Microbes in Human Welfare ISC Class 12 Biology: Notes

What Microbes in Human Welfare is about

Weightage: Microbes in Human Welfare is part of the unit Biology and Human Welfare, which carries 14 of the 70 marks in the ISC Biology Paper 1 (Theory), shared with 1 other chapter. Questions on it can appear in any section of the paper.Source: CISCE ISC Biology syllabus, examination year 2027, distribution of marks

Key concepts

Microbes in household products

Curd: Lactobacillus and other lactic acid bacteria (LAB) added as a small amount of curd (inoculum or 'starter') to milk multiply and convert lactose to lactic acid, which coagulates and partially digests milk proteins (casein). LAB also increase vitamin B₁₂ and, in the stomach, help check disease-causing microbes. Bread: dough is fermented by baker's yeast, Saccharomyces cerevisiae; the CO₂ released makes the dough rise (puff up). Dough for dosa and idli is fermented by bacteria (puffy appearance due to CO₂). Toddy, a traditional drink of southern India, is made by fermenting sap from palms. Cheese: different microbes give distinct textures, flavours and tastes: Propionibacterium sharmanii produces CO₂ that forms the large holes in Swiss cheese; Roquefort cheese is ripened by specific fungi (Penicillium roqueforti) for its flavour. Microbes are also used to ferment fish, soybean and bamboo shoots.

Worked example

Why does curd have more vitamin B₁₂ than milk, and why does Swiss cheese have big holes?

Try it first, then show the solutionSolution

1. Lactic acid bacteria that ferment milk into curd synthesise vitamin B₁₂.
2. Propionibacterium sharmanii in Swiss cheese releases large amounts of CO₂, which forms the holes.

Answer: LAB make B₁₂; Propionibacterium produces CO₂

Fermented beverages and antibiotics

Beverages: Saccharomyces cerevisiae (brewer's yeast) ferments malted cereals and fruit juices to produce ethanol. Without distillation: wine and beer (lower alcohol content). With distillation of the fermented broth: whisky, brandy and rum (higher alcohol content). Antibiotics: chemical substances produced by some microbes that kill or retard the growth of other (disease-causing) microbes. Penicillin was the first antibiotic discovered: Alexander Fleming (1928), working on Staphylococcus, noticed that a mould growing on an unwashed culture plate prevented bacterial growth around it; the mould was Penicillium notatum, and the chemical was named penicillin. Ernst Chain and Howard Florey established its full potential as an effective antibiotic; it was used extensively to treat soldiers wounded in World War II; the three shared the 1945 Nobel Prize. Antibiotics have greatly reduced deaths from plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog).

Worked example

Name the microbe used in brewing and distinguish beverages made with and without distillation, with two examples each.

Try it first, then show the solutionSolution

1. Saccharomyces cerevisiae ferments sugars to ethanol.
2. Without distillation (lower alcohol): wine, beer. With distillation of the fermented broth (higher alcohol): whisky, brandy (also rum).

Answer: Saccharomyces cerevisiae; wine/beer vs whisky/brandy

Organic acids, alcohols, enzymes and bioactive molecules

Organic acids (source → product): Aspergillus niger (fungus) → citric acid; Acetobacter aceti (bacterium) → acetic acid; Clostridium butylicum (bacterium) → butyric acid; Lactobacillus (bacterium) → lactic acid. Alcohol: Saccharomyces cerevisiae → ethanol (industrial). Enzymes: lipases are used in detergent formulations to remove oily stains from laundry; pectinases and proteases clarify bottled fruit juices (they break down pectin and proteins that make juice cloudy); streptokinase, from the bacterium Streptococcus (modified by genetic engineering), is a 'clot buster' that dissolves blood clots in the blood vessels of patients with myocardial infarction (heart attack). Bioactive molecules: cyclosporin-A, from the fungus Trichoderma polysporum, is an immunosuppressive agent used in organ-transplant patients. Statins, from the yeast Monascus purpureus, are blood-cholesterol-lowering agents: they competitively inhibit the enzyme (HMG-CoA reductase) responsible for cholesterol synthesis.

Worked example

Why are microbial lipases added to laundry detergents, and why does very hot wash water make them less useful?

Try it first, then show the solutionSolution

1. Lipases hydrolyse fats and oils into fatty acids and glycerol, which wash away, so oily stains are removed.
2. Lipases are proteins; very high temperatures denature them, destroying their activity.

Answer: They digest oily stains; heat denatures the enzyme

Sewage treatment

Sewage is municipal waste water containing large amounts of organic matter and microbes, many pathogenic; it is treated in sewage treatment plants (STPs) before release into rivers. Primary treatment (physical): removal of particles, large and small, by filtration and sedimentation in stages: floating debris removed by sequential filtration; grit (soil, pebbles) by sedimentation. Settled solids form the primary sludge; the supernatant is the effluent. Secondary (biological) treatment: the primary effluent goes to large aeration tanks where it is constantly agitated mechanically and air is pumped in. Useful aerobic microbes grow rapidly into flocs (masses of bacteria associated with fungal filaments forming mesh-like structures). They consume the major part of the organic matter, greatly reducing the 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. A high BOD means high polluting potential. When BOD has fallen enough, the effluent passes to a settling tank where the flocs sediment as activated sludge. A small part of it is pumped back to the aeration tank as inoculum; the major part is pumped to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge, producing a mixture of gases (methane, H₂S and CO₂): biogas. The effluent is released into natural water bodies. India's Ministry of Environment and Forests started the Ganga Action Plan and Yamuna Action Plan to build STPs.

Worked example

Suppose the BOD of raw sewage is 320 mg L⁻¹ and after secondary treatment it is 24 mg L⁻¹. Find the percentage reduction and explain what caused it.

Try it first, then show the solutionSolution

1. Reduction = (320 − 24)/320 × 100 = 92.5%.
2. Aerobic microbes in flocs in the aeration tank oxidised most of the organic matter, so far less oxygen is now demanded.

Answer: 92.5% reduction, due to aerobic flocs

Biogas production

Biogas is a mixture of gases (predominantly methane) produced by microbial activity, used as fuel. Composition (approximate): methane 50-70%, CO₂ 30-40%, with small amounts of H₂S, H₂ and N₂. Methanogens: certain anaerobic bacteria, e.g. Methanobacterium, which grow anaerobically on cellulosic material and produce large amounts of CH₄ with CO₂ and H₂. They are found in the anaerobic sludge of sewage treatment and in the rumen of cattle, where they help break down cellulose; so cattle dung (gobar) is rich in them. Biogas (gobar gas) plant: a concrete tank 10-15 feet deep into which bio-wastes and dung are fed as a slurry; a floating cover rises as gas is produced; an outlet pipe takes biogas to nearby houses; another outlet removes the spent slurry, used as manure. Process (in three stages): hydrolysis of complex organic matter by anaerobic bacteria, conversion to acids (acidogenesis), and formation of methane by methanogens. Biogas is used for cooking and lighting. The technology was developed in India mainly through the efforts of IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission).

Worked example

Why is cattle dung the main raw material in rural biogas plants, and what are the two products of the plant?

Try it first, then show the solutionSolution

1. Dung is rich in methanogens (from the cattle rumen) and partly digested cellulose.
2. Products: biogas (mainly methane) for fuel, and spent slurry used as manure.

Answer: Rich in methanogens; biogas and manure

Biocontrol agents and IPM

Biocontrol: use of biological methods to control plant diseases and pests, instead of chemicals. Chemical pesticides are toxic and harmful to humans and animals, kill useful organisms (pollinators, predators), persist and biomagnify, pollute soil and water, and pests develop resistance. Examples: ladybird beetle (red and black) eats aphids; dragonflies eat mosquitoes. Bacillus thuringiensis (Bt) spores are sprayed on plants (e.g. brassicas, fruit trees) to control butterfly caterpillars: eaten by larvae, the toxin is released in the gut and kills them while sparing other insects; Bt toxin genes are also introduced into crops (Bt cotton). Trichoderma, a free-living fungus common in root ecosystems, is an effective biocontrol agent of several plant pathogens. Baculoviruses (genus Nucleopolyhedrovirus) attack insects and other arthropods; they are species-specific, narrow-spectrum, and have no negative effects on plants, mammals, birds, fish or non-target insects, so they are ideal for integrated pest management (IPM), especially in ecologically sensitive areas. IPM: combines biological, cultural and limited chemical methods, based on knowledge of the life forms in the field (predators and pests), keeping pests at manageable levels rather than eradicating everything ('organic farming' approach).

Worked example

Why are baculoviruses preferred in integrated pest management programmes?

Try it first, then show the solutionSolution

1. They are species-specific, narrow-spectrum insecticidal agents.
2. They do not harm plants, mammals, birds, fish or non-target (beneficial) insects, conserving natural predators.

Answer: Species-specific with no harm to non-target organisms

Biofertilisers

Biofertilisers are organisms that enrich the nutrient quality of the soil, reducing the need for chemical fertilisers. Main sources: bacteria, fungi and cyanobacteria. Rhizobium: symbiotic bacteria in root nodules of leguminous plants; fix atmospheric nitrogen into organic forms used by the plant. Azospirillum and Azotobacter: free-living bacteria in the soil that fix atmospheric nitrogen, enriching soil nitrogen. Mycorrhiza: symbiotic association of fungi (many members of the genus Glomus) with plant roots; the fungus absorbs phosphorus from the soil and passes it to the plant; mycorrhizal plants also show resistance to root-borne pathogens, tolerance to salinity and drought, and overall increased growth. Cyanobacteria (blue-green algae): autotrophic microbes in aquatic and terrestrial environments, many fixing atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria. In paddy fields they are important biofertilisers; they also add organic matter to the soil, improving its fertility and water-holding capacity. In India many biofertilisers are available commercially.

Worked example

A farmer grows a legume one season and wheat the next. Why does the legume need little nitrogen fertiliser, and why does the following wheat crop also benefit?

Try it first, then show the solutionSolution

1. Rhizobium in the root nodules of the legume fixes atmospheric nitrogen into compounds the plant uses.
2. Nitrogen-rich roots, nodules and crop residues left in the soil decompose, enriching the soil for the next crop.

Answer: Symbiotic Rhizobium fixes nitrogen; residues enrich the soil

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 in Household and Industrial Products
• Microbes as Biofertilisers

Microbes in Human Welfare: key facts

Fermentation by yeast

$C_6{H}_{12}{O}_6\to 2C_2{H}_{5}OH+2C{O}_2$

Bread rises from CO₂; beverages from ethanol.

Lactic acid fermentation

$C_6{H}_{12}{O}_6\to 2C_3{H}_6{O}_3$

Lactobacillus; curd formation.

BOD reduction

$\%\text{reduction}=\frac{BO{D}_{in}-BO{D}_{out}}{BO{D}_{in}}\times 100$

Measures efficiency of secondary treatment.

Biogas composition

$C{H}_{4}50\text{-}70\%,C{O}_{2}30\text{-}40\%,\text{traces of }{H}_{2}S,H_2,N_2$

Produced by methanogens.

Sources of products

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

Organic acids.

Bioactive molecules

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

Immunosuppressant; cholesterol-lowering; clot buster.

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.

• Swiss holes by Propionibacterium

  Pro-PION: puts holes in cheese.

  Use it when: Cheese questions.

• CAB-L acids

  Citric (Aspergillus), Acetic (Acetobacter), Butyric (Clostridium), Lactic (Lactobacillus).

  Use it when: Organic acid sources.

• Tricky Cyclo, Mona Statin

  Trichoderma → cyclosporin A; Monascus → statins.

  Use it when: Bioactive molecules.

• Primary = Physical, Secondary = Biological

  P-P, S-B.

  Use it when: Sewage treatment.

• RAAG-C biofertilisers

  Rhizobium, Azospirillum, Azotobacter, Glomus, Cyanobacteria.

  Use it when: Listing biofertilisers.

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 5-mark question split into 1 + 1 + 1 + 2 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: Vague reason for clear bottled juice

  CISCE examiners' comments, 2024: candidates mentioned chemicals and preservatives instead of enzymes, confused it with fermentation, or wrote 'enzymes' in general without naming pectinase.

  Wrong working

  Bottled juice has preservatives.

  Right working

  Pectinase (and protease) break down pectin and proteins, clarifying the juice.

  ✓Do this: Name the enzyme.

  Usually costs: 1 mark.

• ✗Mistake: Misspelt or non-binomial names

  CISCE examiners' comments, 2024: candidates did not follow binomial nomenclature and misspelt Saccharomyces (Sarcomyces, Sacromyces) and Monascus.

  Wrong working

  sacromyces Cerevisiae

  Right working

  Saccharomyces cerevisiae (genus capitalised, species small, both underlined when handwritten).

  ✓Do this: Practise writing the ten key scientific names.

  Usually costs: 1 mark each.

• ✗Mistake: Confusing biofertilisers with biopesticides

  CISCE examiners' comments, 2025: for a paddy biofertiliser candidates said it would decrease water-holding capacity, kill microbes or protect from sunlight, used general terms (bacteria, fungi), and named methanogens, Agrobacterium, Bacillus thuringiensis or Baculovirus.

  Wrong working

  Biofertiliser made from Bacillus thuringiensis.

  Right working

  Cyanobacteria (Anabaena, Nostoc) or Azotobacter; they add nitrogen and organic matter.

  ✓Do this: Biofertilisers add nutrients; biocontrol agents kill pests.

  Usually costs: 1 mark each.

• ✗Mistake: Muddled sewage treatment stages

  CISCE examiners' comments, 2025: candidates listed sedimentation and filtration for both stages, called secondary treatment chemical rather than biological, wrote 'sludge' for activated sludge, identified tanks the wrong way round, named the digester as the flocs tank, and vaguely said microbes treat microbes; in another item they wrote raw items (milk, rice) instead of processed dairy products.

  Wrong working

  Flocs are added in the sludge digester.

  Right working

  Flocs grow in the aeration tank (secondary, biological treatment); activated sludge settles in the settling tank.

  ✓Do this: Draw the flow diagram once before answering.

  Usually costs: 1 mark each.

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

   The large holes in Swiss cheese are due to:
   (a) Lactobacillus
   (b) Propionibacterium sharmanii
   (c) Saccharomyces
   (d) Aspergillus

   Hint

   CO₂ production.

   Answer

   (b)

2. Q2 · 1 mark · MCQBoard level

   Which is used as a biocontrol agent against butterfly caterpillars?
   (a) Trichoderma
   (b) Bacillus thuringiensis
   (c) Glomus
   (d) Rhizobium

   Hint

   Bt toxin.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerBoard level

   Name the gas that makes bread dough rise.

   Hint

   Yeast fermentation.

   Answer

   Carbon dioxide

4. Q4 · 1 mark · Very short answerBoard level

   Expand BOD.

   Hint

   Oxygen.

   Answer

   Biochemical oxygen demand

5. Q5 · 2 marks · Short answerHarder board level

   Why are whisky and brandy stronger in alcohol than wine and beer?

   Hint

   Distillation.

   Answer

   Whisky and brandy are produced by distillation of the fermented broth, which concentrates the ethanol; wine and beer are not distilled, so their alcohol content stays lower (yeast stops fermenting at moderate ethanol levels).

6. Q6 · 2 marks · Short answerHarder board level

   Why are methanogens found in the rumen of cattle, and what is their role there?

   Hint

   Cellulose.

   Answer

   The rumen is anaerobic and rich in cellulose. Methanogens (with other bacteria) help break down cellulose, aiding the animal's nutrition, and release methane.

7. Q7 · 2 marks · Short answerBoard level

   Name the microbes producing (a) acetic acid, (b) butyric acid, and give one use of lipase.

   Hint

   Bacteria.

   Answer

   (a) Acetobacter aceti (b) Clostridium butylicum. Lipase is added to detergents to remove oily stains.

8. Q8 · 3 marks · Short answerHarder board level

   Give three reasons why biocontrol is preferred to chemical pesticides.

   Hint

   Specificity, safety.

   Answer

   Biocontrol agents are specific and spare beneficial organisms; they are not toxic to humans and animals and do not pollute soil and water; they do not persist or biomagnify, and pests are less likely to develop resistance.

9. Q9 · 2 marks · Short answerHarder board level

   A sewage plant reduces BOD from 400 to 40 mg L⁻¹ in the aeration tank. Calculate the percentage removal and name the structures responsible.

   Hint

   (in − out)/in.

   Answer

   (400 − 40)/400 × 100 = 90%; aerobic bacterial flocs (bacteria with fungal filaments).

10. Q10 · 2 marks · Short answerBoard level

    What is the role of Trichoderma in agriculture and in medicine?

    Hint

    Two species roles.

    Answer

    Agriculture: free-living fungi that act as biocontrol agents against plant pathogens. Medicine: Trichoderma polysporum produces cyclosporin-A, an immunosuppressant for transplant patients.

11. Q11 · 5 marks · Long answerHarder board level

    (a) Describe the structure and working of a biogas plant. [3]
    (b) Give the composition of biogas. [1]
    (c) Name the institutions that developed biogas technology in India. [1]

    Hint

    Floating cover.

    Answer

    (a) A concrete tank 10-15 ft deep receives dung slurry; methanogens (Methanobacterium) ferment it anaerobically, producing gas that collects under a floating cover which rises; gas is piped out for cooking and lighting; spent slurry is removed through an outlet and used as manure. (b) Methane 50-70%, CO₂ 30-40%, traces of H₂S, H₂, N₂. (c) IARI and KVIC.

12. Q12 · 2 marks · Short answerHOTS / competency

    Antibiotics are called 'against life'. Why are they nevertheless life-saving for humans? Why should they not be overused?

    Hint

    Selective toxicity; resistance.

    Answer

    They kill or inhibit disease-causing bacteria without harming human cells much, curing infections such as plague and diphtheria. Overuse selects resistant bacteria, making antibiotics ineffective.

13. Q13 · 2 marks · Short answerHOTS / competency

    Why is Nucleopolyhedrovirus suitable for ecologically sensitive areas but Bt toxin genes in crops raise some concerns?

    Hint

    Specificity and spread.

    Answer

    NPV is species-specific, narrow-spectrum and harmless to non-target organisms. Bt crops express toxin continuously, which may select resistant pests and possibly affect some non-target insects, so their use needs monitoring.

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.Microbe that converts milk to curd?Show answer

   Lactobacillus (lactic acid bacteria).

2. 2.Who discovered penicillin?Show answer

   Alexander Fleming.

3. 3.Source of citric acid?Show answer

   Aspergillus niger.

4. 4.What are flocs?Show answer

   Masses of aerobic bacteria associated with fungal filaments in the aeration tank.

5. 5.Name a methanogen.Show answer

   Methanobacterium.

6. 6.Which fungus forms mycorrhiza commonly?Show answer

   Glomus.

7. 7.Use of streptokinase?Show answer

   Clot buster: dissolves blood clots in heart-attack patients.

8. 8.Baculoviruses belong to which genus?Show answer

   Nucleopolyhedrovirus.

Quick revision checklist

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

• ☐Can you name the microbes for curd, bread and Swiss cheese?
• ☐Can you distinguish distilled and undistilled beverages?
• ☐Can you tell the penicillin story?
• ☐Can you match four organic acids, four enzymes, cyclosporin-A and statins with sources and uses?
• ☐Can you explain primary and secondary treatment, flocs, BOD and activated sludge?
• ☐Can you describe a biogas plant and give biogas composition?
• ☐Can you list five biocontrol agents with targets?
• ☐Can you list six biofertilisers and mycorrhizal benefits?

Microbes in Human Welfare: frequently asked questions