Microbes in Human Welfare Class 12 Notes

Microorganisms have been used in traditional food preparation across cultures for thousands of years. Key examples relevant to NCERT:

• Curd (yoghurt): Lactobacillus bacteria convert lactose in milk to lactic acid, coagulating milk protein (casein) and giving curd its texture and tangy flavour.
• Dough for idli and dosa: natural fermentation by yeasts and Leuconostoc converts sugars to CO2, making the batter rise and giving a soft, spongy texture.
• Bread: baker's yeast (Saccharomyces cerevisiae) ferments sugars, producing CO2 that inflates the dough. The gas is trapped by gluten, giving bread its airy crumb.
• Cheese: specific microbes determine the type — Lactobacillus for cottage cheese, Propionibacterium shermanii for Swiss cheese (CO2 holes and nutty flavour), Penicillium roqueforti for Roquefort and Camembert.
• Toddy: natural fermentation of palm sap by yeasts produces a mild alcoholic beverage traditional in parts of South India.

• Organic acids: Citric acid by Aspergillus niger; acetic acid by Acetobacter aceti (vinegar); lactic acid by Lactobacillus.
• Enzymes: lipase (detergents, fat hydrolysis), pectinase (juice clarification, fruit processing), streptokinase (clot-dissolving enzyme from Streptococcus — used as a 'clot buster' in heart attack treatment).
• Vitamins: vitamin B12 (cobalamin) is produced by Propionibacterium freudenreichii and Pseudomonas denitrificans.
• Statins: Monascus purpureus (a yeast) produces statins — cholesterol-lowering molecules that competitively inhibit HMG-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis.

Sewage treatment plants (STPs) use microbes to decompose organic waste in municipal wastewater before releasing it into water bodies. The process has two main stages:

• In India, the KVIC (Khadi and Village Industries Commission) and DST have popularised family-size biogas plants using cattle dung (gobar gas plants).
• The slurry left after biogas production is an excellent organic manure rich in nutrients.
• Large-scale biogas plants and biomass gasifiers are used for community-level energy generation.

• Trichoderma: a free-living fungus that colonises plant roots and protects them from soil-borne fungal pathogens (Fusarium, Rhizoctonia) through antibiosis and competition.
• Baculovirus (Nucleopolyhedrovirus — NPV): a virus that specifically infects and kills insect pests (e.g. Helicoverpa armigera). It has no effect on non-target organisms, making it highly specific and safe.
• Ladybird beetle (Coccinella): a predatory insect that feeds on aphids — natural biological pest control in gardens and farms.
• Cryptolaemus montrouzieri: a predatory beetle whose larvae mimic mealybugs (woolly white covering) and feed on mealybug pests. Called the 'mealybug destroyer'.
• Bacillus thuringiensis (Bt): produces crystal protein (Cry toxin) that is toxic to specific insect larvae when ingested. The Bt gene is widely used in transgenic crops (Bt cotton, Bt brinjal).

• Rhizobium: symbiotic nitrogen-fixing bacteria that form root nodules in leguminous plants (peas, beans, clover). They convert atmospheric N2 into plant-usable NH3 using the enzyme nitrogenase.
• Azospirillum and Azotobacter: free-living nitrogen-fixing bacteria in the soil. Azospirillum also promotes plant growth through phytohormone production.
• Cyanobacteria (blue-green algae): Anabaena, Nostoc, Aulosira fix atmospheric nitrogen in paddy fields and other wet soils. Anabaena also forms symbiotic associations with the water fern Azolla (Anabaena-Azolla biofertiliser for rice).
• Mycorrhiza (Glomus): symbiotic association between fungal hyphae and plant roots. The fungus increases water and mineral (especially phosphorus) absorption; the plant provides sugars. Mycorrhizal plants show better drought tolerance.

Example: A sewage treatment plant reduces the BOD of effluent before discharge. Explain the role of activated sludge in secondary treatment and why BOD decreases.

Solution: In secondary treatment, the primary effluent is aerated and mixed with activated sludge — a consortium of aerobic heterotrophic bacteria and protozoa. These organisms metabolise dissolved organic matter, using it as a food source and consuming oxygen in the process. As the organic matter is decomposed, the amount of biodegradable material drops, so less oxygen is needed by decomposers downstream. This reduces the BOD. The clear supernatant after settling has a much lower BOD and can be safely discharged.

Example: Why are mycorrhizal plants more drought-tolerant than non-mycorrhizal plants?

Solution: In a mycorrhizal association, the fungal hyphae (Glomus) extend far beyond the root zone, greatly increasing the surface area for water and mineral absorption. The hyphae can access water from smaller soil pores that root hairs cannot reach. This enhanced water uptake capacity makes mycorrhizal plants more resilient during drought conditions.

Exercises (NCERT)

1. 1
   Bacteria cannot be seen by the naked eye. How can they be made visible?
2. 2
   Name the microbes used in the production of curd and cheese.
3. 3
   What is the role of lactic acid bacteria in the human body?
4. 4
   Name the microbe used in the production of bread.
5. 5
   Name the microbe used in the production of ethanol.
6. 6
   What are antibiotics? Name the first antibiotic.
7. 7
   Who discovered penicillin?
8. 8
   What is sewage? Describe the two stages of sewage treatment.
9. 9
   What is the role of anaerobic bacteria in sewage treatment?
10. 10
    What is biogas? Name the microbes involved in its production.
11. 11
    What are biocontrol agents? Give two examples.
12. 12
    What is a biofertiliser? Name two types.
13. 13
    What is the role of Rhizobium in agriculture?
14. 14
    What is the role of mycorrhiza?
15. 15
    What is the 'Biochemical Oxygen Demand' (BOD)?
16. 16
    Why is the BOD a measure of organic pollution in water?
17. 17
    What is the cheese-making process?

Answers at a glance

1. 1
   Bacteria can be seen with a microscope (they are single-celled microorganisms).
2. 2
   Lactic acid bacteria (LAB) such as Lactobacillus are used for curd; Streptococcus and Penicillium (mould) play roles in cheese.
3. 3
   LAB improve the nutritional quality of curd by increasing vitamin B12 and converting lactose to lactic acid.
4. 4
   Yeast (Saccharomyces cerevisiae) is used in bread-making; it produces CO2 that makes the dough rise.
5. 5
   Yeast (Saccharomyces cerevisiae) is used for ethanol production.
6. 6
   Antibiotics are chemical substances produced by microorganisms that kill or inhibit other microbes; the first was penicillin.
7. 7
   Alexander Fleming discovered penicillin from Penicillium notatum.
8. 8
   Sewage is municipal wastewater; treatment has primary (physical) and secondary (biological) stages.
9. 9
   Anaerobic bacteria digest the sludge and produce biogas.
10. 10
    Biogas (mainly methane) is produced by methanogens such as Methanobacterium.
11. 11
    Biocontrol agents are organisms that control pests; e.g. ladybird (aphids) and Bacillus thuringiensis.
12. 12
    Biofertilisers are organisms that enrich the soil nutrients; e.g. Rhizobium (bacterial) and mycorrhiza (fungal).
13. 13
    Rhizobium fixes atmospheric nitrogen in the root nodules of legumes.
14. 14
    Mycorrhiza (fungus + root) improves phosphorus uptake and drought resistance.
15. 15
    BOD is the amount of oxygen consumed by bacteria to oxidise organic matter in water.
16. 16
    A high BOD means more organic matter and hence more pollution.
17. 17
    Cheese-making: LAB produce acid to curdle milk, then specific moulds/bacteria ripen the cheese for flavour and texture.

MCQ Questions

2 Mark Questions

1. 1
   Name the microbes used in the production of curd and cheese.
2. 2
   What is the role of lactic acid bacteria in the human body?
3. 3
   What are biofertilisers? Name two types.
4. 4
   How does a biocontrol agent control pests?
5. 5
   What is BOD?

3 Mark Questions

1. 1
   Describe the two stages of sewage treatment.
2. 2
   Describe the production of biogas.
3. 3
   Explain the role of Rhizobium and mycorrhiza in agriculture.
4. 4
   Describe the process of cheese-making.
5. 5
   What are the uses of microbes in household products?

5 Mark Questions

1. 1
   Describe the role of microbes in industrial production.
2. 2
   Describe sewage treatment and biogas production.
3. 3
   Explain the use of microbes as biocontrol agents and biofertilisers.