Chapter 18 — Microbes in Human Welfare Exercise Solutions
This chapter involves memorising specific microorganism-product pairs. Organise them by category: food, medicine, industrial, agriculture, environment.
How to Use This Page
1. Start with the Key Concepts table below to quickly revise the main terms and definitions you must know. 2. Work through the exercise sections in order — MCQs first, then fill-in-the-blanks, true/false, matching, short answers, and detailed answers. Each section tests a different depth of understanding. 3. Check your answers against the bold/correct responses immediately after each question. Do not skip — this is where most exam marks are lost. 4. Review Common Mistakes to Avoid before every exam. The ❌/✅ format shows exactly what examiners look for. 5. Use the Exam Checklist as a final revision sheet — if you can tick every item, you are exam-ready.
Key Concepts (Quick Recap)
| Term | Definition |
|---|---|
| Fermentation | Metabolic process where microbes convert organic substrates (sugars) into products like acids, gases, or alcohol in the absence of oxygen |
| Antibiotics | Chemical substances produced by microorganisms that kill or inhibit the growth of other microorganisms at low concentrations |
| Methanogens | Strict anaerobic archaea (e.g. Methanobacterium) that produce methane from organic compounds in biogas digesters |
| Biogas | Mixture of CH₄ (60–70%), CO₂ (~30%), H₂S, and H₂ produced by anaerobic digestion of organic waste by methanogens |
| Biofertilisers | Microorganisms (e.g. Rhizobium, Azotobacter, mycorrhiza) that enrich soil nutrient quality by nitrogen fixation or phosphate solubilisation |
| BOD (Biochemical Oxygen Demand) | Amount of dissolved oxygen needed by aerobic bacteria to decompose organic matter in water; high BOD = more pollution |
| Activated Sludge | Flocs of aerobic bacteria formed during secondary sewage treatment that consume organic matter and reduce BOD |
| Bioremediation | Use of microorganisms to degrade environmental pollutants (oil spills, heavy metals, chlorinated solvents) |
| Bioaugmentation | Adding specific cultured microorganisms to a contaminated site to enhance pollutant degradation |
| Biostimulation | Adding nutrients (nitrogen, phosphorus, oxygen) to stimulate growth of existing indigenous microbes at a polluted site |
| Cyclosporin A | Immunosuppressant drug produced by Trichoderma polysporum; used to prevent organ transplant rejection |
| Statin | Cholesterol-lowering drug produced by Monascus purpureus (a yeast); competitively inhibits HMG-CoA reductase |
| Primary Treatment | Physical stage of sewage treatment — screening and sedimentation to remove large solids and grit |
| Secondary Treatment | Biological stage where aerobic microbes (activated sludge) decompose organic matter, lowering BOD |
| Tertiary Treatment | Final chemical stage — chlorination, filtration, or UV to remove remaining pathogens and pollutants |
Microbes in Food Industry
| Product | Microorganism | Process |
|---|---|---|
| Curd/Yoghurt | Lactobacillus acidophilus | Lactic acid fermentation |
| Bread | Saccharomyces cerevisiae | CO2 causes dough to rise |
| Wine/Beer | Saccharomyces cerevisiae | Alcohol fermentation (anaerobic) |
| Vinegar | Acetobacter aceti | Acetic acid fermentation |
| Cheese | Lactobacillus, Propionibacterium | Curdling, ripening |
| Idli/Dosa | Leuconostoc mesenteroides | Lactic acid + CO2 fermentation |
| Soy sauce | Aspergillus oryzae | Fermentation of soybean |
| Citric acid | Aspergillus niger | Fermentation |
Microbes in Medicine
| Product | Microorganism |
|---|---|
| Penicillin | Penicillium notatum |
| Streptomycin | Streptomyces griseus |
| Tetracycline | Streptomyces species |
| Chloramphenicol | Streptomyces venezuelae |
| Statin (cholesterol-lowering) | Monascus purpureus |
| Cyclosporin A (immunosuppressant) | Trichoderma polysporum |
Microbes in Biogas Production
Biogas composition: CH4 (~60–70%), CO2 (~30%), trace H2S, H2
Organisms: Methanobacterium, Methanococcus (methanogens) — strict anaerobes
Types of biogas plants:
- Fixed dome type (KVIC model — India)
- Floating drum type
Slurry remaining after biogas production is used as nitrogen-rich fertiliser.
Microbes in Agriculture (Biofertilisers)
| Organism | Role |
|---|---|
| Rhizobium | Nitrogen fixation in legume root nodules |
| Azotobacter | Free-living nitrogen fixation in soil |
| Azospirillum | Nitrogen fixation associated with roots |
| Anabaena, Nostoc | Nitrogen fixation in paddy fields |
| Azolla | Water fern + Anabaena — nitrogen fixation in rice paddies |
| Mycorrhiza | Fungal association with roots; phosphate absorption |
Microbes in Sewage Treatment
Primary treatment: Physical — screening, sedimentation (removes large particles). Secondary treatment: Biological — aerobic bacteria (activated sludge) degrade organic matter; reduces BOD. Tertiary treatment: Chemical — chlorination, removes residual pollutants and pathogens.
Bioremediation
- Use of microorganisms to degrade environmental pollutants.
- Pseudomonas putida — degrades hydrocarbons (oil spills).
- Bacteria that degrade chlorinated solvents, heavy metals.
- Bioaugmentation: Adding microbes to contaminated site.
- Biostimulation: Adding nutrients to stimulate existing microbes.
Worked Examples
Example 1 — Microbe → product matching
Example 2 — Biogas (model answer)
Biogas is produced by methanogens (e.g. Methanobacterium) that anaerobically digest cattle dung/sewage. It is a mixture of gases — mainly methane (CH₄) with CO₂, H₂, and H₂S — used as fuel; the slurry left behind is manure.
Example 3 — Microbes in sewage treatment
I. Choose the Best Answer
1. Which bacterium is responsible for curd formation?
*Answer: (a) Lactobacillus acidophilus*** — converts lactose to lactic acid, coagulating milk proteins to form curd.
- (a) Lactobacillus acidophilus
- (b) Acetobacter aceti
- (c) Saccharomyces cerevisiae
- (d) Streptomyces griseus
2. Penicillin is produced by
*Answer: (b) Penicillium notatum*** — a fungus, not a bacterium; discovered by Alexander Fleming in 1928.
- (a) Streptomyces griseus
- (b) Penicillium notatum
- (c) Aspergillus niger
- (d) Trichoderma polysporum
3. The main component of biogas is
- (a) CO₂
- (b) H₂S
- (c) CH₄ (methane)
- (d) CO
4. Rhizobium forms symbiotic associations with
- (a) Cereal roots
- (b) Legume root nodules
- (c) Stem nodules
- (d) Leaf surfaces
5. Which microorganism is used in the production of vinegar?
*Answer: (c) Acetobacter aceti*** — oxidises ethanol to acetic acid (vinegar) under aerobic conditions.
- (a) Lactobacillus
- (b) Saccharomyces cerevisiae
- (c) Acetobacter aceti
- (d) Clostridium
6. BOD stands for
- (a) Biological Oxygen Deficit
- (b) Biochemical Oxygen Demand
- (c) Bacterial Organic Decomposition
- (d) Biochemical Oxidation Depth
7. Pseudomonas putida is used in
- (a) Antibiotic production
- (b) Biogas generation
- (c) Oil spill bioremediation
- (d) Cheese making
8. Statins, which lower blood cholesterol, are produced by
*Answer: (b) Monascus purpureus*** — a yeast; statins competitively inhibit HMG-CoA reductase in cholesterol synthesis.
- (a) Penicillium notatum
- (b) Monascus purpureus
- (c) Aspergillus niger
- (d) Lactobacillus
9. Which of the following is a free-living nitrogen-fixing bacterium?
*Answer: (b) Azotobacter** — free-living in soil; Rhizobium is symbiotic; Anabaena is a cyanobacterium; Methanobacterium* is a methanogen.
- (a) Rhizobium
- (b) Azotobacter
- (c) Anabaena
- (d) Methanobacterium
10. The immunosuppressant Cyclosporin A is produced by
*Answer: (b) Trichoderma polysporum*** — a fungus; used to suppress immune response during organ transplants.
- (a) Saccharomyces cerevisiae
- (b) Trichoderma polysporum
- (c) Streptomyces griseus
- (d) Penicillium notatum
II. Fill in the Blanks
1. The bacterium that converts milk into curd is \_\_\_\_\_\_. — Lactobacillus acidophilus 2. Biogas contains approximately \_\_\_\_\_\_% methane. — 60–70 3. The fungus that produces penicillin is \_\_\_\_\_\_. — Penicillium notatum 4. \_\_\_\_\_\_ is a free-living nitrogen-fixing bacterium found in soil. — Azotobacter 5. The slurry left after biogas production is rich in \_\_\_\_\_\_ and used as fertiliser. — nitrogen 6. Pseudomonas putida is used in \_\_\_\_\_\_ to degrade oil spills. — bioremediation 7. The secondary treatment of sewage involves \_\_\_\_\_\_ bacteria that form activated sludge. — aerobic 8. Azolla is a water fern that harbours the cyanobacterium \_\_\_\_\_\_ for nitrogen fixation in rice paddies. — Anabaena 9. Monascus purpureus produces \_\_\_\_\_\_, which lowers blood cholesterol. — statins 10. BOD stands for \_\_\_\_\_\_. — Biochemical Oxygen Demand
III. True or False (with corrections)
1. Statement: Penicillin is produced by the bacterium Streptomyces. False. Penicillin is produced by the fungus Penicillium notatum. Streptomyces produces streptomycin.
2. Statement: Biogas is purely methane gas. False. Biogas is a mixture — methane (60–70%), CO₂ (~30%), and traces of H₂S and H₂.
3. Statement: Rhizobium is a free-living nitrogen-fixing bacterium. False. Rhizobium fixes nitrogen symbiotically inside legume root nodules. Azotobacter is free-living.
4. Statement: In sewage treatment, primary treatment involves biological degradation by bacteria. False. Primary treatment is physical — screening and sedimentation. Biological degradation occurs in secondary treatment by aerobic bacteria.
5. Statement: Saccharomyces cerevisiae is used in both bread making and alcohol production. True. Yeast ferments sugars anaerobically to produce CO₂ (bread leavening) and ethanol (alcoholic beverages).
6. Statement: BOD increases after effective sewage treatment. False. BOD decreases after treatment. A fall in BOD indicates that organic matter has been decomposed and the water is cleaner.
7. Statement: Mycorrhiza is a fungal association that helps plants absorb phosphate from soil. True. Mycorrhizal fungi form symbiotic associations with plant roots and enhance phosphate uptake.
8. Statement: Bioaugmentation involves adding nutrients to a polluted site to stimulate existing microbes. False. Bioaugmentation involves adding cultured microorganisms to a contaminated site. Adding nutrients is called biostimulation.
IV. Match the Following
| Column A | Column B |
|---|---|
| 1. Lactobacillus | a. Immunosuppressant drug |
| 2. Penicillium notatum | b. Nitrogen fixation in legume nodules |
| 3. Methanobacterium | c. Curd formation |
| 4. Trichoderma polysporum | d. Oil spill cleanup |
| 5. Pseudomonas putida | e. Antibiotic — penicillin |
| 6. Rhizobium | f. Biogas production |
| 7. Monascus purpureus | g. Statins (cholesterol-lowering) |
| 8. Acetobacter aceti | h. Vinegar (acetic acid) |
V. Answer Briefly (2-mark)
Q1. Name two microorganisms used in food production and state their roles. Two microorganisms used in food production are: (1) Lactobacillus acidophilus — converts lactose in milk to lactic acid, causing coagulation of milk proteins to form curd/yoghurt. (2) Saccharomyces cerevisiae (yeast) — ferments sugars anaerobically to produce CO₂ for bread leavening and ethanol for alcoholic beverages like wine and beer.
Q2. What is the difference between bioaugmentation and biostimulation? Bioaugmentation involves adding specific cultured microorganisms to a contaminated site to enhance degradation of pollutants. Biostimulation involves adding nutrients such as nitrogen, phosphorus, or oxygen to stimulate the growth and activity of indigenous microbes already present at the site. Both are bioremediation strategies but differ in whether new microbes are introduced or existing ones are encouraged.
Q3. Why is the slurry left after biogas production useful as fertiliser? The slurry remaining after biogas production is rich in nitrogen and other minerals because the methanogens mainly convert carbon compounds into methane and CO₂, leaving behind nitrogenous compounds. This nutrient-rich slurry serves as an excellent organic fertiliser, improving soil fertility without the need for chemical fertilisers.
Q4. Differentiate between primary and secondary sewage treatment. Primary sewage treatment is a physical process involving screening to remove large debris and sedimentation in primary clarifiers where settleable solids form sludge. Secondary treatment is a biological process where aerobic bacteria (in activated sludge or trickling filters) decompose dissolved organic matter, significantly reducing the BOD of the effluent before it undergoes tertiary treatment.
*Q5. What is the role of Anabaena in paddy fields? Anabaena is a nitrogen-fixing cyanobacterium that lives in symbiosis with the water fern Azolla* in rice paddies. It fixes atmospheric nitrogen into ammonia, which enriches the paddy soil with nitrogen. This natural biofertiliser reduces the need for chemical nitrogen fertilisers in rice cultivation.
VI. Answer in Detail (5-mark)
Q1. Describe the role of microbes in industrial products with suitable examples. Microbes play a vital role in producing several industrial products:
- Fermented foods: Lactobacillus converts milk to curd (lactic acid fermentation). Leuconostoc mesenteroides ferments rice batter for idli/dosa. Aspergillus oryzae ferments soybean to produce soy sauce.
- Alcoholic beverages: Saccharomyces cerevisiae ferments fruit juices and grain mash to produce ethanol — basis for wine, beer, and toddy.
- Organic acids: Aspergillus niger produces citric acid; Acetobacter aceti oxidises ethanol to acetic acid (vinegar).
- Antibiotics: Penicillium notatum produces penicillin; Streptomyces griseus produces streptomycin — both revolutionised medicine.
- Bioactive molecules: Trichoderma polysporum produces cyclosporin A (immunosuppressant); Monascus purpureus produces statins (cholesterol-lowering).
Each microbe-product pair must be memorised with both the organism and its specific product.
Q2. Explain the process of biogas production and the role of methanogens. Biogas production occurs in a biogas digester through anaerobic digestion in three stages:
- Stage 1 — Hydrolysis: Complex organic polymers (cellulose, proteins, fats) in cattle dung or agricultural waste are broken down by hydrolytic bacteria into simpler monomers (sugars, amino acids, fatty acids).
- Stage 2 — Acidogenesis: Acidogenic bacteria convert these monomers into organic acids (acetic acid, propionic acid) and CO₂ and H₂.
- Stage 3 — Methanogenesis: Methanogens (Methanobacterium, Methanococcus) — strict anaerobic archaea — convert acetic acid and CO₂ + H₂ into methane (CH₄) and CO₂.
The resulting biogas contains 60–70% CH₄, ~30% CO₂, and traces of H₂S and H₂. It is used as fuel for cooking and electricity generation. The nitrogen-rich slurry left behind is an excellent organic fertiliser. Two common plant designs in India are the fixed-dome type (KVIC model) and the floating-drum type.
Q3. Describe the role of microbes in sewage treatment. Sewage treatment involves three stages to make wastewater safe for discharge:
- Primary (Physical) treatment: Wastewater passes through screens to remove large objects (rags, sticks), then through grit chambers and primary sedimentation tanks. Settleable solids sink as primary sludge; floating materials (oils, grease) are skimmed off.
- Secondary (Biological) treatment: The effluent enters aeration tanks where aerobic bacteria form flocs (activated sludge). These microbes consume organic matter, reducing BOD significantly. The mixture flows to a secondary clarifier where flocs settle; some are recycled back to the aeration tank to maintain microbial population. A small portion of the settled sludge is routed to anaerobic digesters to produce biogas.
- Tertiary treatment: The treated effluent is disinfected — typically by chlorination or UV radiation — to kill remaining pathogens before discharge into water bodies.
The key indicator of effective treatment is a significant drop in BOD.
Q4. Discuss the importance of biofertilisers in sustainable agriculture. Biofertilisers are microorganisms that enrich soil nutrients naturally, reducing dependence on chemical fertilisers:
- Symbiotic nitrogen fixers: Rhizobium forms nodules on legume roots and fixes atmospheric N₂ into ammonia, benefiting crops like pulses, groundnut, and soybean.
- Free-living nitrogen fixers: Azotobacter and Azospirillum fix N₂ in the rhizosphere of cereals, vegetables, and cash crops without forming nodules.
- Cyanobacterial biofertilisers: Anabaena, Nostoc, and the Azolla–Anabaena symbiosis fix N₂ in rice paddies, making them particularly important for wetland rice cultivation.
- Mycorrhiza: Vesicular-arbuscular mycorrhizae (VAM) form associations with 80% of plant roots, greatly enhancing phosphate absorption from soil — critical since phosphate is often the limiting nutrient.
- Benefits: Biofertilisers are eco-friendly, cost-effective, improve soil structure, do not cause pollution, and are sustainable over the long term. They are essential components of organic farming.
Q5. What is bioremediation? Explain with examples. Bioremediation is the use of microorganisms to degrade or detoxify environmental pollutants into harmless substances:
- Oil spill cleanup: Pseudomonas putida (also called "superbug") degrades hydrocarbons in crude oil. It was used in the cleanup of the Exxon Valdez oil spill and is the most cited example.
- Pesticide degradation: Certain bacteria can degrade chlorinated pesticides like DDT and organophosphates in contaminated soil.
- Heavy metal removal: Some microbes absorb or precipitate heavy metals (mercury, lead, arsenic) from industrial effluents through biosorption or bioprecipitation.
- Sewage treatment: Aerobic bacteria in activated sludge degrade organic waste in wastewater — a large-scale bioremediation process.
- Approaches: Bioaugmentation adds specific cultured microbes to a contaminated site. Biostimulation adds nutrients (N, P, O₂) to stimulate indigenous microbes. Both approaches aim to accelerate natural biodegradation processes without harming the environment.
Common Mistakes
- Confusing Penicillin (from fungus Penicillium) with Streptomycin (from bacteria Streptomyces).
- Saying biogas is only methane — it is a mixture; methane is the main component.
- Forgetting Rhizobium is found in legume root nodules only; Azotobacter is free-living.
Common Mistakes to Avoid
- ❌ Saying "biogas is methane" → ✅ Biogas is a mixture of gases — 60–70% methane, ~30% CO₂, and traces of H₂S and H₂.
- ❌ Confusing Penicillium (fungus) with Streptomyces (bacterium) as antibiotic sources → ✅ Penicillin comes from the fungus Penicillium notatum; streptomycin comes from the bacterium Streptomyces griseus.
- ❌ Writing that Rhizobium is free-living → ✅ Rhizobium is symbiotic — it fixes nitrogen only inside legume root nodules. Azotobacter is the free-living nitrogen fixer.
- ❌ Saying primary sewage treatment is biological → ✅ Primary treatment is physical (screening, sedimentation). Secondary treatment is biological (activated sludge, aerobic bacteria).
- ❌ Writing that BOD increases after sewage treatment → ✅ BOD decreases after effective treatment. Lower BOD = cleaner water.
- ❌ Confusing bioaugmentation with biostimulation → ✅ Bioaugmentation = adding new microbes to a site; Biostimulation = adding nutrients to stimulate existing microbes.
- ❌ Stating Anabaena lives alone in paddy fields → ✅ Anabaena lives in symbiosis with Azolla (a water fern) in rice paddies; both together fix nitrogen.
- ❌ Writing that Monascus purpureus produces antibiotics → ✅ It produces statins (cholesterol-lowering drugs), not antibiotics.
Exam Checklist
- [ ] Can I list at least 6 microbe-product pairs for food industry (curd, bread, wine, vinegar, cheese, idli/dosa, soy sauce, citric acid)?
- [ ] Can I name 4 antibiotics with their source microorganisms (Penicillium notatum, Streptomyces griseus, Streptomyces sp., Streptomyces venezuelae)?
- [ ] Do I know that statins come from Monascus purpureus and cyclosporin A from Trichoderma polysporum?
- [ ] Can I explain biogas composition (CH₄ 60–70%, CO₂ ~30%) and name the methanogens (Methanobacterium, Methanococcus)?
- [ ] Can I describe the three stages of sewage treatment — primary (physical), secondary (biological/activated sludge), tertiary (chlorination)?
- [ ] Do I understand BOD — what it measures, why it must decrease after treatment?
- [ ] Can I differentiate symbiotic (Rhizobium) from free-living (Azotobacter) nitrogen fixers?
- [ ] Can I explain Azolla–Anabaena symbiosis and its significance in rice paddies?
- [ ] Do I know the difference between bioaugmentation (adding microbes) and biostimulation (adding nutrients)?
- [ ] Can I write a 5-mark answer on biogas production covering the three stages (hydrolysis, acidogenesis, methanogenesis)?
- [ ] Can I name Pseudomonas putida for oil spill bioremediation and explain its role?
- [ ] Can I list the types of biogas plants (fixed dome/KVIC, floating drum) and the use of slurry as fertiliser?