Microbes in Human Welfare Class 12: Concepts, Diagrams and Board Answers

What Microbes in Human Welfare is about

Weightage: This chapter is part of Unit VIII, Biology and Human Welfare, which carries 12 of the 70 theory marks together with Human Health and Disease.Source: CBSE Biology (044) syllabus 2026-27, course structure, Unit VIII Biology and Human Welfare

In real board papers: asked in 8 of the 8 board papers we hold (2023 to 2026), on average 4.9 marks per paper. See the questions

Key concepts

Microbes in household products

NCERT section 8.1 Microbes in Household Products

Curd from milk:
1. Lactobacillus and other lactic acid bacteria (LAB) grow in milk.
2. They produce acids that coagulate and partially digest milk proteins, turning milk into curd.
3. A little curd added to fresh milk is the inoculum (starter), containing millions of LAB that multiply at a suitable temperature.
4. Curd has better nutritional quality: more vitamin B₁₂.
5. In our stomach, LAB also check disease-causing microbes.

Fermented dough:

• Idli and dosa batter is fermented by bacteria; the puffed-up look comes from CO₂ produced during fermentation.
• Bread dough is fermented by baker's yeast, Saccharomyces cerevisiae, which also releases CO₂.

Other traditional foods and drinks:

• Toddy: a traditional drink of parts of southern India, made by fermenting palm sap.
• Microbes ferment fish, soyabean and bamboo shoots into foods.

Cheese (one of the oldest microbial foods): each variety's texture, flavour and taste comes from the microbes used.

• Swiss cheese: its large holes come from a large amount of CO₂ produced by the bacterium Propionibacterium sharmanii.
• Roquefort cheese: ripened by growing a specific fungus on it, which gives a particular flavour.

Worked example

Explain how a spoonful of curd turns warm milk into curd, and give two benefits of LAB to humans. (2 marks)

Try it first, then show the solutionSolution

1. The curd acts as an inoculum with millions of lactic acid bacteria; at a suitable temperature they multiply and produce acids that coagulate and partly digest milk proteins. (1)
2. Benefits: curd has more vitamin B₁₂ (better nutrition); in the stomach LAB check disease-causing microbes. (1)

Answer: LAB in the starter make acids that coagulate milk proteins; curd gains vitamin B₁₂ and LAB suppress gut pathogens.

Fermented beverages and industrial fermentation

NCERT section 8.2 Microbes in Industrial Products (8.2.1 Fermented Beverages; 8.2.3 Chemicals)

Industrial production needs microbes grown in very large vessels called fermentors.

Fermented beverages: yeasts have been used since ancient times for wine, beer, whisky, brandy and rum.

• The yeast is Saccharomyces cerevisiae (the same as baker's yeast), here called brewer's yeast.
• It ferments malted cereals and fruit juices to produce ethanol.
• The type of drink depends on the raw material and on processing:
• Without distillation: wine and beer.
• With distillation of the fermented broth: whisky, brandy and rum (higher alcohol content).

Chemicals made by microbes:

Product	Microbe	Type
Citric acid	Aspergillus niger	Fungus
Acetic acid	Acetobacter aceti	Bacterium
Butyric acid	Clostridium butylicum	Bacterium
Lactic acid	Lactobacillus	Bacterium
Ethanol (commercial)	Saccharomyces cerevisiae	Yeast

Worked example

Name the microbe used to make both bread and beer. Why is whisky stronger in alcohol than beer, though both are made by the same kind of fermentation? (2 marks)

Try it first, then show the solutionSolution

1. Saccharomyces cerevisiae (baker's/brewer's yeast). (1)
2. Beer is not distilled; whisky is made by distilling the fermented broth, which concentrates the ethanol. (1)

Answer: Saccharomyces cerevisiae; whisky is distilled, beer is not.

Antibiotics and their judicious use

NCERT section 8.2.2 Antibiotics (with resistance from 6.3)

Antibiotics are chemical substances produced by some microbes that kill or retard the growth of other (disease-causing) microbes. 'Anti' = against, 'bio' = life: against the life of pathogens, but 'pro life' for humans. They are among the most significant discoveries of the 20th century.

Discovery of penicillin (the first antibiotic, a chance discovery):
1. Alexander Fleming, working on Staphylococci, saw a mould growing on an unwashed culture plate; Staphylococci could not grow around it.
2. He found a chemical made by the mould and named it penicillin, after Penicillium notatum.
3. Its full potential as an effective antibiotic was established later by Ernest Chain and Howard Florey.
4. It was used extensively to treat American soldiers wounded in World War II.
5. Fleming, Chain and Florey won the Nobel Prize in 1945.

Impact: antibiotics greatly improved treatment of killers such as plague, diphtheria (gal ghotu), whooping cough (kali khansi) and leprosy (kusht rog); these once took millions of lives.

Judicious use (syllabus focus): in Chapter 6, NCERT explains that using antibiotics and drugs selects resistant microbes in months or years. So antibiotics should be taken only when prescribed, in the correct dose, and for the full course; misuse and overuse speed up the spread of resistant bacteria and make the drugs useless.

Worked example

Describe how penicillin was discovered and who established its value. Why is it important to complete a prescribed antibiotic course? (3 marks)

Try it first, then show the solutionSolution

1. Fleming, working on Staphylococci, noticed that a mould (Penicillium notatum) on an unwashed plate stopped their growth; he named the chemical penicillin. (1)
2. Ernest Chain and Howard Florey later established its full potential as an antibiotic; it treated soldiers in World War II; all three shared the 1945 Nobel Prize. (1)
3. Stopping early or misusing antibiotics leaves the more resistant bacteria alive, and they multiply, so resistance spreads (natural selection); completing the course prevents this. (1)

Answer: Chance observation by Fleming (Penicillium notatum); Chain and Florey proved its value; full courses avoid selecting resistant bacteria.

Enzymes and bioactive molecules from microbes

NCERT section 8.2.3 Chemicals, Enzymes and other Bioactive Molecules

Enzymes:

• Lipases: added to detergents to remove oily stains from laundry.
• Pectinases and proteases: clarify bottled fruit juices (why bought juices look clearer than home-made).
• Streptokinase: from the bacterium Streptococcus, modified by genetic engineering; used as a 'clot buster' to remove clots from blood vessels of patients who have had a myocardial infarction (heart attack).

Bioactive molecules:

• Cyclosporin A: from the fungus Trichoderma polysporum; an immunosuppressive agent for organ-transplant patients.
• Statins: from the yeast Monascus purpureus; lower blood cholesterol by competitively inhibiting the enzyme responsible for cholesterol synthesis.

Link to Chapter 7: transplant patients need lifelong immunosuppressants because cell-mediated immunity rejects grafts; cyclosporin A is one such drug.

Worked example

Name the source organism and use of (a) streptokinase, (b) cyclosporin A, (c) statins. How do statins work? (2 marks)

Try it first, then show the solutionSolution

1. (a) Streptococcus (genetically modified); clot buster after myocardial infarction. (b) Trichoderma polysporum; immunosuppressant in organ transplants. (1)
2. (c) Monascus purpureus (yeast); lowers blood cholesterol by competitively inhibiting the enzyme that synthesises cholesterol. (1)

Answer: (a) Streptococcus, clot buster (b) Trichoderma polysporum, immunosuppressant (c) Monascus purpureus, cholesterol-lowering by competitive inhibition.

Microbes in sewage treatment and BOD

NCERT section 8.3 Microbes in Sewage Treatment

Sewage: municipal waste water, mostly human excreta, with large amounts of organic matter and microbes, many pathogenic. It cannot be discharged directly into rivers, so it is treated in sewage treatment plants (STPs) by heterotrophic microbes naturally present in it.

Primary treatment (physical):
1. Floating debris removed by sequential filtration.
2. Grit (soil, small pebbles) removed by sedimentation.
3. Settled solids = primary sludge; the liquid above = effluent, which goes to secondary treatment.

Secondary treatment (biological):
1. Primary effluent goes into large aeration tanks, constantly agitated with air pumped in.
2. Useful aerobic microbes grow vigorously into flocs (masses of bacteria with fungal filaments forming mesh-like structures).
3. Flocs consume most of the organic matter, which greatly reduces the BOD.
4. When BOD is low enough, effluent passes to a settling tank where flocs sediment as activated sludge.
5. Some of this activated sludge (a small part) is returned to the aeration tank to act as inoculum.
6. The major part goes to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi of the sludge, producing methane, hydrogen sulphide and CO₂: biogas, an inflammable energy source.
7. The secondary effluent is usually released into rivers and streams.

BOD (biochemical oxygen demand): the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The BOD test measures the rate of oxygen uptake by microbes, so it indirectly measures organic matter. Higher BOD = more polluting.

The problem: urbanisation produces far more sewage than existing STPs can treat, so untreated sewage enters rivers, causing pollution and water-borne diseases. The Ministry of Environment and Forests started the Ganga Action Plan and Yamuna Action Plan to build many STPs so that only treated sewage reaches these rivers. Microbial sewage treatment has been used for over a century and no man-made technology has matched it.

Worked example

Three unlabelled samples, river water, untreated sewage and secondary effluent from an STP, gave BOD values of P = 280 mg/L, Q = 6 mg/L and R = 35 mg/L. Identify each, assuming the river water is fairly clean, and find the percentage by which treatment lowered the BOD. (2 marks)

Try it first, then show the solutionSolution

1. Higher BOD means more organic matter: P (280 mg/L) is untreated sewage, the most polluted; Q (6 mg/L, lowest) is river water; R (35 mg/L) is secondary effluent, much lower than sewage but higher than clean river water. (1)
2. Fall in BOD = 280 − 35 = 245 mg/L; 245 ÷ 280 × 100 = 87.5%. Check: 280 × 0.125 = 35. (1)

Answer: P = untreated sewage, Q = river water, R = secondary effluent; treatment lowered BOD by 87.5%.

Microbes in biogas production

NCERT section 8.4 Microbes in Production of Biogas

Biogas: a mixture of gases, mainly methane, produced by microbial activity and used as fuel. The gas a microbe makes depends on the microbe and its substrate: in dough, cheese and beverages the main gas is CO₂.

Methanogens: bacteria that grow anaerobically on cellulosic material and produce large amounts of methane along with CO₂ and H₂; a common one is Methanobacterium.

• Found in the anaerobic sludge of sewage treatment.
• Found in the rumen (part of the stomach) of cattle, where they help break down cellulose and so help cattle nutrition (humans cannot digest cellulose).
• So cattle dung (gobar) is rich in methanogens and can be used to make gobar gas.

The biogas plant:
1. A concrete tank (10–15 feet deep) where bio-wastes are collected and a slurry of dung is fed.
2. A floating cover over the slurry rises as gas is produced by microbial activity.
3. A gas outlet pipe supplies biogas to nearby houses for cooking and lighting.
4. Spent slurry is removed through another outlet and used as fertiliser.

Why rural areas: cattle dung is available in large quantities there. Biogas technology in India was developed mainly by IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission).

Worked example

Why is cattle dung used for biogas? Name the group of bacteria involved and the main gas produced, and state two uses of the products of a biogas plant. (3 marks)

Try it first, then show the solutionSolution

1. Methanogens such as Methanobacterium live in the rumen of cattle, helping digest cellulose, so dung is rich in them and in cellulosic matter. (1)
2. Under anaerobic conditions they produce mainly methane, with CO₂ and H₂. (1)
3. Biogas is used for cooking and lighting; the spent slurry is used as fertiliser. (1)

Answer: Dung carries methanogens; methane is the main gas; uses: fuel for cooking/lighting and slurry as fertiliser.

Microbes as biocontrol agents

NCERT section 8.5 Microbes as Biocontrol Agents

Biocontrol: using biological methods to control plant diseases and pests. Chemical insecticides, pesticides and weedicides are toxic to humans and animals and pollute soil, ground water, fruits, vegetables and crops.

The organic farmer's view:

• Biodiversity furthers health: the more variety in a landscape, the more sustainable it is.
• Pests are not eradicated but kept at manageable levels by checks and balances in a living ecosystem. Eradicating them is undesirable, because beneficial predatory and parasitic insects depend on them as food or hosts.
• Conventional farming kills useful and harmful forms indiscriminately; organic farming is holistic, based on knowing the field's organisms, their life cycles, feeding patterns and habitats.

Examples:

• Ladybird (red and black beetle) eats aphids; dragonflies eat mosquitoes.
• Bacillus thuringiensis (Bt): a bacterium used against butterfly caterpillars. Sold as dried spores in sachets, mixed with water and sprayed on plants such as brassicas and fruit trees. Larvae eat them; the toxin is released in the larval gut and kills them, leaving other insects unharmed. Bt toxin genes have been put into plants, e.g. Bt-cotton, grown in some Indian states.
• Trichoderma: free-living fungi common in root ecosystems; effective biocontrol of several plant pathogens.
• Baculoviruses (mostly genus Nucleopolyhedrovirus): attack insects and other arthropods; species-specific, narrow-spectrum insecticides with no negative impact on plants, mammals, birds, fish or non-target insects. Valuable in integrated pest management (IPM) and in ecologically sensitive areas.

Benefit: biocontrol greatly reduces dependence on toxic chemicals.

Worked example

Explain how Bacillus thuringiensis controls caterpillars and why baculoviruses are preferred in integrated pest management. (3 marks)

Try it first, then show the solutionSolution

1. Dried Bt spores are mixed with water and sprayed on plants; caterpillars eat them, and the toxin is released in the larval gut, killing the larvae but leaving other insects unharmed. (1½)
2. Baculoviruses (Nucleopolyhedrovirus) are species-specific and narrow-spectrum. (½)
3. They do not harm plants, mammals, birds, fish or non-target (beneficial) insects, so useful insects are conserved in IPM. (1)

Answer: Bt toxin kills caterpillars in the gut; baculoviruses are species-specific and spare non-target organisms.

Microbes as biofertilisers

NCERT section 8.6 Microbes as Biofertilisers

Overuse of chemical fertilisers contributes to pollution, so there is pressure to switch to organic farming with biofertilisers: organisms that enrich the nutrient quality of the soil. Main sources: bacteria, fungi and cyanobacteria.

Bacteria (nitrogen fixers):

• Rhizobium: forms symbiotic root nodules on leguminous plants; fixes atmospheric nitrogen into organic forms the plant uses.
• Azospirillum and Azotobacter: fix nitrogen while free-living in soil.

Fungi (mycorrhiza):

• Symbiotic association of fungi with plant roots; many members of the genus Glomus form mycorrhiza.
• The fungus absorbs phosphorus from soil and passes it to the plant.
• Other benefits to the plant: resistance to root-borne pathogens, tolerance to salinity and drought, overall better growth.
• The fungus, in return, gets food (organic compounds) from the plant.

Cyanobacteria (blue-green algae):

• Autotrophic microbes in water and on land; many fix nitrogen, e.g. Anabaena, Nostoc, Oscillatoria.
• Important biofertilisers in paddy fields; they also add organic matter and increase soil fertility.

Many biofertilisers are sold commercially in India; farmers use them to replenish soil nutrients and reduce chemical fertilisers.

Worked example

How do Rhizobium, Glomus and Anabaena each enrich soil or help crops? (3 marks)

Try it first, then show the solutionSolution

1. Rhizobium: lives in root nodules of legumes and fixes atmospheric nitrogen into organic forms used by the plant. (1)
2. Glomus: forms mycorrhiza; the fungus absorbs phosphorus from soil for the plant and gives resistance to root pathogens and tolerance to salinity and drought. (1)
3. Anabaena: a nitrogen-fixing cyanobacterium in paddy fields; it also adds organic matter, increasing fertility. (1)

Answer: Rhizobium fixes N symbiotically; Glomus supplies phosphorus; Anabaena fixes N and adds organic matter.

Microbes in Human Welfare in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Sewage Treatment and BOD
• Microbes as Biocontrol Agents
• Microbes as Biofertilisers

Microbes in Human Welfare: key facts

Organic acid producers

$\text{Aspergillus niger}\to \text{citric acid};\text{Acetobacter aceti}\to \text{acetic acid};\text{Clostridium butylicum}\to \text{butyric acid};\text{Lactobacillus}\to \text{lactic acid}$

Only Aspergillus is a fungus; the rest are bacteria.

Bioactive molecules

$\text{Streptococcus}\to \text{streptokinase};\text{Trichoderma polysporum}\to \text{cyclosporin A};\text{Monascus purpureus}\to \text{statins}$

Clot buster; immunosuppressant; cholesterol-lowering.

Beverages

$\text{wine, beer (no distillation)};\text{whisky, brandy, rum (distilled)}$

All by Saccharomyces cerevisiae (brewer's yeast).

Sewage treatment sequence

$\text{filtration}\to \text{sedimentation}\to \text{aeration (flocs)}\to \text{settling (activated sludge)}\to \text{anaerobic digester (biogas)}$

BOD falls during aeration; higher BOD = more polluted.

Biogas composition

$\text{biogas}=\text{mainly }C{H}_4+C{O}_2+H_2(\text{and }{H}_{2}S\text{ from sludge digesters})$

Made by methanogens such as Methanobacterium.

Biofertilisers

$\text{Rhizobium}\text{ (symbiotic N)};\text{Azotobacter},\text{Azospirillum}\text{ (free-living N)};\text{Glomus}\text{ (P)};\text{Anabaena},\text{Nostoc}\text{ (N, paddy)}$

Mycorrhiza also gives drought and salinity tolerance.

Microbes in Human Welfare: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Microbes in Human Welfare

The question patterns to expect, and the method that works for each.

Tricks to remember Microbes in Human Welfare

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Acids: 'A Citric Aspergillus, A Acetic Acetobacter, Butyric Clostridium, Lactic Lactobacillus'

  Citric–Aspergillus niger; Acetic–Acetobacter aceti; Butyric–Clostridium butylicum; Lactic–Lactobacillus.

  Use it when: Organic acid producers.

• Swiss holes from 'Propionibacterium', 'P for Pops of gas'

  Large CO₂ bubbles in Swiss cheese from Propionibacterium sharmanii; Roquefort ripened by a fungus.

  Use it when: Cheese examples.

• S-C-S: Streptokinase, Cyclosporin, Statins

  Streptococcus busts clots; Trichoderma polysporum (T for Transplant) gives cyclosporin A; Monascus (M for Must lower cholesterol) gives statins.

  Use it when: Bioactive molecules.

• Sewage: 'Filter, Settle, Bubble, Settle, Digest'

  Filtration, sedimentation (primary); aeration with flocs, settling to activated sludge, anaerobic digestion (secondary).

  Use it when: Sewage treatment order.

• Wine and Beer are 'WB: Without Boiling' (no distillation)

  Whisky, brandy, rum are distilled.

  Use it when: Distilled versus undistilled beverages.

• Biofertilisers N-P-N

  Bacteria give N (Rhizobium, Azotobacter, Azospirillum); fungi give P (Glomus); cyanobacteria give N (Anabaena, Nostoc, Oscillatoria).

  Use it when: Biofertiliser roles.

Microbes in Human Welfare: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Microbes in Human Welfare

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying primary sewage treatment is biological

  Wrong working

  In primary treatment, bacteria digest the organic matter and reduce BOD.

  Right working

  Primary treatment is physical (filtration and sedimentation); BOD is reduced in secondary (biological) treatment by aerobic flocs.

  ✓Do this: Primary = physical; secondary = biological.

  Usually costs: 1 mark.

• ✗Mistake: Writing that activated sludge goes entirely to the digester

  ✓Do this: A small part is pumped back into the aeration tank as inoculum; the major part goes to anaerobic sludge digesters.

  Usually costs: ½ to 1 mark.

• ✗Mistake: Thinking a high BOD means clean water

  ✓Do this: High BOD means more organic matter and more pollution; clean water has low BOD.

  Usually costs: 1 mark.

• ✗Mistake: Naming Penicillium chrysogenum or giving Fleming alone the Nobel Prize

  ✓Do this: NCERT names Penicillium notatum; Fleming, Chain and Florey shared the 1945 Nobel Prize.

  Usually costs: ½ mark.

• ✗Mistake: Calling Trichoderma a bacterium

  ✓Do this: Trichoderma is a free-living fungus of root ecosystems (biocontrol); Trichoderma polysporum also gives cyclosporin A.

  Usually costs: ½ mark.

• ✗Mistake: Saying Bt kills all insects

  ✓Do this: Bt toxin kills specific larvae (caterpillars) in their gut, leaving other insects unharmed.

  Usually costs: 1 mark.

NCERT exercise guide: Microbes in Human Welfare

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Microbes in Human Welfare: previous year questions

• Asked in 8 of the 8 board papers we hold (2023 to 2026), on average 4.9 marks per paper; 17 questions in all, counting OR alternatives. In 3 of 4 sample papers.

Counted from the real papers on our previous year papers page, not estimated. Average marks are over every board paper we hold, with an OR pair counted once. Chapter tags are ours.

Solved board questions from Microbes in Human Welfare

Real questions from recent papers. Try each one first, then open our step-by-step solution with marks per step.

Every Microbes in Human Welfare question, by year

Practice questions on Microbes in Human Welfare

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which microbe is used in the commercial production of citric acid?
   (a) Acetobacter aceti (b) Aspergillus niger (c) Clostridium butylicum (d) Lactobacillus

   Hint

   A fungus.

   Answer

   (b) Aspergillus niger

2. Q2 · 1 mark · Assertion-reasonBoard level

   Assertion (A): Wine and beer are produced without distillation.
   Reason (R): Whisky, brandy and rum are produced by distillation of the fermented broth.
   (a) Both A and R are true and R is the correct explanation of A.
   (b) Both A and R are true but R is not the correct explanation of A.
   (c) A is true but R is false.
   (d) A is false but R is true.

   Hint

   Both facts; does one explain the other?

   Answer

   (b) Both true, but R does not explain A.

3. Q3 · 1 mark · MCQHarder board level

   Methanogens are found in all of these EXCEPT:
   (a) anaerobic sludge (b) rumen of cattle (c) cattle dung (d) aeration tank of an STP

   Hint

   Methanogens are anaerobic.

   Answer

   (d) aeration tank (it is aerobic)

4. Q4 · 2 marks · Very short answerBoard level

   Name two traditional Indian foods made with microbial fermentation and the gas responsible for their texture. (2 marks)

   Hint

   Rice and black gram batter.

   Answer

   Idli and dosa (also bread, toddy, or fermented bamboo shoots) (1). CO₂ produced during fermentation makes the batter puff up (1).

5. Q5 · 2 marks · Very short answerBoard level

   Why are bottled fruit juices clearer than home-made ones, and how do lipases help in laundry? (2 marks)

   Hint

   Enzymes.

   Answer

   Bottled juices are clarified with pectinases and proteases (1). Lipases in detergents remove oily stains (1).

6. Q6 · 2 marks · Very short answerHarder board level

   Explain why the floating cover of a biogas plant rises during operation, and why the spent slurry is valuable. (2 marks)

   Hint

   Gas collects.

   Answer

   Methanogens digesting the dung produce gas (mainly methane), which collects under the cover and pushes it up (1). The spent slurry is rich in nutrients and is used as fertiliser (1).

7. Q7 · 3 marks · Short answerHarder board level

   Describe primary treatment of sewage. What are primary sludge and primary effluent? (3 marks)

   Hint

   Physical removal in stages.

   Answer

   Primary treatment physically removes particles in stages (1): floating debris by sequential filtration, then grit (soil, small pebbles) by sedimentation (1). Settled solids form the primary sludge; the supernatant is the effluent, which goes for secondary treatment (1).

8. Q8 · 3 marks · Short answerHarder board level

   How do biofertilisers enrich soil? Give one example each of a symbiotic nitrogen fixer, a free-living nitrogen fixer and a phosphorus supplier. (3 marks)

   Hint

   Bacteria, fungi, cyanobacteria.

   Answer

   Biofertilisers are organisms that enrich soil nutrients by fixing nitrogen, supplying phosphorus or adding organic matter (1). Symbiotic N-fixer: Rhizobium in legume nodules; free-living: Azotobacter or Azospirillum (1). Phosphorus: Glomus (mycorrhiza); also cyanobacteria (Anabaena, Nostoc) fix N and add organic matter in paddy (1).

9. Q9 · 5 marks · Long answerHarder board level

   (a) What are antibiotics? Describe the discovery of penicillin. (3)
   (b) Explain how misuse of antibiotics leads to resistant bacteria and suggest two ways to prevent it. (2) (5 marks)

   Hint

   Natural selection in microbes.

   Answer

   (a) Antibiotics are chemicals produced by some microbes that kill or retard the growth of other, disease-causing microbes (1). Fleming, working on Staphylococci, saw that a mould (Penicillium notatum) on an unwashed plate stopped their growth and named the chemical penicillin (1). Chain and Florey established its potential; it treated World War II soldiers; Nobel Prize 1945 (1). (b) A bacterial population has variation; overuse or incomplete courses kill sensitive cells and leave resistant ones, which multiply, so resistance appears in months or years (1). Prevention: take antibiotics only when prescribed, in the right dose and for the full course; do not use them for viral infections such as the common cold (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose a river downstream of a town shows BOD rising from 5 mg/L to 60 mg/L and many dead fish. Explain the link, and suggest a microbial solution. (3 marks)

    Hint

    Organic load uses up oxygen.

    Answer

    Untreated sewage adds organic matter, so microbes decomposing it consume dissolved oxygen: BOD rises from 5 to 60 mg/L (1). Low dissolved oxygen kills fish (1). Building STPs where aerobic microbes (flocs) consume organic matter before discharge, as in the Ganga and Yamuna Action Plans, would lower BOD (1).

11. Q11 · 3 marks · Short answerHOTS / competency

    A farmer sprays a broad-spectrum chemical insecticide and finds his crop pests return worse the next year, while ladybirds have disappeared. Explain, and suggest a better approach. (3 marks)

    Hint

    Natural enemies.

    Answer

    Broad-spectrum chemicals kill useful predators such as ladybirds (which eat aphids) along with pests (1). Without natural enemies, surviving or returning pests multiply unchecked (and resistant pests may be selected) (1). Biocontrol and IPM, using ladybirds, Bt for caterpillars or species-specific baculoviruses, keeps pests at manageable levels while conserving beneficial insects (1).

12. Q12 · 2 marks · Very short answerHarder board level

    Why is Swiss cheese different from Roquefort cheese? (2 marks)

    Hint

    Bacterium versus fungus.

    Answer

    Swiss cheese has large holes because the bacterium Propionibacterium sharmanii produces large amounts of CO₂ (1). Roquefort cheese is ripened by growing a specific fungus on it, which gives it a particular flavour (1).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which vitamin increases when milk turns to curd?Show answer

   Vitamin B₁₂.

2. 2.What is toddy?Show answer

   A traditional drink of parts of southern India made by fermenting palm sap.

3. 3.What makes the large holes in Swiss cheese?Show answer

   CO₂ produced in large amounts by Propionibacterium sharmanii.

4. 4.Name the enzymes used to clarify fruit juices.Show answer

   Pectinases and proteases.

5. 5.What is used as a clot buster?Show answer

   Streptokinase from Streptococcus, modified by genetic engineering.

6. 6.What are flocs?Show answer

   Masses of bacteria associated with fungal filaments forming mesh-like structures in the aeration tank.

7. 7.Define BOD.Show answer

   The amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.

8. 8.Which gases are produced in anaerobic sludge digesters?Show answer

   Methane, hydrogen sulphide and carbon dioxide.

9. 9.Name a common methanogen.Show answer

   Methanobacterium.

10. 10.Which organisations developed biogas technology in India?Show answer

    IARI and KVIC.

11. 11.Which genus do most biocontrol baculoviruses belong to?Show answer

    Nucleopolyhedrovirus.

12. 12.Which fungus forms mycorrhiza and what does it supply?Show answer

    Glomus; it absorbs phosphorus from soil for the plant.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you explain how LAB make curd and why curd is more nutritious?
• ☐Can you name the microbes behind Swiss cheese, bread, idli and toddy?
• ☐Can you tell distilled from undistilled beverages?
• ☐Can you narrate the discovery of penicillin and explain why antibiotics must be used judiciously?
• ☐Can you match four acids and three bioactive molecules with their microbes?
• ☐Can you describe primary and secondary sewage treatment and define BOD?
• ☐Can you draw a biogas plant and explain the role of methanogens?
• ☐Can you give two examples each of biocontrol agents and biofertilisers with how they work?

Microbes in Human Welfare: frequently asked questions