to chemical fertilisers.
- Rhizobium: lives in root nodules of legumes; fixes atmospheric N2 into ammonia (NH3) usable by plants — a mutualistic relationship.
- Azospirillum and Azotobacter: free-living nitrogen-fixing bacteria in soil.
- Cyanobacteria (blue-green algae) like Anabaena, Nostoc, Oscillatoria: fix nitrogen in paddy fields. Nostoc and Anabaena also form symbiotic association with the fern Azolla, which is used as biofertiliser in rice paddies.
- Mycorrhizae: symbiotic association between fungi and plant roots. The fungal hyphae greatly increase the surface area for absorption of water and minerals (especially phosphorus). Glomus is a common mycorrhizal fungus.
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Why does curd form when Lactobacillus is added to warm milk?
Lactobacillus ferments lactose to lactic acid. The lower pH (more acidic) causes milk proteins (casein) to coagulate (denature), forming the semi-solid texture of curd.
How did Alexander Fleming discover penicillin?
Fleming observed that the mould Penicillium notatum growing on a bacterial culture plate (Staphylococcus) had a clear zone around it — the mould had secreted something that killed the bacteria. This substance was later isolated as penicillin.
What is BOD and what does a high BOD indicate?
BOD (Biochemical Oxygen Demand) is the amount of dissolved oxygen needed by aerobic microorganisms to decompose organic matter in one litre of water at a specific temperature. A high BOD means more organic pollution and severe depletion of dissolved oxygen.
Explain how Bacillus thuringiensis acts as a biocontrol agent.
Bt produces proteinaceous crystalline (Cry) proteins during sporulation. When insect larvae ingest these crystals, gut enzymes activate the toxin, which binds to gut epithelial cells, creates pores, and kills the larva. It is safe for humans, birds, and other organisms.
Distinguish between Rhizobium and Azotobacter in nitrogen fixation.
Rhizobium is a symbiotic nitrogen fixer — it lives inside root nodules of leguminous plants. Azotobacter is a free-living nitrogen fixer in soil. Both convert atmospheric N2 into ammonia, but through different associations.
What is mycorrhiza and why is it important in agriculture?
Mycorrhiza is a mutualistic symbiosis between fungi (like Glomus) and plant roots. Fungal hyphae extend beyond the root zone to absorb water and minerals (especially phosphorus). This improves plant growth and reduces the need for chemical phosphate fertilisers.
Why is methane produced in biogas plants, and what organisms are responsible?
Methanogens (strictly anaerobic bacteria-like archaea, e.g., Methanobacterium) break down complex organic matter in the absence of oxygen, releasing methane (CH4), CO2, and H2S. Methane is the combustible component of biogas used as fuel.
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Summary
- BOD: higher value = greater organic pollution
- Antibiotics inhibit bacterial growth (bacteriostatic) or kill bacteria (bactericidal); penicillin disrupts cell wall synthesis
- Nitrogen fixation: N2 + 8H+ + 8e- + 16 ATP => 2NH3 + H2 + 16 ADP + 16 Pi (enzyme: nitrogenase)
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Common mistakes
Students confuse Bt toxin with chemical pesticides — Bt toxin is a biological protein specific to insects, not a synthetic chemical. Do not confuse activated sludge (secondary treatment) with primary sludge (primary treatment). Mycorrhiza improves phosphorus absorption, NOT nitrogen fixation. Remember that statins come from yeast (Monascus), not bacteria.
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Summary
Microbes are indispensable allies: they ferment food, produce antibiotics and industrial chemicals, treat sewage, generate biogas, serve as biocontrol agents, and enrich soil as biofertilisers. Understanding their applications is key to sustainable human welfare.