Class 12 Biology · NCERT chapter 10 · 6 topics · free to read
Microbes in Household Products
Everyday foods like curd, bread, dosa, cheese, and toddy are all made using microbial fermentation — Lactobacillus, yeast, and fungi are the invisible kitchen helpers.
Curd Formation
Lactobacillus bacteria convert lactose in milk into lactic acid, which coagulates the milk protein casein to form curd.
When you add a small amount of curd (inoculum/starter) to warm milk, Lactobacillus bacteria multiply rapidly. They ferment lactose (milk sugar) into lactic acid. This acid lowers the pH, causing the milk protein casein to coagulate — and you get curd.
Lactic acid coagulates casein protein arrow curd forms
Curd is more nutritious than milk — LAB produce vitamin B _ 12
LAB also check disease-causing microbes in the stomach
Bread, Dosa & Idli
Baker's yeast (Saccharomyces cerevisiae) ferments sugars in dough, producing CO₂ gas that creates the fluffy, puffed-up texture.
When you knead dough with yeast and keep it warm, Saccharomyces cerevisiae (baker's yeast) ferments the sugars and produces CO _2 gas. These gas bubbles get trapped in the gluten network, making the dough rise and become fluffy. This is why bread is soft and spongy.
Microbe: Saccharomyces cerevisiae (baker's yeast)
Process: Sugar arrow Ethanol + CO _2 (alcoholic fermentation)
CO _2 gas makes dough rise — ethanol evaporates during baking
Same principle in dosa and idli batter — bacteria + yeast cause fermentation
South Indian fermented foods also involve Leuconostoc (LAB) along with yeast
Cheese Ripening
Cheese is made by coagulating milk with rennet/acid, then specific microbes ripen it to create distinct flavours, textures, and aromas.
Cheese-making starts with curdling milk (using rennet or acid). The curd is then processed and ripened using specific microbes that give each cheese its unique character. Different microbes = completely different cheese.
Swiss cheese (big holes): Ripened by Propionibacterium shermanii — produces large amounts of CO _2 that creates the characteristic holes
Roquefort cheese (blue-veined): Ripened by Penicillium roqueforti — gives the blue-green veins and sharp flavour
Camembert cheese: Ripened by Penicillium camemberti — gives the soft, creamy texture and white rind
Big holes (eyes)
Blue-green veins
Propionibacterium shermanii
Penicillium roqueforti
CO _2 creates holes
Fungal hyphae create veins
Mild, nutty flavour
Sharp, tangy flavour
Toddy & Fermented Beverages
Toddy is made by fermenting palm sap using naturally occurring yeasts; many traditional beverages worldwide rely on microbial fermentation.
Toddy is a traditional alcoholic beverage made from the sap of palms (coconut, palmyra, date palm). The sap is collected in earthen pots and left to ferment naturally. Wild yeasts present in the sap and on the pot walls convert sugars into ethanol.
Toddy is fermented palm sap (neera) — contains natural sugars
Other fermented foods: soy sauce (Aspergillus), vinegar (Acetobacter), tempeh (Rhizopus)
All fermented foods rely on anaerobic or partially anaerobic microbial metabolism
Quick recall
What is the common microbial process behind curd, bread, toddy, and cheese?
All involve fermentation — microbes breaking down organic substrates anaerobically to produce useful products (lactic acid, CO _2 , ethanol, flavour compounds).
Industrial Products from Microbes
Microbes are industrial powerhouses — they produce antibiotics, organic acids, enzymes, ethanol, and cholesterol-lowering statins on a massive scale in fermentors.
Fermented Beverages
The same species Saccharomyces cerevisiae produces different alcoholic beverages depending on the starting material (substrate) and processing.
All alcoholic beverages use Saccharomyces cerevisiae (brewer's yeast) to ferment sugars into ethanol. What makes beer different from wine? The substrate. What makes whisky different from beer? Distillation.
Wine: Fermentation of grape juice (no distillation)
Beer: Fermentation of malted cereals + hops flavouring (no distillation)
Whisky: Fermented grain mash arrow distilled for higher alcohol
Alexander Fleming's accidental discovery of Penicillium notatum killing bacteria in 1928 led to antibiotics that have saved millions of lives.
In 1928, Alexander Fleming noticed that the mould Penicillium notatum was killing Staphylococcus bacteria on his culture plate. The chemical it produced was named penicillin — the world's first antibiotic. Later, Ernst Chain and Howard Florey purified it for mass production. All three won the Nobel Prize in 1945.
Penicillin: Produced by *Penicillium notatum* (also *P. chrysogenum*)
Antibiotics = chemicals produced by microbes that kill or inhibit other microbes
Full-scale production began during World War II to treat wounded soldiers
Streptomycin: Produced by *Streptomyces griseus* — treats TB
Tetracycline: Produced by *Streptomyces* species — broad-spectrum antibiotic
Organic Acids, Enzymes & Bioactive Molecules
Industry uses microbes to mass-produce citric acid, enzymes like lipases and proteases, streptokinase for clot-busting, cyclosporin A for immunosuppression, and statins for cholesterol control.
Microbes are cheaper and more efficient than chemical synthesis for producing many compounds. Industrial fermentors (huge steel tanks, 100-1000 litres) grow microbes under controlled conditions of temperature, pH, and aeration.
Citric acid: *Aspergillus niger* — used in food/beverage industry
Acetic acid (vinegar): *Acetobacter aceti*
Butyric acid: *Clostridium butylicum*
Lactic acid: *Lactobacillus*
Ethanol: *Saccharomyces cerevisiae* — also used as industrial solvent and fuel
Lipases: Used in detergents to remove oily stains
Pectinases + Proteases: Used to clarify bottled fruit juices
Streptokinase: Produced by *Streptococcus* — used as a clot buster (thrombolytic agent) for heart attacks
Cyclosporin A: Produced by *Trichoderma polysporum* — immunosuppressant used in organ transplants
Statins: Produced by *Monascus purpureus* (a yeast) — lowers blood cholesterol by inhibiting the enzyme for cholesterol synthesis
Quick recall
Which microbe produces statins, and what do statins do?
Monascus purpureus (a yeast) produces statins. Statins are blood cholesterol-lowering agents — they competitively inhibit the enzyme responsible for cholesterol synthesis.
Sewage Treatment
Sewage is cleaned in stages using microbes — they eat organic matter, reduce BOD, and produce biogas as a by-product. Without microbes, our rivers would be open drains.
High BOD arrow Microbial action arrow Low BOD (safe to discharge)
Primary Treatment
The first stage of sewage treatment uses physical methods (filtration, sedimentation) to remove floating debris and settleable solids — no microbes involved yet.
Raw sewage first passes through physical treatment steps to remove the big stuff before microbes can work on the dissolved organic matter.
Grit chamber: Sewage flows slowly — soil, sand, and small pebbles settle by gravity
Sedimentation tank (primary settling tank): Solids that settle = primary sludge (bottom). Supernatant liquid = effluent
Primary treatment removes only physical impurities — dissolved organic matter still present
BOD is still HIGH after primary treatment
Primary treatment is purely physical/mechanical. No biology here — just gravity and filtration doing the work.
Secondary Treatment
The biological heart of sewage treatment — aerobic microbes in aeration tanks consume organic waste, forming flocs that settle out as activated sludge.
Secondary treatment is biological treatment. The primary effluent is pumped into large aeration tanks where vigorous agitation and air supply encourage aerobic microbes to grow and consume the dissolved organic matter.
Primary effluent enters aeration tank — constantly agitated mechanically + air pumped in
Microbes form masses called flocs — mesh of bacteria + fungal filaments
BOD drops significantly as microbes digest organic waste
When BOD is reduced sufficiently, effluent moves to settling tank
In the settling tank, flocs sediment out as activated sludge. The clear supernatant is the treated water. A small part of activated sludge is recycled back to the aeration tank as inoculum (to seed fresh sewage with microbes). The major part goes to the anaerobic sludge digester.
BOD
BOD measures the amount of dissolved oxygen microbes need to decompose organic matter in water. High BOD = very polluted; low BOD = clean water.
BOD (Biochemical Oxygen Demand) is the amount of dissolved oxygen that aerobic microbes would consume to break down the organic matter present in one litre of water. It's the gold standard for measuring water pollution.
BOD = oxygen consumed by microbes to decompose organic waste in water
High BOD = lots of organic matter = heavily polluted water
Low BOD = little organic matter = clean water
Untreated sewage: BOD is very high (300-400 mg/L)
After secondary treatment: BOD drops drastically (to <30 mg/L)
Safe discharge into rivers requires sufficiently low BOD
Quick recall
If sewage is dumped into a river and the BOD of downstream water increases sharply, what happens to fish?
Fish die. High BOD means microbes consume most dissolved oxygen, leaving too little for fish and aquatic organisms. This is why untreated sewage kills river ecosystems.
Anaerobic Sludge Digester
The excess activated sludge is pumped into anaerobic digesters where methanogens break it down, producing a mixture of gases (methane, CO₂, H₂S) called biogas.
The major portion of activated sludge that isn't recycled goes into anaerobic sludge digesters — sealed tanks with no oxygen. Here, anaerobic bacteria (including methanogens) break down the organic matter in sludge.
Anaerobic bacteria decompose organic matter in absence of oxygen
Products: biogas (mixture of methane, CO _2 , H _2 S)
Biogas can be used as fuel — sewage treatment plants often use it to generate electricity
The remaining dried sludge (after digestion) can be used as manure or disposed safely
Effluent from secondary treatment can undergo tertiary treatment (chemical) before release
Biogas Production
Methanogens like Methanobacterium anaerobically digest organic waste (especially cow dung) to produce biogas — a clean fuel rich in methane. India's gobar gas plants are a classic example.
Methanobacterium and other methanogens are strict anaerobes found in cattle dung, marshes, and sewage. They break down cellulosic material to produce methane-rich biogas.
Methanogens are anaerobic bacteria (actually archaea) that produce methane (CH _4 ) as a metabolic by-product. They thrive wherever oxygen is absent and organic matter is abundant — cattle intestines, swamps, paddy fields, and sewage sludge.
Key genus: Methanobacterium — an obligate anaerobe
Found in: rumen of cattle, anaerobic sludge, marshes, flooded paddy fields
They digest cellulosic material that no other organism can easily break down
Biogas = ~60-70% methane (CH _4 ) + ~30% CO _2 + traces of H _2 S
Methane is the combustible component — burns with a clean blue flame
Biogas Plant
A biogas plant has a concrete tank (digester) where slurry ferments anaerobically. Gas collects under a floating dome and is piped out for cooking/lighting.
The technology for biogas production was developed in India largely due to the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC). The design is simple and low-cost.
Slurry preparation: Cow dung (or other bio-waste) mixed with water arrow fed into the digester through inlet
Digester tank: Underground concrete tank — anaerobic conditions maintained
Floating gas holder (dome): Collects biogas rising from the fermenting slurry
Gas outlet pipe: Connects to kitchen stoves/lamps for cooking and lighting
Spent slurry outlet: Digested slurry removed — used as nutrient-rich manure
Commonly called gobar gas plant in rural India (gobar = cow dung)
Quick recall
Why is biogas considered a superior fuel compared to burning raw dung cakes?
Biogas burns cleanly (no smoke, no soot), has higher calorific value than raw dung, and the spent slurry is a better fertilizer than raw dung. Burning raw dung wastes its manure value and causes indoor air pollution.
Biocontrol Agents
Instead of chemical pesticides, we can use living organisms — Bt toxin from Bacillus thuringiensis, baculoviruses, ladybirds, dragonflies, and Trichoderma — to control pests biologically.
Biological Control
Biocontrol uses predators, parasites, or pathogens to suppress pest populations naturally, avoiding the ecological damage caused by chemical pesticides.
Chemical pesticides are a blunt weapon — they kill pests but also kill beneficial organisms, pollute soil and water, and enter the food chain. Biological control (biocontrol) is the smarter approach: use the pest's natural enemies to keep its population in check.
Biocontrol = using living organisms to control pest populations
Part of Integrated Pest Management (IPM) — combines biological, cultural, and minimal chemical methods
Key advantage: species-specific — targets pests without harming beneficial insects
No toxic residues in soil, water, or food
Predators, parasitoids, and microbial pathogens are all used
IPM doesn't mean zero chemicals — it means using chemicals only as a last resort and in targeted amounts, combined with biological methods.
Bt Toxin
Bacillus thuringiensis (Bt) produces insecticidal crystal proteins (Cry proteins) during sporulation. These are toxic to specific insect larvae but harmless to other organisms.
Bacillus thuringiensis (Bt) is a soil bacterium that produces crystal (Cry) proteins during spore formation. These proteins are toxic to insect larvae (caterpillars, beetles, mosquito larvae) but completely harmless to humans, animals, and beneficial insects.
Bt produces Cry proteins (crystal proteins) — also called -endotoxins
Cry proteins are inactive in the crystalline form (protoxin)
When an insect larva eats Bt spores, the alkaline gut pH dissolves the crystal and activates the toxin
Active toxin punches holes in the gut lining arrow larva stops feeding arrow dies
Available as dried spores — sprayed on crops like insecticide
Also: Bt genes have been engineered into crop plants (Bt cotton, Bt brinjal)
Baculoviruses (NPV)
Nucleopolyhedrovirus (NPV) is a baculovirus used as a species-specific biocontrol agent. It infects and kills only the target insect pest.
Baculoviruses (especially the genus Nucleopolyhedrovirus, NPV) are viruses that attack insects and arthropods. They are excellent biocontrol agents because they are extremely species-specific — they only infect the target pest species.
NPV = Nucleopolyhedrovirus — a genus of baculoviruses
Attacks insects and arthropods only — harmless to plants, animals, birds, fish, humans
Species-specific: each NPV strain targets a narrow range of insect species
No negative impact on non-target organisms — ideal for IPM
Particularly useful in ecologically sensitive areas where chemical pesticides would cause damage
Biocontrol Examples
Natural predators (ladybird beetles, dragonflies) and antagonistic fungi (Trichoderma) are used as biocontrol agents against crop pests and plant diseases.
Nature already has a built-in pest control system — predators that feed on pest organisms. Biocontrol simply uses this natural system more strategically.
Ladybird beetles (ladybugs): Voracious predators of aphids — used in gardens and greenhouses
Dragonflies: Natural predators of mosquitoes — control mosquito populations near water bodies
Trichoderma (fungus): Free-living in soil, acts as biocontrol against several plant pathogens (soil-borne fungal diseases)
Baculoviruses (NPV): Kill specific insect pests without affecting beneficial insects
Kill target AND non-target
Kill only target pest (species-specific)
Pollute soil and water
No environmental pollution
Pests develop resistance
Resistance development is rare
Toxic residues in food
No toxic residues
Fast-acting
Slower but sustainable
Quick recall
Match: (a) Aphids, (b) Mosquitoes, (c) Soil-borne plant fungi — with their biocontrol agents.
Biofertilizers are organisms that enrich the nutrient quality of soil. Unlike chemical fertilizers, they don't pollute and actually improve soil health over time.
Biofertilizers are living microorganisms that, when applied to soil, seeds, or plant surfaces, colonize the rhizosphere (root zone) and promote plant growth by increasing the supply of nutrients. They are the eco-friendly alternative to chemical fertilizers.
Biofertilizers = living organisms that enhance soil fertility naturally
Main nutrient supplied: Nitrogen (via biological nitrogen fixation)
Also help with: Phosphorus solubilization and uptake
Three main categories: Bacteria (Rhizobium, Azotobacter), Cyanobacteria (Anabaena, Nostoc), Fungi (mycorrhiza)
Advantages: no pollution, improve soil structure, sustainable, cost-effective
Synthetic chemicals (urea, DAP)
Living microorganisms
Instant nutrient supply
Slow, sustained nutrient supply
Deplete soil health over time
Improve soil health and structure
Cause water pollution (eutrophication)
No pollution
Expensive in long run
Cost-effective and renewable
NCERT emphasizes biofertilizers as the sustainable alternative. Chemical fertilizers are a short-term fix; biofertilizers are the long-term solution.
Rhizobium
Rhizobium bacteria form root nodules on legumes (peas, beans, alfalfa) and fix atmospheric N₂ into ammonia using the enzyme nitrogenase. This is the most important symbiotic nitrogen fixation.
Rhizobium is a symbiotic nitrogen-fixing bacterium. It lives inside root nodules of leguminous plants (peas, beans, lentils, groundnut, soybean). The plant provides food and shelter; Rhizobium provides fixed nitrogen. Classic mutualism.
Rhizobium = symbiotic, lives in root nodules of legumes
Fixes atmospheric N _2 arrow NH _3 (ammonia) using nitrogenase enzyme
Nitrogenase is oxygen-sensitive — nodules contain leghaemoglobin to protect it
Leghaemoglobin gives nodules a pinkish colour — scavenges oxygen
This is why farmers rotate legumes with cereal crops — legumes naturally enrich soil nitrogen
Azotobacter & Azospirillum
Unlike Rhizobium, these bacteria fix nitrogen independently in the soil without needing a plant partner. They enrich soil nitrogen around any crop's roots.
Not all nitrogen fixers need a legume partner. Azotobacter and Azospirillum are free-living soil bacteria that fix atmospheric N _2 independently. They live in the rhizosphere (soil around roots) of virtually any crop.
Azotobacter: Free-living, aerobic, fixes N _2 in soil — found in crop fields worldwide
Both enrich soil nitrogen for non-leguminous crops too
These can be applied to any crop — not restricted to legumes like Rhizobium
Lives inside root nodules
Lives freely in soil/rhizosphere
Only in legumes
Any crop
Fixes more N _2 per plant
Fixes less N _2 per area
Requires specific host plant
No host requirement
Obligate symbiont
Independent organisms
Cyanobacteria
Cyanobacteria like Anabaena, Nostoc, and Oscillatoria are autotrophic nitrogen fixers. They are especially important as biofertilizers in waterlogged paddy (rice) fields.
Cyanobacteria (blue-green algae) are unique — they can photosynthesize (make their own food) AND fix atmospheric nitrogen. This double ability makes them invaluable as biofertilizers, especially in paddy fields where waterlogged conditions favour their growth.
Important genera: Anabaena, Nostoc, Oscillatoria
They are autotrophic — need only light, CO _2 , and N _2 to grow
Fix N _2 using heterocysts — specialized thick-walled cells that exclude O _2
Especially important in rice paddy fields (waterlogged conditions suit them)
Also add organic matter to soil, improving soil texture and water-holding capacity
Anabaena forms symbiotic association with Azolla (aquatic fern) — used in paddy fields
Quick recall
Why are cyanobacteria ideal biofertilizers for paddy fields specifically?
Paddy fields are waterlogged — perfect for cyanobacteria which are aquatic. They photosynthesize (need no external food), fix N _2 (free fertilizer), and add organic matter. Three benefits from one organism.
Mycorrhiza
Mycorrhiza is a symbiotic association between fungi (especially Glomus) and plant roots. The fungus extends the root's reach and dramatically increases phosphorus absorption.
Mycorrhiza (literally 'fungus root') is a symbiotic association between a fungus and plant roots. The fungal hyphae extend far into the soil — much farther than root hairs can reach — and absorb phosphorus and other minerals which they share with the plant. In return, the plant gives the fungus sugars.
Glomus is the most common mycorrhizal fungus (forms AM — arbuscular mycorrhiza)
Fungal hyphae act as extensions of the root system — absorb P, Zn, Cu, S from soil
Phosphorus uptake is the primary benefit — P is often the limiting nutrient in soil
Mycorrhiza also provides resistance to root-borne pathogens
Improves plant tolerance to salinity and drought
Found in ~80% of land plants — this is one of the most common symbioses on Earth
Quick recall
A farmer finds that his crop shows phosphorus deficiency despite adding phosphorus fertilizer. What biofertilizer would you recommend?
Mycorrhiza (genus Glomus). The fungal hyphae can access phosphorus that roots alone cannot reach. Mycorrhiza increases the effective absorption area of roots by 10-1000 times for phosphorus uptake.