Microbes in Human Welfare Class 12 Notes मानव कल्याण में सूक्ष्मजीव — कक्षा 12 जीव विज्ञान अध्याय 8

Revise these first (prerequisites)

• Basic idea of bacteria and fungi (Class 8 'Microorganisms')
• Simple meaning of fermentation / anaerobic respiration (Class 11)
• Chapter 7: idea of pathogens and antibiotics

1. Not every microbe is an enemy

   The word microbes (सूक्ष्मजीव) reminds us of disease, but many microbes are our friends. Curd, bread, cheese, medicines, sewage treatment and farming — microbes are useful in all of these.

2. Microbes in the home kitchen

   Milk is converted into curd by Lactobacillus (LAB) — it also increases vitamin B₁₂. Idli/dosa batter is fermented by bacteria, and bread dough rises due to baker's yeast (Saccharomyces cerevisiae) because CO₂ is produced.

3. Microbes in industry

   Microbes are used to produce antibiotics (such as penicillin), acids (citric acid), enzymes and alcohol on a large scale. They are grown in large machines called fermenters, which work like a small 'microbe factory'.

4. Sewage treatment

   If a city's dirty water (sewage) goes directly into a river, it causes pollution. In a sewage treatment plant, debris is first removed physically, and then bacteria consume the organic waste and clean the water. In other words, microbes act as the 'cleaning staff' here.

5. Biogas — gas from cow dung

   In cattle dung, bacteria called methanogens produce methane gas without oxygen. This biogas can be used in villages for cooking and lighting, and the leftover slurry serves as manure in the fields.

6. Microbes – the farmer's friends

   Some microbes kill pests (biocontrol) and some supply nitrogen/phosphorus to the soil (biofertilisers). This reduces the need for chemical pesticides and fertilisers, and keeps both soil and water safe.

Quick check ✔

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As per NCERT 2026-27 — every point you need for the board exam.

1. Household products

   Lactobacillus (LAB) converts milk into curd — increases Vitamin B₁₂ and checks harmful microbes in the stomach. Idli/dosa batter: fermented by bacteria (CO₂). Bread: Saccharomyces cerevisiae (baker's yeast). Toddy: fermentation of palm sap.

2. Cheese

   Large holes in Swiss cheese: due to CO₂ from Propionibacterium sharmanii. Roquefort cheese: ripened by fungi.

3. Fermented beverages

   Wine, beer, whisky, brandy, rum — Saccharomyces cerevisiae (brewer's yeast). Wine/beer without distillation; whisky, brandy, rum with distillation.

4. Antibiotics

   Penicillin — while working on Staphylococci, Alexander Fleming noticed that bacteria did not grow near Penicillium notatum. Its full potential was established by Chain and Florey; it was used to treat wounded soldiers in WW-II. Fleming, Chain and Florey received the Nobel Prize in 1945. Judicious use of antibiotics is essential — misuse leads to resistance.

5. Organic acids & alcohol

   Citric acid — Aspergillus niger (fungus); acetic acid — Acetobacter aceti; butyric acid — Clostridium butylicum; lactic acid — Lactobacillus. Ethanol — yeast.

6. Enzymes & bioactive molecules

   Lipases — detergents (oily stains). Pectinases, proteases — clarifying juices. Streptokinase (Streptococcus) — clot buster. Cyclosporin A (Trichoderma polysporum) — immunosuppressant in organ transplants. Statins (Monascus purpureus) — lower cholesterol.

7. Sewage treatment

   Primary: physical — filtration, sedimentation → primary sludge. Secondary (biological): flocs of aerobic microbes in the aeration tank → BOD decreases → activated sludge in the settling tank → anaerobic sludge digesters (biogas: methane, H₂S, CO₂). Ganga Action Plan, Yamuna Action Plan.

8. BOD

   Biochemical Oxygen Demand = the amount of O₂ consumed when the organic matter in 1 litre of water is oxidised by bacteria. Higher BOD = more pollution.

9. Biogas

   Methanogens (e.g., Methanobacterium) live in anaerobic conditions; found in the rumen of cattle and in cow dung. Biogas plant: concrete tank, slurry, floating cover on top. Technology developed by IARI and KVIC.

10. Biocontrol agents

    Ladybird (aphids), dragonflies (mosquitoes). Bacillus thuringiensis (Bt) — against caterpillars. Trichoderma — against plant pathogens. Baculoviruses (genus Nucleopolyhedrovirus) — species-specific, ideal for IPM.

11. Biofertilisers

    Rhizobium (N₂ fixation in root nodules of legumes), Azotobacter, Azospirillum (free-living). Mycorrhiza (Glomus) — absorbs phosphorus, gives drought resistance. Cyanobacteria — Anabaena, Nostoc, Oscillatoria (in paddy fields).

The concept in depth — the why and the how. Everything stays within the NCERT syllabus.

1. The activated sludge cycle

   In secondary treatment, aerobic bacteria and fungal filaments in the aeration tank together form flocs, which consume organic matter and reduce the BOD. Once the BOD is reduced significantly, the water is passed into a settling tank, where the flocs settle down to form activated sludge. A small part of it is sent back to the aeration tank as inoculum, and the rest goes to anaerobic sludge digesters, where bacteria produce biogas (methane, H₂S, CO₂).

2. BOD = measure of pollution

   BOD is the amount of oxygen consumed by bacteria in oxidising all the organic matter in 1 litre of water. The more organic waste, the more oxygen consumed — so higher BOD = more pollution. The success of a treatment plant is measured by the decrease in BOD.

3. From rumen to biogas — the same microbe group

   Methanogens (e.g., Methanobacterium) are found in anaerobic places — in the anaerobic sludge of sewage and in the rumen of cattle, where they help break down cellulose. That is why they are already present in cow dung and are useful in biogas plants. Biogas technology in India was developed through the efforts of IARI and KVIC.

4. Bioactive molecules — microbes as medicine

   Streptokinase (from Streptococcus, modified by genetic engineering) breaks down clots in blood vessels — a 'clot buster' for patients with myocardial infarction (heart attack). Cyclosporin A (Trichoderma polysporum) is an immunosuppressive agent used in organ transplants. Statins (Monascus purpureus) lower blood cholesterol by competitively inhibiting the enzyme that synthesises cholesterol.

5. Why is biocontrol better?

   Chemical pesticides kill both useful and harmful organisms. Biocontrol is targeted: Bacillus thuringiensis against butterfly caterpillars, ladybirds against aphids, dragonflies against mosquitoes. Baculoviruses (genus Nucleopolyhedrovirus) are species-specific and narrow-spectrum, so they do not harm non-target organisms — they are ideal for IPM and ecologically sensitive areas. This same Bt toxin gene is used to make Bt cotton in Chapter 10.

Common confusions — cleared up

1. Primary vs Secondary sewage treatment

   Primary = physical (filtration, sedimentation) → primary sludge. Secondary = biological (aerobic microbes, flocs) → reduces BOD.

2. Wine/beer vs Whisky/rum

   Wine and beer are produced without distillation; whisky, brandy and rum are produced by distillation of the fermented broth.

3. Biofertiliser vs Biocontrol agent

   A biofertiliser improves the nutrient quality of the soil (Rhizobium, Azotobacter, Glomus). A biocontrol agent controls pests/pathogens (Bt, Trichoderma, baculovirus).

4. Penicillin — who did what?

   Alexander Fleming discovered penicillin from Penicillium notatum; Ernest Chain and Howard Florey established its potential as an antibiotic.

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