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Microbes in Human Welfare

NCERT Class 12 · Biology Based on NCERT Class 12 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the beneficial roles of microbes in human life, from household products like curd and bread to industrial products like antibiotics and enzymes, as well as their critical functions in sewage treatment, biogas production, biocontrol, and biofertilisers. It exists in the syllabus to counter the misconception that all microbes are harmful, highlighting their indispensable contributions to health, agriculture, and environmental management. The one big idea a student must walk away with is that microbes are not just disease-causing agents but are essential tools for sustainable living, waste management, and industrial production.
Key Concepts (32)
Lactic Acid Bacteria (LAB)
Micro-organisms such as Lactobacillus that grow in milk and convert it to curd by producing acids that coagulate and partially digest milk proteins.
e.g. Adding a small amount of curd (starter) to fresh milk to make curd at home.
Baker's yeast
Saccharomyces cerevisiae, a yeast used for fermenting dough to make bread, producing CO2 that gives the dough a puffed-up appearance.
e.g. Using yeast in bread-making to make the dough rise.
Brewer's yeast
The same yeast Saccharomyces cerevisiae used for fermenting malted cereals and fruit juices to produce ethanol in alcoholic beverages.
e.g. Fermenting grape juice to produce wine.
Antibiotics
Chemical substances produced by some microbes that can kill or retard the growth of other (disease-causing) microbes.
e.g. Penicillin produced by the mould Penicillium notatum.
Penicillin
The first antibiotic to be discovered, produced by the mould Penicillium notatum, discovered by Alexander Fleming.
e.g. Used to treat bacterial infections in World War II soldiers.
Biochemical Oxygen Demand (BOD)
The amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria; a measure of the organic matter present in water.
e.g. A BOD of 400 mg/L indicates highly polluted water like untreated sewage.
Flocs
Masses of bacteria associated with fungal filaments to form mesh-like structures that grow in aeration tanks during secondary sewage treatment.
e.g. In secondary treatment, flocs consume organic matter and reduce BOD.
Activated sludge
The sediment of bacterial flocs formed in the settling tank after secondary treatment, part of which is used as inoculum for the aeration tank.
e.g. The sludge that settles after aeration and is partially recycled.
Methanogens
Bacteria that grow anaerobically on cellulosic material and produce large amounts of methane along with CO2 and H2; e.g., Methanobacterium.
e.g. Found in the rumen of cattle and in anaerobic sludge digesters.
Biogas
A mixture of gases (predominantly methane) produced by microbial activity, used as fuel; also called gobar gas when produced from cattle dung.
e.g. Using biogas for cooking and lighting in rural areas.
Biocontrol
The use of biological methods for controlling plant diseases and pests, relying on natural predation rather than introduced chemicals.
e.g. Using Bacillus thuringiensis (Bt) to kill butterfly caterpillars.
Biofertilisers
Organisms that enrich the nutrient quality of the soil, such as bacteria, fungi, and cyanobacteria.
e.g. Rhizobium in root nodules of leguminous plants fixing atmospheric nitrogen.
Mycorrhiza
A symbiotic association between fungi (e.g., Glomus) and plant roots, where the fungus absorbs phosphorus from soil and passes it to the plant.
e.g. Plants with mycorrhiza show increased resistance to root pathogens and tolerance to salinity.
Cyanobacteria
Autotrophic microbes that can fix atmospheric nitrogen, e.g., Anabaena, Nostoc, Oscillatoria; used as biofertilisers in paddy fields.
e.g. Blue-green algae in rice paddies add organic matter and fix nitrogen.
Cyclosporin A
A bioactive molecule used as an immunosuppressive agent in organ-transplant patients, produced by the fungus Trichoderma polysporum.
e.g. Given to transplant patients to prevent organ rejection.
Statins
Blood-cholesterol lowering agents produced by the yeast Monascus purpureus; they competitively inhibit the enzyme responsible for cholesterol synthesis.
e.g. Used as medication to lower high cholesterol levels.
Streptokinase
An enzyme produced by the bacterium Streptococcus, modified by genetic engineering, used as a 'clot buster' for removing clots from blood vessels in heart attack patients.
e.g. Administered to patients who have undergone myocardial infarction.
Bacillus thuringiensis (Bt)
A bacterium used as a microbial biocontrol agent; its dried spores are mixed with water and sprayed onto plants, and the toxin released in the insect gut kills the larvae.
e.g. Bt-cotton, a genetically engineered plant resistant to insect pests.
Trichoderma
A free-living fungus common in root ecosystems, used as an effective biocontrol agent against several plant pathogens.
e.g. Applied to soil to control fungal diseases in crops.
Nucleopolyhedrovirus
A type of baculovirus used as a biological control agent; it is species-specific, narrow-spectrum, and has no negative impact on non-target organisms.
e.g. Used in integrated pest management (IPM) programmes to target specific insect pests.
Primary sludge
The solids that settle during primary sewage treatment after filtration and sedimentation.
e.g. The sediment removed from the primary settling tank.
Anaerobic sludge digesters
Large tanks where the remaining major part of activated sludge is digested by anaerobic bacteria, producing biogas (methane, hydrogen sulphide, carbon dioxide).
e.g. The tank where sludge is broken down to produce methane.
Propionibacterium sharmanii
A bacterium that produces large amounts of CO2, responsible for the large holes in Swiss cheese.
e.g. Swiss cheese has characteristic holes due to this bacterium.
Roquefort cheese
A cheese ripened by growing a specific fungi on it, giving it a particular flavour.
e.g. A type of blue cheese with a distinct taste.
Toddy
A traditional drink made by fermenting sap from palms using microbes.
e.g. A popular beverage in parts of southern India.
Fermentors
Very large vessels used for growing microbes on an industrial scale to produce products like beverages and antibiotics.
e.g. Used in the production of beer and penicillin.
Pectinases and Proteases
Enzymes used to clarify bottled fruit juices; pectinases break down pectin, proteases break down proteins.
e.g. Bottled juices are clearer than homemade ones due to these enzymes.
Lipases
Enzymes used in detergent formulations to help remove oily stains from laundry.
e.g. Added to laundry detergents for stain removal.
Gobar gas
Common name for biogas produced from cattle dung (gobar).
e.g. Used for cooking and lighting in rural India.
Rhizobium
A symbiotic bacterium that forms nodules on the roots of leguminous plants and fixes atmospheric nitrogen into organic forms.
e.g. Found in root nodules of peas and beans.
Azospirillum and Azotobacter
Free-living bacteria that fix atmospheric nitrogen while living in the soil, enriching its nitrogen content.
e.g. Used as biofertilisers for non-leguminous crops.
Glomus
A genus of fungi that forms mycorrhizal associations with plants, helping in phosphorus absorption.
e.g. A mycorrhizal fungus used in biofertiliser formulations.
Formulas & Equations (3)
C6H12O6 → 2 C2H5OH + 2 CO2
Reads as: Glucose yields ethanol plus carbon dioxide
Variables: C6H12O6 = glucose; C2H5OH = ethanol; CO2 = carbon dioxide
Use when: Explaining alcoholic fermentation by yeast (Saccharomyces cerevisiae) in production of beverages or bread-making.
⚠ Students often write the equation as incomplete or forget to balance it, e.g., writing C6H12O6 → C2H5OH + CO2 without the coefficients.
BOD = amount of O2 consumed (in mg) per litre of water
Reads as: Biochemical oxygen demand equals the amount of oxygen consumed in milligrams per litre of water
Variables: BOD = Biochemical Oxygen Demand (mg/L); O2 = oxygen consumed by microbes to oxidise organic matter
Use when: Assessing the polluting potential of sewage or waste water; higher BOD indicates more organic matter and higher pollution.
⚠ Students confuse BOD with COD (Chemical Oxygen Demand) or think BOD measures dissolved oxygen directly, not the oxygen consumed.
CO2 + H2 → CH4 + H2O (simplified methanogenesis)
Reads as: Carbon dioxide plus hydrogen yields methane plus water
Variables: CO2 = carbon dioxide; H2 = hydrogen; CH4 = methane; H2O = water
Use when: Explaining biogas production by methanogens like Methanobacterium in anaerobic digesters or cattle rumen.
⚠ Students forget that methanogens produce methane along with CO2 and H2, not just methane alone.
How It Works
Curd formation by LAB
  1. Step 1: Lactic acid bacteria (LAB) such as Lactobacillus are added to fresh milk as inoculum or starter.
  2. Step 2: At suitable temperatures, LAB multiply and produce acids.
  3. Step 3: The acids coagulate and partially digest milk proteins, converting milk to curd.
  4. Step 4: The nutritional quality improves due to an increase in vitamin B12.
💡 LAB are already present in the curd used as starter; they are not added separately from an external source.
Fermentation of dough for idli/dosa and bread
  1. Step 1: Dough is prepared using rice and black gram (for idli/dosa) or wheat flour (for bread).
  2. Step 2: Bacteria (for idli/dosa) or baker's yeast (Saccharomyces cerevisiae) (for bread) ferment the dough.
  3. Step 3: CO2 gas is produced during fermentation, giving the dough a puffed-up appearance.
  4. Step 4: The dough is then cooked to make idli, dosa, or bread.
💡 For idli/dosa, the bacteria come from the environment or previous batch; for bread, yeast is added intentionally.
Cheese production using microbes
  1. Step 1: Milk is curdled using microbes or rennet.
  2. Step 2: Specific bacteria or fungi are added to impart characteristic texture, flavour, and taste.
  3. Step 3: For Swiss cheese, Propionibacterium sharmanii produces large amounts of CO2, creating holes.
  4. Step 4: For Roquefort cheese, specific fungi are grown on the cheese to give it a particular flavour.
💡 The specificity of cheese varieties comes from the type of microbe used, not just the milk source.
Production of alcoholic beverages
  1. Step 1: Malted cereals or fruit juices are used as raw material.
  2. Step 2: Brewer's yeast (Saccharomyces cerevisiae) ferments the sugars to produce ethanol.
  3. Step 3: For wine and beer, the fermented broth is used without distillation.
  4. Step 4: For whisky, brandy, and rum, the fermented broth is distilled to concentrate the alcohol.
💡 Distillation is the key difference between beverages like wine/beer (no distillation) and spirits like whisky/brandy/rum (distillation).
Sewage treatment (primary and secondary)
  1. Step 1 (Primary treatment): Floating debris is removed by sequential filtration; grit (soil and small pebbles) is removed by sedimentation. Solids settle as primary sludge; supernatant is the effluent.
  2. Step 2 (Secondary treatment): The primary effluent is passed into aeration tanks where it is constantly agitated and air is pumped in.
  3. Step 3: Aerobic microbes grow into flocs (masses of bacteria and fungal filaments) and consume organic matter, reducing BOD.
  4. Step 4: The effluent is passed into a settling tank where bacterial flocs sediment to form activated sludge.
  5. Step 5: A small part of activated sludge is pumped back to the aeration tank as inoculum; the remaining major part goes to anaerobic sludge digesters.
  6. Step 6: In anaerobic digesters, bacteria digest the sludge and produce biogas (methane, hydrogen sulphide, carbon dioxide).
  7. Step 7: The treated effluent is released into natural water bodies.
💡 Primary treatment is physical (filtration and sedimentation); secondary treatment is biological (aerobic microbial digestion).
Biogas production in a biogas plant
  1. Step 1: Bio-wastes (e.g., cattle dung) are collected and a slurry is fed into a concrete tank (10-15 feet deep).
  2. Step 2: A floating cover is placed over the slurry.
  3. Step 3: Methanogens (e.g., Methanobacterium) in the slurry anaerobically degrade cellulosic material, producing methane, CO2, and H2.
  4. Step 4: The gas accumulates under the floating cover, causing it to rise.
  5. Step 5: Biogas is supplied through an outlet pipe to nearby houses for cooking and lighting.
  6. Step 6: The spent slurry is removed through another outlet and used as fertiliser.
💡 Biogas is predominantly methane; the technology was developed in India by IARI and KVIC.
Biocontrol using Bacillus thuringiensis (Bt)
  1. Step 1: Dried spores of B. thuringiensis are mixed with water and sprayed onto vulnerable plants (e.g., brassicas, fruit trees).
  2. Step 2: Insect larvae eat the spores along with plant material.
  3. Step 3: In the gut of the larvae, the toxin is released.
  4. Step 4: The toxin kills the larvae, but leaves other insects unharmed.
💡 Bt toxin genes have been introduced into plants (e.g., Bt-cotton) through genetic engineering for built-in pest resistance.
Nitrogen fixation by Rhizobium in leguminous plants
  1. Step 1: Rhizobium bacteria form a symbiotic association with roots of leguminous plants, forming nodules.
  2. Step 2: The bacteria fix atmospheric nitrogen into organic forms (e.g., ammonia).
  3. Step 3: The plant uses these nitrogen compounds as nutrients.
  4. Step 4: In return, the plant provides carbohydrates and shelter to the bacteria.
💡 Rhizobium is a symbiotic nitrogen-fixer; free-living nitrogen-fixers include Azospirillum and Azotobacter.
Mycorrhizal association for phosphorus uptake
  1. Step 1: Fungi (e.g., Glomus) form symbiotic associations with plant roots (mycorrhiza).
  2. Step 2: The fungal symbiont absorbs phosphorus from the soil.
  3. Step 3: Phosphorus is passed to the plant, improving its nutrition.
  4. Step 4: The plant provides carbohydrates to the fungus.
💡 Mycorrhiza also provides benefits like resistance to root-borne pathogens, tolerance to salinity and drought, and overall increased plant growth.
Exam Traps — Don't Make These Mistakes
⚠ Confusing LAB with yeast in curd formation
✗ Yeast is used to make curd from milk.
→ Curd is made by lactic acid bacteria (Lactobacillus), not yeast. Yeast (Saccharomyces cerevisiae) is used for bread and alcoholic fermentation.
✓ Lactic acid bacteria (LAB) such as Lactobacillus convert milk to curd by producing acids that coagulate milk proteins.
⚠ Mixing up primary and secondary sewage treatment
✗ Primary treatment uses microbes to break down organic matter.
→ Primary treatment is physical (filtration and sedimentation), not biological. Secondary treatment is biological, using aerobic microbes in aeration tanks.
✓ Primary treatment involves physical removal of solids through filtration and sedimentation. Secondary treatment is a biological process where aerobic microbes in flocs consume organic matter, reducing BOD.
⚠ Thinking BOD measures dissolved oxygen directly
✗ BOD is the amount of oxygen dissolved in water.
→ BOD is the amount of oxygen consumed by microbes to oxidise organic matter in water, not the dissolved oxygen itself. Higher BOD indicates more organic pollution.
✓ BOD (Biochemical Oxygen Demand) is the amount of oxygen that would be consumed if all organic matter in one litre of water were oxidised by bacteria; it is an indirect measure of organic matter present.
⚠ Confusing the source of cyclosporin A and statins
✗ Cyclosporin A is produced by Monascus purpureus, and statins by Trichoderma polysporum.
→ These are swapped. Cyclosporin A is from Trichoderma polysporum (a fungus), and statins are from Monascus purpureus (a yeast).
✓ Cyclosporin A is produced by the fungus Trichoderma polysporum. Statins are produced by the yeast Monascus purpureus.
⚠ Assuming all microbes in sewage are harmful
✗ Sewage contains only harmful pathogenic microbes.
→ Sewage contains both pathogenic and beneficial microbes. Heterotrophic microbes naturally present in sewage are used in treatment to break down organic matter.
✓ Sewage contains large amounts of organic matter and microbes, many of which are pathogenic, but also contains beneficial heterotrophic microbes that are used in secondary treatment to consume organic matter.
⚠ Thinking biogas is only CO2
✗ Biogas is mainly carbon dioxide.
→ Biogas is predominantly methane (CH4), along with CO2 and H2S. Methane is the inflammable component that makes it useful as fuel.
✓ Biogas is a mixture of gases predominantly containing methane, along with carbon dioxide and hydrogen sulphide, produced by methanogens.
Compare & Contrast
Type of process
Primary sewage treatment
Secondary sewage treatment
Primary treatment is physical (filtration and sedimentation), while secondary treatment is biological (aerobic microbial digestion using flocs).
Mode of nitrogen fixation
Rhizobium
Azospirillum / Azotobacter
Rhizobium fixes nitrogen symbiotically in root nodules of leguminous plants, whereas Azospirillum and Azotobacter fix nitrogen while free-living in the soil.
Method of production
Wine and beer
Whisky, brandy, and rum
Wine and beer are produced without distillation of the fermented broth, while whisky, brandy, and rum are produced by distillation of the fermented broth.
Target of biocontrol
Bacillus thuringiensis (Bt)
Trichoderma
Bt is used to control insect pests (e.g., butterfly caterpillars) by producing a toxin in their gut, while Trichoderma is used to control plant pathogens (fungal diseases).
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (2-3 marks), 1 Long Answer (5 marks) — typically covering sewage treatment or biogas production as a 5-mark question.
Most asked: Role of LAB in curd formation · Sewage treatment process (primary vs secondary) · Biogas production and methanogens · Antibiotics (Penicillin discovery and significance) · Biofertilisers (Rhizobium, mycorrhiza, cyanobacteria) · Biocontrol agents (Bt, Trichoderma, baculoviruses)
Don't skip: Explain the process of sewage treatment with a diagram (5 marks); Describe the role of microbes in biogas production (3-5 marks); Compare and contrast primary and secondary sewage treatment (3 marks); Explain the role of microbes as biofertilisers with examples (3 marks)
One-Line Revision (read aloud before the exam)
  1. Lactic acid bacteria (LAB) convert milk to curd and improve its nutritional quality by increasing vitamin B12.
  2. Baker's yeast (Saccharomyces cerevisiae) is used to ferment dough for bread, producing CO2 that makes the dough rise.
  3. Swiss cheese has large holes due to CO2 produced by Propionibacterium sharmanii.
  4. Penicillin, the first antibiotic, was discovered by Alexander Fleming from the mould Penicillium notatum.
  5. BOD (Biochemical Oxygen Demand) measures the oxygen consumed to oxidise organic matter in water; higher BOD means more pollution.
  6. Primary sewage treatment is physical (filtration and sedimentation); secondary treatment is biological using aerobic microbes in flocs.
  7. Activated sludge is the sediment of bacterial flocs from secondary treatment; part is reused as inoculum, the rest goes to anaerobic digesters.
  8. Methanogens like Methanobacterium produce biogas (methane, CO2, H2) from cellulosic material in anaerobic conditions.
  9. Biogas plants were developed in India by IARI and KVIC; biogas is used for cooking and lighting in rural areas.
  10. Bacillus thuringiensis (Bt) is a biocontrol agent that kills insect larvae by releasing a toxin in their gut.
  11. Trichoderma is a fungus used as a biocontrol agent against plant pathogens.
  12. Rhizobium fixes atmospheric nitrogen symbiotically in root nodules of leguminous plants.
  13. Mycorrhiza (e.g., Glomus) helps plants absorb phosphorus from soil and provides resistance to pathogens and drought.
  14. Cyanobacteria like Anabaena and Nostoc fix atmospheric nitrogen and are used as biofertilisers in paddy fields.
  15. Cyclosporin A (immunosuppressant) is from Trichoderma polysporum; statins (cholesterol-lowering) are from Monascus purpureus.
  16. Streptokinase from Streptococcus is used as a clot buster for heart attack patients.
  17. Baculoviruses (Nucleopolyhedrovirus) are species-specific biocontrol agents with no impact on non-target organisms.
  18. Fermented beverages like wine and beer are made without distillation; whisky, brandy, and rum require distillation.
  19. LAB in our stomach play a beneficial role in checking disease-causing microbes.
  20. The puffed-up appearance of idli/dosa dough is due to CO2 produced by bacterial fermentation.

MCQs — 10 Questions with Answers

Q1. Lactobacillus converts milk into curd by producing which type of compound?

  • A. Organic acids that coagulate milk proteins ✓
  • B. Enzymes that break down fats
  • C. Gases that inflate the curd
  • D. Hormones that preserve milk

Answer: A — LAB produce lactic acid during fermentation, which lowers pH and causes milk proteins to coagulate and partially digest, forming curd.

Q2. Which yeast is used for both bread-making and commercial beverage production?

  • A. Candida albicans
  • B. Saccharomyces cerevisiae ✓
  • C. Schizosaccharomyces pombe
  • D. Pichia pastoris

Answer: B — Saccharomyces cerevisiae (baker's yeast or brewer's yeast) ferments dough and fruit juices to produce ethanol and CO₂ for bread puffing and beverage fermentation.

Q3. Large holes in Swiss cheese are produced by which bacterium?

  • A. Lactobacillus delbrueckii
  • B. Propionibacterium sharmanii ✓
  • C. Streptococcus thermophilus
  • D. Bacillus subtilis

Answer: B — Propionibacterium sharmanii produces large amounts of CO₂ gas during cheese fermentation, creating the characteristic large holes.

Q4. Penicillin was first discovered by Alexander Fleming in which year and from which organism?

  • A. 1920, from Bacillus subtilis
  • B. 1928, from Penicillium notatum ✓
  • C. 1945, from Streptomyces coelicolor
  • D. 1935, from Aspergillus fumigatus

Answer: B — Fleming discovered Penicillin in 1928 by observing a mould (Penicillium notatum) on an unwashed culture plate where Staphylococci could not grow.

Q5. Which of the following is NOT correctly matched with its industrial product?

  • A. Aspergillus niger — citric acid
  • B. Acetobacter aceti — acetic acid
  • C. Lactobacillus — butyric acid ✓
  • D. Clostridium butylicum — butyric acid

Answer: C — Lactobacillus produces lactic acid, not butyric acid; Clostridium butylicum is the correct source of butyric acid.

Q6. During dough fermentation for dosa and idli, the puffed-up appearance results from which gas?

  • A. Oxygen released by bacteria
  • B. Nitrogen from air incorporation
  • C. Carbon dioxide produced by bacterial fermentation ✓
  • D. Hydrogen released during protein breakdown

Answer: C — Bacteria ferment carbohydrates in dough to produce CO₂ gas, which creates the puffed-up appearance and improves texture.

Q7. Assertion: Lipases produced by microbes are used in detergent formulations. Reason: Lipases help remove oily stains from laundry by breaking down lipids.

  • A. Both assertion and reason are true, and reason explains assertion ✓
  • B. Both assertion and reason are true, but reason does not explain assertion
  • C. Assertion is true, but reason is false
  • D. Both assertion and reason are false

Answer: A — Both statements are correct: lipases are used in detergents, and they work by enzymatically breaking down oily/lipid stains on fabric.

Q8. If 100 kg of cattle dung is fermented anaerobically by methanogenic bacteria, which product is the primary energy source?

  • A. Carbon dioxide only
  • B. Methane (CH₄) and carbon dioxide (CO₂) mixture, with CH₄ being the fuel ✓
  • C. Hydrogen gas
  • D. Ethanol

Answer: B — Anaerobic decomposition of organic matter produces biogas (mixture of CH₄ ~60% and CO₂ ~40%), where methane is the primary fuel for energy.

Q9. Rhizobium bacteria fix atmospheric nitrogen in legume plants through symbiotic relationship. Which statement is correct?

  • A. Rhizobium acts as a biofertiliser by converting N₂ into plant-available nitrates in root nodules ✓
  • B. Rhizobium produces nitrogen-containing antibiotics to protect the plant
  • C. Rhizobium breaks down organic nitrogen from soil into ammonia
  • D. Rhizobium is a fungal partner in mycorrhizal association

Answer: A — Rhizobium fixes atmospheric N₂ via nitrogenase enzyme in root nodules, converting it to ammonia and eventually nitrates usable by legume plants.

Q10. Bacillus thuringiensis (Bt) toxin kills insect pests through which mechanism?

  • A. Inhibiting insect nerve impulse transmission
  • B. Cry proteins are activated by insect gut protease, forming pores in epithelium → cell lysis → death ✓
  • C. Blocking ATP synthesis in insect mitochondria
  • D. Preventing nutrient absorption in the insect digestive system

Answer: B — Bt toxin exists as inactive Cry proteins; insect gut proteases cleave and activate them, forming ion channels in epithelial cells causing lysis and death.

Flashcards

Which bacterium converts milk into curd by producing acids?

Lactobacillus (lactic acid bacteria or LAB) produces acids that coagulate and partially digest milk proteins.

Name the yeast used in bread-making and beverage fermentation.

Saccharomyces cerevisiae, also called baker's yeast or brewer's yeast, ferments dough and fruit juices to produce ethanol and CO₂.

Why do large holes appear in Swiss cheese?

Propionibacterium sharmanii produces large amounts of CO₂ gas during fermentation, creating the characteristic holes.

What is Penicillin and which organism produces it?

Penicillin is an antibiotic chemical produced by the mould Penicillium notatum that kills or retards growth of disease-causing bacteria.

Name one industrial use of Aspergillus niger.

Aspergillus niger produces citric acid commercially on industrial scale in large fermentors.

What is the role of lactic acid bacteria in the human stomach?

LAB in the stomach check the growth of disease-causing microbes, providing a beneficial role in digestion and health.

Which gas is produced during dough fermentation for dosa and idli?

Carbon dioxide (CO₂) gas is produced by bacteria during fermentation, causing the puffed-up appearance of dough.

Define antibiotics in one sentence.

Antibiotics are chemical substances produced by microbes that can kill or retard the growth of disease-causing microbes.

Name two acids produced by microbes industrially.

Citric acid (Aspergillus niger), acetic acid (Acetobacter aceti), butyric acid (Clostridium butylicum), and lactic acid (Lactobacillus) are examples.

What is the metabolic pathway by which yeast produces ethanol in beverages?

Yeast undergoes anaerobic fermentation (glycolysis + fermentation) of sugars to produce ethanol and CO₂ as end products.

Important Board Questions

Define antibiotic and name any two microorganisms that produce them industrially. Give one example of disease each antibiotic helps treat. [2 marks]

Define antibiotics as chemicals produced by microbes that kill pathogens. Name Penicillium notatum (penicillin—treats bacterial infections like plague) and Streptomyces species. State one disease each.

Explain how Lactobacillus converts milk into curd. Describe the biochemical changes occurring during this fermentation and name one additional benefit of curd consumption. [5 marks]

LAB ferment lactose via anaerobic pathway → produce lactic acid → lower pH → coagulate casein proteins → curdle milk. Mention partial protein digestion improves digestibility. State vitamin B12 increase improves nutrition.

Describe the industrial production and mode of action of Bt toxin in transgenic crops. Explain why Bt toxin is selective against insect pests and safe for humans. Draw a labelled diagram showing how Cry proteins form pores in insect epithelial cells. [6 marks]

Bacillus thuringiensis produces Cry proteins (δ-endotoxins); these are protoxins activated only by specific insect gut proteases at alkaline pH. Human stomach is acidic; human digestive enzymes do not activate Cry proteins → safe. Diagram: show inactive protoxin → protease cleavage → active toxin fragment → pore formation in epithelium → K⁺ efflux → cell lysis → insect death. Mention selectivity due to pH and protease specificity.

Assertion-Reason Questions for Microbes in Human Welfare

Q1. Assertion (A):

Lactobacillus used in curd production improves the nutritional quality of milk by increasing vitamin B12 content.

Reason (R):

Lactic acid bacteria synthesize B vitamins as secondary metabolites during fermentation.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true. LAB produce B vitamins, including B12, during milk fermentation through their metabolic processes. The reason correctly explains why vitamin B12 increases in curd.

Q2. Assertion (A):

Saccharomyces cerevisiae is used for both bread-making and alcoholic beverage production.

Reason (R):

This yeast produces the same end products (ethanol and CO₂) in both fermentation processes, making it equally suitable for both applications.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — The assertion is true—the same yeast species is used for both bread and beverages (called baker's and brewer's yeast respectively). However, the reason is partially misleading. While ethanol and CO₂ are produced in both, bread fermentation primarily requires CO₂ for leavening, while beverage production requires ethanol as the main product. The reason oversimplifies the mechanisms.

Q3. Assertion (A):

Propionibacterium sharmanii is used in Swiss cheese production to create large holes through gas formation.

Reason (R):

This bacterium undergoes anaerobic respiration producing propionic acid, which creates the holes through mechanical expansion.

Answer: C — A is true but R is false — The assertion is correct—P. sharmanii produces CO₂ (not propionic acid) that creates the characteristic holes. The reason is false because propionic acid doesn't create mechanical holes; it's the CO₂ gas that does. Students often confuse the end products of different bacterial fermentations.

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Case-Based Questions (CBQ) for Microbes in Human Welfare

Case Study:

A rural cooperative in Tamil Nadu wants to start a cottage industry producing curd from locally sourced milk. They learned that adding a small amount of previous batch curd as inoculum significantly accelerates fermentation at 37°C compared to starting from raw milk alone. The curd produced has a slightly sour taste due to lactic acid production. The cooperative notices that their curd spoils quickly in summer if stored at room temperature but stays fresh for 5-7 days when refrigerated. They want to improve shelf-life and consistency. An advisor suggests they use a pure culture of Lactobacillus starter instead of using previous batch curd.

Q1. Why does adding previous batch curd (inoculum) accelerate fermentation compared to raw milk alone?

  • A. Previous batch curd contains millions of viable LAB cells that start multiplying immediately ✓
  • B. Previous batch curd contains vitamins that stimulate milk proteins
  • C. Previous batch curd is already acidic, which triggers faster coagulation
  • D. Previous batch curd contains enzymes that substitute for LAB action

Answer: A

Q2. The sour taste of curd comes from the production of which compound during LAB fermentation?

  • A. Ethanol
  • B. Lactic acid ✓
  • C. Acetic acid
  • D. Propionic acid

Answer: B

Q3. Why does curd spoil quickly at room temperature in summer but remains fresh when refrigerated?

  • A. Higher temperature increases spoilage microbe growth rate and reduces LAB viability ✓
  • B. Refrigeration removes water from curd
  • C. Room temperature increases moisture content
  • D. LAB cannot survive at any temperature below 10°C

Answer: A

Q4. The advisor suggests using pure Lactobacillus starter instead of previous batch curd. What is the primary advantage of this approach?

  • A. Pure cultures eliminate the need for refrigeration
  • B. Pure cultures produce more lactic acid than mixed populations
  • C. Pure cultures ensure consistent microbial composition and predictable fermentation kinetics ✓
  • D. Pure cultures can ferment at room temperature without spoilage

Answer: C

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Previous Year Questions (PYQ) for Microbes in Human Welfare

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 57/4/2 · 2 marks

A laboratory assistant collected three water samples, namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant. But the laboratory attendant did not note which was which. The samples were labelled A, B and C and subjected to a BOD test. The BOD values of the three samples A, B and C were recorded as 30 mg/L, 8 mg/L and 400 mg/L respectively. (a) Which sample of the water is most polluted? (b) Can you assign the correct label to each, assuming the river water is relatively clean?

CBSE 2026 · Set 57/1/1 · 1 mark

Assertion (A): Flocs are masses of bacteria associated with fungal filaments in secondary treatment of sewage. Reason (R): Flocs help in digestion of solid waste by anaerobic respiration.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 57/1/1 · 5 marks

(b) (i) What are bio-fertilizers? (1 mark) (ii) Name the different types of microorganisms used as bio-fertilizers in organic farming and explain how each contributes to soil fertility. (3 marks) (iii) Write two advantages of using bio-fertilizers over chemical fertilizers. (1 mark)

CBSE 2026 · Set 57/1/2 · 1 mark

Microbes commonly used in kitchen are

  • Lactobacillus and Yeast
  • Penicillium and Yeast
  • Microspora and E.coli
  • Rhizopus and Lactobacillus

CBSE 2026 · Set 57/1/2 · 2 marks

(a) What is the role of LAB (Lactic Acid Bacteria) in preparation of curd? (1 mark) (b) Mention two health benefits of consuming curd. (1 mark)

CBSE 2026 · Set 57/2/1 · 1 mark

Assertion (A) : Microbes are used for commercial production of alcohol and beverages. Reason (R) : Yeast converts glucose into ethanol and carbon dioxide during fermentation.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not correct explanation for Assertion (A).
  • Assertion (A) is true, Reason (R) is false.
  • Assertion (A) is false, Reason (R) is true.

CBSE 2026 · Set 57/2/1 · 2 marks

After significant reduction of BOD during secondary treatment of sewage, what further processes are carried out ? Mention any one important outcome of these processes.

CBSE 2026 · Set 57/2/1 · 5 marks

(b) (i) Why are microbes considered effective biocontrol agents in sustainable agriculture ? (ii) Explain the role of the following as biocontrol agents naming the pests or diseases they help to control : (1) A Bacterium (2) A Fungus (3) Virus

CBSE 2026 · Set 57/2/2 · 5 marks

Why are microbes considered effective biocontrol agents in sustainable agriculture? Explain the role of the following as biocontrol agents naming the pests or diseases they help to control: (1) A Bacterium (2) A Fungus (3) Virus

CBSE 2026 · Set 57/2/3 · 1 mark

Caenorhabditis elegans is a nematode which is

  • Free living and non-pathogenic
  • Endoparasitic and pathogenic
  • Free living and pathogenic
  • Ecto-parasitic and non-pathogenic

CBSE 2026 · Set 57/2/3 · 1 mark

Assertion (A): Microbes are used for commercial production of alcohol and beverages. Reason (R): Yeast converts glucose into ethanol and carbon dioxide during fermentation.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not correct explanation for Assertion (A).
  • Assertion (A) is true, Reason (R) is false.
  • Assertion (A) is false, Reason (R) is true.

CBSE 2026 · Set 57/2/3 · 2 marks

Study the flow chart and answer the following questions: (a) Identify A, B and C in the flow chart. (b) Give one another example of bio-fertilizer B.

See all 92 board PYQs for this chapter — open PYQ tab →

Class 12 Biology Previous Year Questions, Chapter-wise

All 1074 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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