Microbes in Human Welfare Class 12 Notes

Microbes in Household Products

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.

Industrial Production — Fermented Beverages, Antibiotics and More

• 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

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:

Biogas Production

• 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.

Biocontrol Agents

• 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).

Biofertilisers

• 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.

Solved Examples

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.