Microbes in Human Welfare - Class 12 Biology

Class 12 Biology | Unit IX — Biology in Human Welfare

Chapter 10: Microbes in Human Welfare

Household • Industrial • Sewage Treatment • Biogas • Biocontrol • Biofertilisers

1. Microbes in Household Products

1.1 Curd (Yoghurt)

Organism: Lactobacillus (Lactic Acid Bacteria, LAB). Converts milk → curd via fermentation (lactose → lactic acid). Lactic acid coagulates milk proteins (casein), forming curd. LAB also grow in our gut and produce vitamins (B₁₂) and inhibit harmful bacteria.

1.2 Bread / Idli / Dosa

Bread: Saccharomyces cerevisiae (baker's yeast) ferments dough → CO₂ makes it porous (leavening). Ethanol produced is evaporated during baking.
Idli / Dosa: Batter of rice and black gram fermented overnight by naturally occurring microbes — Leuconostoc mesenteroides and other bacteria produce CO₂ that makes batter rise (porous texture).
Toddy (Neera): Traditional drink of South India. Made from fermenting sap of palms (date/coconut). Fermented by microbes naturally present.

1.3 Cheese

Roquefort cheese: Ripened with Penicillium roqueforti (mould). Gives characteristic flavour. Aged in specific conditions.
Swiss/Emmental cheese: Large holes formed by CO₂ produced by Propionibacterium shermanii. Characteristic flavour + hole formation.

⚠️ NEET Focus (2013, 2016, 2018, 2021): Curd = Lactobacillus (LAB). Bread = Saccharomyces cerevisiae. Roquefort cheese = Penicillium roqueforti. Swiss cheese holes = CO₂ by Propionibacterium shermanii. Toddy = fermented palm sap. LAB = vitamins B₁₂, inhibit pathogens.

2. Microbes in Industrial Products

2.1 Fermented Beverages

Product	Microorganism	Process
Wine, Beer	Saccharomyces cerevisiae	Fermentation of grape juice/malt without distillation. Wine: <13% alcohol.
Whiskey, Brandy, Rum	Saccharomyces cerevisiae	Distilled beverages (higher alcohol%). Rum from molasses (sugarcane), whiskey from cereals, brandy from fruit juice.

2.2 Antibiotics

Antibiotics: Chemical substances produced by some microorganisms that inhibit the growth of, or kill, other microorganisms.

Antibiotic	Produced by	Discovered by / year	Target
Penicillin	Penicillium notatum	Alexander Fleming (1928), accidentally (lysis of Staphylococcus on contaminated Petri dish). Ernest Chain & Howard Florey developed it for clinical use (1940). Nobel Prize 1945.	Bacteria (inhibits cell wall synthesis)
Streptomycin	Streptomyces griseus	Selman Waksman (1943)	Bacteria (tuberculosis, gram-negative)
Tetracycline	Streptomyces aureofaciens	Benjamin Duggar	Broad-spectrum
Chloramphenicol	Streptomyces venezuelae	—	Broad-spectrum
Cephalosporin	Cephalosporium acremonium	Giuseppe Brotzu	Bacteria (penicillin-resistant)

2.3 Organic Acids

Product	Microorganism	Use
Citric acid	Aspergillus niger	Food & beverage industry
Acetic acid (vinegar)	Acetobacter aceti	Food preservative, condiment
Butyric acid	Clostridium butyricum	—
Lactic acid	Lactobacillus	Food preservation

2.4 Enzymes from Microbes

Enzyme	Source	Application
Lipases	Bacteria/Fungi	Used in detergents to remove fat/oil stains
Pectinases, Proteases	Bacteria/Fungi	Clarification of fruit juices (pectinases break pectin)
Streptokinase	Streptococcus	“Clot Buster” — dissolves blood clots in heart attack patients
Collagenase, Keratinase	Microbes	Medical applications, wound healing

2.5 Bioactive Molecules

Molecule	Source	Use
Cyclosporin A	Trichoderma polysporum (fungus)	Immunosuppressant used in organ transplants (prevents rejection)
Statins	Monascus purpureus (yeast)	Blood cholesterol lowering drugs (competitive inhibitors of HMG-CoA reductase). Lovastatin.

⚠️ NEET Focus (2014, 2017, 2019, 2022): Penicillin = Penicillium notatum, discovered by Fleming (1928). Streptokinase = clot buster (Streptococcus). Cyclosporin A = Trichoderma polysporum (immunosuppressant). Statins = Monascus purpureus (cholesterol). Citric acid = Aspergillus niger. Acetic acid = Acetobacter aceti.

3. Microbes in Sewage Treatment

Sewage: Municipal wastewater containing human and industrial waste. Contains large amounts of organic matter and pathogenic microbes. Must be treated before release into natural water bodies.

3.1 Primary Treatment (Physical)

Physical removal of large and small particles by filtration and sedimentation.
Effluent passes through sequential filtration (bar screens) → grit chamber → primary settling tank.
Floatable material skimmed; settled material = primary sludge.
Supernatant = primary effluent → secondary treatment.

3.2 Secondary Treatment (Biological)

Biological treatment using microbes to decompose organic matter (BOD reduction). Two main methods:

Activated Sludge Method: Primary effluent pumped into large aeration tanks → air pumped in → aerobic microbes form flocs (masses of bacteria + fungal filaments = mesh-like structure) → decompose organic matter → BOD drops significantly → effluent passes to settling tank → activated sludge collected (some recycled as inoculum to aeration tank).
Trickling filter / Bio-filter method: Sewage trickled over a bed of gravel/rocks covered with microbial biofilm → microbes decompose organic matter.

3.3 Biogas / Anaerobic Sludge Digestion

Sludge from settling tank transferred to large anaerobic sludge digesters.
Anaerobic bacteria (methanogens) decompose organic matter → produce biogas (mainly methane + CO₂ + H₂S).
Digested sludge used as manure.
Effluent can be released into water bodies (treated water).

BOD (Biochemical Oxygen Demand): The amount of O₂ needed by microbes to decompose organic matter in a given volume of water. Higher BOD = more organic matter = more polluted water.

⚠️ NEET Focus (2015, 2018, 2020, 2022): Primary treatment = physical (filtration, sedimentation). Secondary treatment = biological (microbes + BOD reduction). BOD = measures organic matter; higher BOD = more polluted. Flocs = aggregates of bacteria + fungal filaments in aeration tank. Sludge digester = anaerobic = biogas (methane).

4. Microbes in Production of Biogas

4.1 Biogas

Biogas: A mixture of gases produced by anaerobic fermentation of organic matter (dung, agricultural waste, sewage sludge) by methanogens (methanogenic bacteria). Composition: mainly CH₄ (methane, 50–70%) + CO₂ + H₂S. Used as fuel (cooking, lighting).

4.2 Methanogenic Bacteria (Methanogens)

Examples: Methanobacterium, Methanococcus. Strict anaerobes. Archaebacteria (Domain Archaea).
Found in: rumen of cattle (help digest cellulose, produce CH₄), swamps, marshy areas, sewage sludge digesters, anaerobic biogas plants.
Also present in gut of insects (termites produce large amounts of CH₄).

4.3 Biogas Plant

Biogas plant (Gobar gas plant / KVIC model) developed by KVIC (Khadi and Village Industries Commission) in India.
Components: Large concrete tank (biodegradation chamber), floating gas holder (collects gas), slurry outlet.
Cattle dung + water added → methanogens act anaerobically → biogas collected → used as fuel.
Spent slurry (rich in nitrogen, phosphorus) used as bio-manure.

⚠️ NEET Focus: Methanogens = archaebacteria (not true bacteria). Found in rumen of cattle. Biogas = mainly methane. KVIC developed biogas plant in India. Spent slurry = bio-manure. Termites produce methane via gut methanogens.

5. Microbes as Biocontrol Agents

Biocontrol: Use of biological organisms (microbes, insects, or plants) to control pests, weeds, and pathogens, reducing or replacing chemical pesticides.

Agent	Organism	Target / Application
Bacillus thuringiensis (Bt)	Gram-positive bacterium, spore-forming	Produces Bt toxin (crystal protein / Cry proteins / delta-endotoxin). Kills larvae of Lepidoptera (butterflies/moths: Helicoverpa, bollworms), Diptera (mosquitoes), Coleoptera (beetles). Specific to target pests; safe for mammals. Used in Bt cotton (Bt cry1Ac and cry2Ab genes). Bt spray as biopesticide.
*Trichoderma*	Fungus	Free-living soil fungus. Very effective against several plant pathogens (root rots, wilts). Used as biocontrol agent for plant diseases.
Baculovirus (Nuclear Polyhedrosis Virus, NPV)	Virus (genus Nucleopolyhedrovirus)	Effective against specific insects. Species-specific: does not harm non-target organisms (mammals, birds, plants). Good for IPM (Integrated Pest Management). Used against caterpillars, leaf-eating insects. Autographa californica MNPV commonly used.
*Dragonfly larva*	Insect larva	Predator of mosquito larvae — biocontrol for malaria/dengue.
Gambusia fish	Fish	Feeds on mosquito larvae — used in ponds to control mosquito breeding.

⚠️ NEET Focus (2014, 2017, 2019, 2022): Bt toxin = from Bacillus thuringiensis. Kills larvae of moths/butterflies (Lepidoptera). Bt toxin = protein = pro-toxin (activated by gut pH). Baculovirus = species-specific, safe biopesticide, used in IPM. Trichoderma = biocontrol fungus for plant pathogens.

6. Microbes as Biofertilisers

Biofertilisers: Organisms that enrich the nutrient quality of soil naturally by nitrogen fixation, phosphate solubilisation, or hormone production. Reduce need for chemical fertilisers.

6.1 Nitrogen-Fixing Microbes

Organism	Type	Mechanism
*Rhizobium*	Bacterium, symbiotic	Forms root nodules on legumes (soybean, pea, groundnut). Fixes atmospheric N₂ → NH₃. Gene = nif gene. Enzyme = nitrogenase (requires anaerobic conditions + Mo, Fe).
*Azotobacter*	Bacterium, free-living (aerobic)	Fixes N₂ in soil. Used in biofertiliser preparations.
*Azospirillum*	Bacterium, free-living (or associative)	Associates loosely with roots. N-fixation + hormone production.
*Anabaena, Nostoc, Oscillatoria*	Cyanobacteria (BGA)	Free-living, fix N₂ in paddy fields. Heterocysts = specialised cells for N-fixation (anaerobic microenvironment).
Azolla (water fern)	Fern-cyanobacteria symbiosis	Anabaena azollae in leaf cavities of Azolla fixes N₂. Used as biofertiliser in paddy fields. Doubles rice yield when mixed into soil.

6.2 Mycorrhiza

Mycorrhiza: Symbiotic association between fungi and plant roots. Fungi help in absorbing phosphate and other nutrients from soil (enormous surface area of hyphal network). Protect plant from root-borne pathogens. Enhance plant resistance to salt and drought.

Ectomycorrhiza: Hyphae sheath around root (mantle), between cortical cells. Example: Boletus, Russula with pine trees.
Endomycorrhiza (VAM — Vesicular Arbuscular Mycorrhiza): Hyphae penetrate root cells. Form vesicles and arbuscules. Glomus spp. Most common in agricultural soils.

⚠️ NEET Focus (2015, 2018, 2020, 2022): Rhizobium = symbiotic N-fixation in legume root nodules. Anabaena = free-living cyanobacteria in paddy (heterocysts for N-fixation). Azolla = fern with Anabaena azollae, biofertiliser for paddy. Mycorrhiza = fungi-root symbiosis, absorbs phosphate. VAM = Vesicular Arbuscular Mycorrhiza = endomycorrhiza (Glomus).

🎓 Key NEET Questions (Previous Years)

Q1. [NEET 2022] Which fungus is used to produce cyclosporin A?

(a) Aspergillus niger (b) Trichoderma polysporum (c) Monascus purpureus (d) Penicillium notatum

Answer: (b) Cyclosporin A is produced by the fungus Trichoderma polysporum. It is an immunosuppressant drug used in organ transplant patients to prevent rejection.

Q2. [NEET 2021] Streptokinase produced by Streptococcus is used as:

(a) Immunosuppressant (b) Antibiotic (c) Clot buster (d) Cholesterol-lowering drug

Answer: (c) Streptokinase is a clot buster (thrombolytic enzyme). It dissolves blood clots in patients with myocardial infarction (heart attack). Produced by Streptococcus.

Q3. [NEET 2020] BOD of water increases when:

(a) Photosynthetic activity increases (b) Dissolved O₂ increases (c) Organic pollutants increase (d) Inorganic salts decrease

Answer: (c) BOD (Biochemical Oxygen Demand) is the amount of O₂ needed by microbes to decompose organic matter. When organic pollutants increase, more microbial decomposition occurs, consuming more O₂ → BOD increases. High BOD = more polluted water.

Q4. [NEET 2019] Bacillus thuringiensis (Bt) toxin kills insects by:

(a) Causing liver damage (b) Creating pores in midgut epithelium causing cell lysis (c) Blocking nerve transmission (d) Destroying reproductive organs

Answer: (b) Bt toxin (Cry proteins / delta-endotoxin) is a pro-toxin activated by alkaline gut pH of insects. It binds to epithelial cells of midgut → creates pores → cell swelling and lysis → insect dies. Safe to mammals (acidic gut pH).

Q5. [NEET 2018] Which of the following correctly describes Azolla?

(a) Free-living blue-green alga used in paddy fields (b) Water fern containing nitrogen-fixing Anabaena (c) Symbiotic bacterium in legume root nodules (d) Free-living aerobic N-fixing bacterium

Answer: (b) Azolla is an aquatic fern with leaf cavities containing nitrogen-fixing cyanobacterium Anabaena azollae. Used as biofertiliser in paddy fields. Mixed into soil before or during transplantation.

Q6. [NEET 2017] Penicillin was discovered by:

(a) Selman Waksman (b) Louis Pasteur (c) Alexander Fleming (d) Robert Koch

Answer: (c) Alexander Fleming discovered penicillin in 1928 when he accidentally observed lysis of Staphylococcus around a Penicillium notatum mould contaminating his Petri dish. Ernest Chain and Howard Florey later purified and developed it for clinical use (Nobel Prize 1945).

💡 Rapid Revision — Key Organism–Product Table

Curd = Lactobacillus | Bread = Saccharomyces cerevisiae
Roquefort cheese = Penicillium roqueforti | Swiss cheese holes = Propionibacterium shermanii
Penicillin = Penicillium notatum | Fleming, 1928
Citric acid = Aspergillus niger | Acetic acid = Acetobacter aceti
Streptokinase = clot buster (Streptococcus) | Cyclosporin A = Trichoderma polysporum
Statins (cholesterol) = Monascus purpureus | Lovastatin
BOD: high = polluted; Secondary treatment = biological = reduces BOD
Bt toxin = B. thuringiensis; kills Lepidoptera larvae | Baculovirus = IPM, species-specific
Azolla + Anabaena azollae = paddy biofertiliser | Mycorrhiza = phosphate absorption
Methanogens = archaebacteria; found in rumen; produce CH₄ (biogas)

NCERT Solutions - Microbes in Human Welfare - Class 12

CLASS 12 BIOLOGY | NCERT SOLUTIONS

Chapter 10 — Microbes in Human Welfare

All NCERT Exercise Questions with Detailed Solutions

📋 Note: All questions from NCERT Class 12 Biology Chapter 10 Exercise. Organism names, products, and processes as per NCERT.

NCERT Exercise Questions & Solutions

2 MarksQ1. Bacteria cannot be seen with the naked eye, but these can be seen with the help of a microscope. If you have to carry a sample from your home to a lab for microbiological examination, which way would you carry it?

✓ Answer
To carry a sample for microbiological examination:

Use a sterile, sealed container — to prevent contamination from environmental microbes.
Label clearly with source, time of collection, and name.
Maintain cold chain (ice box/refrigerant) — to prevent excessive microbial growth that could alter the sample composition before examination.
Transport quickly to reduce delay between collection and analysis.
Do not open the container during transport.

Note: Bacteria are microscopic (0.1–10 μm), invisible to naked eye but visible under light microscope (1000x with oil immersion) or electron microscope.

3 MarksQ2. Give examples to prove that microbes release gases during metabolism.

✓ Answer
Microbes release various gases as byproducts of their metabolism:

CO₂ by yeast: Saccharomyces cerevisiae ferments glucose → CO₂ + ethanol. CO₂ makes bread dough rise (leavening) and is responsible for carbonation in beer/wine.
CH₄ by methanogens: Methanobacterium anaerobically breaks down organic matter → methane (CH₄) + CO₂. Methane = biogas. Also produced in rumen of cattle.
CO₂ by Swiss cheese bacteria: Propionibacterium shermanii produces CO₂ during cheese ripening → forms large holes in Swiss/Emmental cheese.
H₂S by anaerobic bacteria: Sulphur-reducing bacteria produce H₂S during decomposition of organic matter.
N₂ by denitrifying bacteria: Pseudomonas converts nitrates → N₂ gas (denitrification).

3 MarksQ3. In which food would you find lactic acid bacteria? Mention some benefits of these bacteria.

✓ Answer
Foods containing Lactic Acid Bacteria (LAB):

Curd / Yoghurt — Lactobacillus ferments milk (lactose → lactic acid → curd formation by casein coagulation).
Idli, Dosa, Dhokla batter (fermented foods)
Sauerkraut (fermented cabbage)
Pickles
Cheese (some varieties)
Buttermilk, kefir

Benefits of LAB:

Improve nutritional quality of food — increase levels of Vitamin B₁₂ in curd.
Probiotic: LAB present in our stomach and intestine help check growth of harmful bacteria.
Produce lactic acid — lowers pH and acts as a natural preservative.
Help maintain healthy gut microflora.
Some produce bacteriocins (antibiotic-like compounds) that inhibit pathogenic bacteria.

5 MarksQ4. Name the microorganism from which cyclosporin A and statins are obtained. Describe their uses.

✓ Answer

Molecule	Source Microorganism	Use / Application
Cyclosporin A	Trichoderma polysporum (a fungus)	Immunosuppressant drug. Used in organ transplant patients to prevent immune rejection of transplanted organs (heart, kidney, liver). Inhibits T-lymphocyte activation, suppressing the immune response that would otherwise attack the foreign organ.
Statins (e.g., Lovastatin)	Monascus purpureus (a yeast / red mould)	Cholesterol-lowering drugs. Act as competitive inhibitors of the enzyme HMG-CoA reductase, which is the rate-limiting enzyme in cholesterol biosynthesis. Reduce LDL cholesterol and triglyceride levels. Reduce risk of cardiovascular disease and heart attacks.

5 MarksQ5. What is sewage? In what way can it be harmful to us?

✓ Answer
Sewage: Municipal wastewater from homes, industries, and commercial establishments. Contains:

Large amounts of organic matter (food waste, excreta, paper).
Pathogenic microorganisms (bacteria, viruses, protozoans, helminths).
Inorganic matter (heavy metals, salts) from industrial runoff.
Nutrients (nitrates, phosphates) — cause eutrophication if released.

Harmful effects of untreated sewage:

Water-borne diseases: Cholera (Vibrio cholerae), typhoid (Salmonella typhi), polio, hepatitis, amoebiasis if humans consume contaminated water.
Eutrophication: Excess nutrients (nitrates/phosphates) promote algal blooms → algae die → decomposition reduces dissolved O₂ → fish and aquatic life suffocate (hypoxia/anoxia).
High BOD: Organic matter increases BOD → depletes dissolved oxygen in water bodies → kills aerobic aquatic organisms.
Soil contamination: Heavy metals and toxins in sewage can accumulate in soil and enter food chain.
Groundwater contamination: Seepage into aquifers contaminates drinking water sources.

5 MarksQ6. What is the difference between the primary and secondary sewage treatment? Which one of them would you consider as a more effective treatment and why?

✓ Answer

Feature	Primary Treatment	Secondary Treatment
Nature	Physical	Biological
Principle	Filtration and sedimentation	Microbial decomposition (aerobic)
Removes	Large particles, suspended solids, grit	Dissolved organic matter, reduces BOD
Process	Bar screens → grit chamber → settling tank	Aeration tank (floc formation) → settling tank → chlorination
BOD reduction	Very little	Significant (60–90% BOD reduction)
Pathogens removed?	Partially	Mostly (further removed by chlorination)

Secondary treatment is more effective because:

It removes dissolved organic matter that primary treatment cannot remove.
It significantly reduces BOD (by 60–90%), making the effluent safe for discharge into water bodies.
Microbial flocs in aeration tanks efficiently decompose complex organic molecules.
Final effluent is cleaner and safer for the environment.

3 MarksQ7. Explain the role of microbes in production of biogas. Name any two microbes involved.

✓ Answer
Role of microbes in biogas production:

Hydrolytic bacteria first break down complex organic molecules (polysaccharides, lipids, proteins) into simpler compounds (sugars, fatty acids, amino acids).
Acidogenic bacteria convert these to organic acids (acetic acid, propionic acid), alcohols, H₂, CO₂.
Methanogenic bacteria (Methanogens) — strictly anaerobic archaebacteria — convert acetic acid and H₂/CO₂ into methane (CH₄) and CO₂ = biogas.

The whole process is called anaerobic digestion and takes place in biogas plants (gobar gas plants).

Two microbes involved:

Methanobacterium — a methanogenic archaebacterium.
Methanococcus — another methanogenic archaebacterium.

Both are strict anaerobes. They are also found in the rumen of cattle where they help in cellulose digestion.

3 MarksQ8. Name any two biofertilisers. Explain how they enrich soil fertility.

✓ Answer
Two biofertilisers:

1. Rhizobium (symbiotic N-fixing bacterium):

Lives in root nodules of leguminous plants (soybean, pea, groundnut, clover, alfalfa).
Has enzyme nitrogenase (encoded by nif gene).
Converts atmospheric N₂ → ammonia (NH₃) → which plant can directly use for amino acid/protein synthesis.
This reduces need for synthetic nitrogen fertilisers. Legume crop rotation enriches soil for subsequent non-legume crops.

2. Azolla + Anabaena azollae (BGA-fern symbiosis):

Azolla is an aquatic fern; its leaf cavities harbour Anabaena azollae (N-fixing cyanobacterium).
Anabaena fixes N₂ via heterocysts — specialised anaerobic cells containing nitrogenase.
Used as biofertiliser in paddy fields (rice cultivation). Mixed/ploughed into soil before or during rice transplantation. Doubles rice yield in experiments.

Facts Capsule - Microbes in Human Welfare - Class 12

CLASS 12 BIOLOGY | NEET RAPID CAPSULE

Facts & High-Yield Points

Chapter 10 — Microbes in Human Welfare | 24 Key Facts for NEET

🏠 Household Products

FACT #01 — Curd & Bread

Curd: Lactobacillus (LAB) — lactose → lactic acid → casein coagulates. Bread: Saccharomyces cerevisiae (baker's yeast) — CO₂ makes dough porous.

FACT #02 — Cheese Types

Roquefort cheese: Penicillium roqueforti (mould ripening, characteristic flavour). Swiss/Emmental cheese: Large holes = CO₂ by Propionibacterium shermanii.

FACT #03 — LAB Benefits

LAB improve nutrition: produce Vitamin B₁₂. Inhibit harmful gut bacteria. Probiotic. Also produce bacteriocins. Idli/Dosa batter fermented by Leuconostoc mesenteroides. Toddy = fermented palm sap.

🏭 Industrial Microbes

FACT #04 — Penicillin Discovery

Alexander Fleming (1928) — accidentally observed Penicillium notatum killing Staphylococcus on contaminated Petri dish. Chain & Florey developed it clinically. Nobel Prize 1945.

FACT #05 — Other Antibiotics

Streptomycin: Streptomyces griseus (Waksman). Tetracycline: S. aureofaciens. Chloramphenicol: S. venezuelae. Cephalosporin: Cephalosporium acremonium.

FACT #06 — Organic Acids

Citric acid: Aspergillus niger. Acetic acid (vinegar): Acetobacter aceti. Butyric acid: Clostridium butyricum. Lactic acid: Lactobacillus.

FACT #07 — Industrial Enzymes

Streptokinase (Streptococcus): clot buster (thrombolytic). Lipases: detergent ingredient (oil stain removal). Pectinases: fruit juice clarification. Proteases: detergent/food industry.

FACT #08 — Bioactive Molecules

Cyclosporin A: Trichoderma polysporum — immunosuppressant (organ transplant, anti-rejection). Statins (Lovastatin): Monascus purpureus — competitive inhibitor of HMG-CoA reductase → lowers LDL cholesterol.

💧 Sewage Treatment

FACT #09 — Primary Treatment

Physical process: bar screens → grit chamber → settling tank. Removes suspended solids, floating material. Does NOT remove dissolved organic matter. Produces primary sludge.

FACT #10 — Secondary Treatment

Biological process. Primary effluent → aeration tank → aerobic microbes form flocs (bacteria + fungal hyphae) → decompose organic matter → BOD drops significantly. More effective.

FACT #11 — BOD

BOD = Biochemical Oxygen Demand. Amount of O₂ needed by microbes to oxidize organic matter. Higher BOD = more organic matter = more polluted water. Secondary treatment reduces BOD by 60–90%.

FACT #12 — Sludge & Biogas

Sludge from settling tank → anaerobic sludge digesters → methanogens produce biogas (CH₄ + CO₂). Digested sludge = manure. Effluent released/reused.

🌿 Biocontrol

FACT #13 — Bt Toxin

Bacillus thuringiensis (Bt): gram-positive, spore-forming bacterium. Produces Cry proteins (delta-endotoxin). Pro-toxin activated at alkaline insect gut pH. Creates pores in midgut epithelium → insect dies. Kills Lepidoptera larvae (moths, butterflies), Diptera (mosquitoes), Coleoptera (beetles). Used in Bt cotton, Bt spray.

FACT #14 — Baculovirus & Trichoderma

Baculovirus (NPV): species-specific, safe for non-target organisms (mammals, birds, fish). Used in IPM. Trichoderma: free-living soil fungus; biocontrol agent against plant pathogens (root rots, wilts). Gambusia fish eats mosquito larvae.

🌿 Biofertilisers

FACT #15 — Rhizobium

Symbiotic N-fixing bacterium in root nodules of legumes. Enzyme: nitrogenase (nif gene). Fixes N₂ → NH₃. Requires anaerobic conditions (leghemoglobin in nodules maintains low O₂; leghemoglobin = pink colour of nodules).

FACT #16 — Cyanobacteria (BGA)

Free-living N-fixers in paddy fields: Anabaena, Nostoc, Oscillatoria. Fix N₂ in heterocysts (anaerobic microenvironment, nitrogenase present). Used as biofertiliser in paddy fields.

FACT #17 — Azolla

Azolla = aquatic water fern. Leaf cavities house Anabaena azollae (N-fixing BGA). Used in paddy fields. Doubles rice yield. Ploughed into soil as green manure or used as floating mat.

FACT #18 — Mycorrhiza

Fungi-root symbiosis. Absorbs phosphate (and other minerals) for plant. Protect from root pathogens. Enhance drought tolerance. VAM (Vesicular Arbuscular Mycorrhiza): Glomus spp. = endomycorrhizal, most common in agricultural soils.

🧠 Mnemonics — Remember Fast

N-Fixers: “RANA” Rhizobium (legume nodules) | Azotobacter (free-living aerobic) | Nostoc/Anabaena (BGA, heterocysts) | Azolla + Anabaena azollae (paddy).

Sewage Treatment Order 1° = Physical (filtration + sedimentation). 2° = Biological (flocs + aeration). Sludge → anaerobic digester → Biogas. Effluent → water body. BOD: high=polluted.

Cheese Mnemonics Roquefort = P. roqueforti (same name, easy!). Swiss = Propionibacterium shermanii (makes propionic acid + CO₂ = holes). Swiss = holes = round Swiss clocks.

📊 Quick Reference — Organism to Product

Organism	Product / Role
Lactobacillus	Curd (lactic acid fermentation); Vitamin B₁₂
Saccharomyces cerevisiae	Bread (leavening), Beer, Wine, Alcohol
Penicillium notatum	Penicillin (antibiotic) — Fleming 1928
Penicillium roqueforti	Roquefort cheese (ripening)
Propionibacterium shermanii	Swiss cheese holes (CO₂)
Aspergillus niger	Citric acid
Acetobacter aceti	Acetic acid (vinegar)
Trichoderma polysporum	Cyclosporin A (immunosuppressant)
Monascus purpureus	Statins / Lovastatin (cholesterol-lowering)
Streptococcus	Streptokinase (clot buster)
Trichoderma sp.	Biocontrol against plant pathogens
Bacillus thuringiensis	Bt toxin (Cry proteins) — kills Lepidoptera larvae
Rhizobium	N-fixation in legume root nodules (symbiotic)
Anabaena / Nostoc	Free-living N-fixation in paddy (BGA, heterocysts)
Azolla + Anabaena azollae	Biofertiliser for paddy (symbiosis)
Glomus spp.	VAM (endomycorrhiza) — phosphate absorption
Methanobacterium	Biogas (CH₄) production; in cattle rumen

🔢 Critical Numbers — Never Forget

1928 — Fleming discovered penicillin 1945 — Nobel Prize: Fleming, Chain, Florey BOD high — more polluted water CH₄ — main component of biogas 2° treatment — reduces BOD 60–90% Heterocysts — N-fixation cells in BGA VAM = Glomus — most common endomycorrhiza

📱 Practice MCQs for this topic inside our App