Chapter Overview & Weightage
Microbes in Human Welfare covers the beneficial roles of microorganisms — in household products, industrial processes, sewage treatment, biogas production, biocontrol agents, and biofertilizers. This is a pure memorisation chapter with easy marks.
This chapter carries 3-4% weightage in NEET with 2-3 questions. Microbe-product associations and sewage treatment steps are the most tested topics.
Key Concepts You Must Know
Tier 1 (Core)
- Household products: curd (Lactobacillus), bread (yeast/Saccharomyces), idli/dosa (Leuconostoc), cheese (Propionibacterium for Swiss cheese holes)
- Fermentation: ethanol (Saccharomyces cerevisiae), antibiotics (Penicillium — penicillin, Streptomyces — streptomycin)
- Sewage treatment: primary (physical — sedimentation), secondary (biological — activated sludge with aerobic bacteria), effluent discharge
- Biogas: produced in biogas plant by methanogenic bacteria (Methanobacterium) from organic waste
Tier 2 (Frequently tested)
- Industrial products: enzymes (lipases for detergent, streptokinase for blood clots), organic acids (citric acid from Aspergillus niger)
- Biocontrol agents: Bacillus thuringiensis (Bt toxin against insects), Trichoderma (against plant pathogens), ladybird beetles (against aphids)
- Biofertilizers: Rhizobium (nitrogen-fixing, symbiotic with legumes), Azospirillum/Azotobacter (free-living N-fixers), mycorrhiza (phosphorus absorption)
- BOD: Biochemical Oxygen Demand — measure of organic pollution in water
Tier 3 (Occasionally tested)
- Toddy from palm sap (Saccharomyces)
- Swiss cheese holes from CO produced by Propionibacterium shermanii
- Cyclosporin A from Trichoderma polysporum (immunosuppressant)
Important Formulas
| Product | Microorganism | Type |
|---|---|---|
| Curd | Lactobacillus | Bacterium |
| Bread, wine, beer | Saccharomyces cerevisiae (yeast) | Fungus |
| Swiss cheese (holes) | Propionibacterium shermanii | Bacterium |
| Roquefort cheese | Penicillium roqueforti | Fungus |
| Penicillin | Penicillium notatum/chrysogenum | Fungus |
| Streptomycin | Streptomyces griseus | Bacterium (actinomycete) |
| Citric acid | Aspergillus niger | Fungus |
| Streptokinase | Streptococcus | Bacterium |
| Biogas | Methanobacterium | Archaebacterium |
| Ethanol | Saccharomyces cerevisiae | Fungus |
| Organism | Type | Function |
|---|---|---|
| Rhizobium | Bacterium (symbiotic) | N-fixation in legume root nodules |
| Azotobacter | Bacterium (free-living) | N-fixation in soil |
| Azospirillum | Bacterium (free-living) | N-fixation in soil |
| Anabaena/Nostoc | Cyanobacteria | N-fixation (also in Azolla) |
| Mycorrhiza (Glomus) | Fungus (symbiotic) | Phosphorus absorption from soil |
The BOD concept is tested numerically: high BOD = lots of organic matter = heavily polluted water. After successful sewage treatment, BOD should decrease significantly. NEET asks “what happens to BOD after secondary treatment?” — it drops.
Solved Previous Year Questions
PYQ 1 — NEET 2024
Problem: Which microorganism is used in the production of biogas?
(A) Saccharomyces (B) Lactobacillus (C) Methanobacterium (D) Aspergillus
Solution:
Methanobacterium (a methanogenic archaebacterium) produces methane () as the major component of biogas. It works under anaerobic conditions in the biogas plant, digesting organic waste (cattle dung, agricultural waste) to produce biogas.
Answer: (C) Methanobacterium
PYQ 2 — NEET 2023
Problem: Bt toxin is produced by:
(A) Trichoderma (B) Bacillus thuringiensis (C) Methanobacterium (D) Rhizobium
Solution:
Bacillus thuringiensis produces Bt toxin (cry proteins) as crystalline inclusions during sporulation. This toxin kills specific insect larvae when ingested — the alkaline gut pH activates the toxin, which creates pores in the gut lining, causing death. This principle is used in Bt crops (Bt cotton, Bt brinjal).
Answer: (B) Bacillus thuringiensis
PYQ 3 — NEET 2022
Problem: The
symbiotic nitrogen-fixing bacterium in legume root nodules is:
(A) Azotobacter (B) Azospirillum (C) Rhizobium (D) Nostoc
Solution:
Rhizobium is the symbiotic nitrogen-fixing bacterium that forms root nodules in leguminous plants. It converts atmospheric to ammonia () using the enzyme nitrogenase. The plant provides organic acids and shelter; Rhizobium provides fixed nitrogen — a mutualistic relationship.
Azotobacter and Azospirillum are free-living nitrogen fixers (not symbiotic).
Answer: (C) Rhizobium
Difficulty Distribution
| Difficulty | % of Questions | What to Expect |
|---|---|---|
| Easy | 60% | Microbe-product matching, biofertilizer identification |
| Medium | 30% | Sewage treatment steps, BOD concept |
| Hard | 10% | Biocontrol mechanisms, industrial enzyme applications |
Expert Strategy
Single session (2-3 hours): This chapter requires one focused study session. Build the microbe-product table and memorise it using flashcards. Know the sewage treatment process (primary → secondary → tertiary) and the biofertilizer table. Solve last 5 years’ PYQs — you’ll see the same organisms repeated.
A mnemonic for nitrogen fixers: RAN = Rhizobium (symbiotic), Azotobacter and Azospirillum (free-living), Nostoc/Anabaena (cyanobacteria). NEET always asks whether a given organism is symbiotic or free-living.
Common Traps
Trap 1 — Rhizobium is symbiotic; Azotobacter is free-living. Both fix nitrogen, but through different mechanisms. Rhizobium needs the legume host plant. Azotobacter can fix nitrogen independently in the soil.
Trap 2 — Penicillium produces penicillin; Streptomyces produces streptomycin. The names sound similar enough to confuse. Match the first letters: Penicillium → Penicillin. Streptomyces → Streptomycin.
Trap 3 — Secondary sewage treatment is biological, not physical. Primary treatment is physical (sedimentation, filtration). Secondary treatment uses aerobic bacteria in the activated sludge process to decompose organic matter. NEET tests which step is biological.
Trap 4 — Mycorrhiza helps with phosphorus absorption, not nitrogen fixation. Mycorrhizal fungi form symbiotic associations with plant roots and enhance phosphorus uptake from the soil. Don’t confuse their role with nitrogen fixers.