Microbes in Human Welfare — Long Notes
Microbes are the workhorses of biology — invisibly making our food, medicines, industrial chemicals, sewage treatment, biofuels, biocontrol agents, and biofertilisers. This chapter surveys the biotechnology of microbes in daily life.
1. Microbes in Household Products
1.1 Curd
- **Lactic Acid Bacteria (LAB), especially Lactobacillus** ferment milk sugar (lactose) → lactic acid.
- Milk proteins coagulate → curd.
- Health benefit: increases Vitamin B₁₂; protects against gut pathogens.
1.2 Dough (idli, dosa, bhaturas)
- Fermented by bacteria + yeasts overnight.
- CO₂ production makes the dough puffy and soft.
1.3 Cheese
- Curdling & ageing of milk under microbial action.
- Swiss cheese — its characteristic large holes come from CO₂ produced by Propionibacterium shermanii during ripening.
- Roquefort cheese — matured with Penicillium roqueforti (blue veins, sharp flavour).
1.4 Bread
- Baker's yeast (Saccharomyces cerevisiae) ferments sugar → CO₂ (raises dough) + ethanol (evaporates during baking).
1.5 Toddy
- Traditional drink of southern India, made by fermenting palm sap with yeast.
2. Microbes in Industrial Products
2.1 Fermented Beverages
- Brewer's yeast (Saccharomyces cerevisiae) ferments sugars from grains, grapes, molasses etc. → ethanol + CO₂.
- Without distillation — wine, beer, cider.
- With distillation — whisky, brandy, rum.
2.2 Antibiotics
- Antibiotic = chemical produced by one microbe that kills or inhibits another.
- Alexander Fleming (1928) noticed a Staphylococcus culture had a bacteria-free zone around a mould contaminant (Penicillium notatum). He named the compound penicillin.
- Ernst Chain and Howard Florey developed penicillin as an antibiotic, saving lives of WWII soldiers.
- All three received the Nobel Prize (1945).
- Antibiotics revolutionised medicine — TB, plague, whooping cough, and diphtheria are now treatable.
2.3 Chemicals, Enzymes, Bioactive Molecules
| Product | Microbe | Application |
|---|---|---|
| Citric acid | Aspergillus niger | Food preservation, soft drinks |
| Acetic acid | Acetobacter aceti | Vinegar |
| Butyric acid | Clostridium butylicum | Chemical industry |
| Lactic acid | Lactobacillus | Food industry |
| Ethanol | Saccharomyces cerevisiae | Beverages, fuel |
| Lipases | Bacteria/fungi | Detergents — hydrolyse oil stains |
| Pectinase, protease | Fungi | Bottle fruit juices (clarify) |
| Streptokinase | Streptococcus (modified by genetic engineering) | Clot buster — dissolves clots in heart-attack patients |
| Cyclosporin-A | Trichoderma polysporum | Immunosuppressant — prevents transplant rejection |
| Statins | Monascus purpureus | Lower blood cholesterol — competitive inhibitors of the enzyme HMG-CoA reductase |
3. Microbes in Sewage Treatment
Sewage = wastewater from cities carrying human excreta and other organic wastes. It has:
- High BOD (biochemical oxygen demand) — organic matter that microbes will consume.
- Pathogenic microbes.
Treatment happens in Sewage Treatment Plants (STPs) in stages:
3.1 Primary Treatment (physical)
- Sewage is passed through filters (screens) that remove solids.
- Sedimentation tanks let heavy particles settle → primary sludge.
- The clear supernatant is the primary effluent.
3.2 Secondary (Biological) Treatment
- Primary effluent goes to a large aeration tank — vigorously stirred with air.
- Naturally occurring aerobic microbes (bacteria + fungi) form flocs — mesh-like aggregates that consume organic matter.
- BOD drops dramatically.
- The mixture then goes to a settling tank → the flocs sediment as activated sludge.
- Some activated sludge is returned to the aeration tank as inoculum; the rest goes to anaerobic sludge digesters.
- In the digester: anaerobic bacteria digest the sludge, producing a mixture of gases — methane (CH₄), hydrogen sulphide (H₂S), carbon dioxide — called biogas.
3.3 Tertiary Treatment (chemical polishing)
- Removes remaining nutrients (nitrogen, phosphorus) and pathogens (chlorination) before release into rivers.
4. Microbes in the Production of Biogas
- Biogas — CH₄, CO₂, H₂, and traces of H₂S.
- Methanogens — anaerobic bacteria (Methanobacterium) that produce methane.
- They live in the rumen of cattle and in anaerobic sludge.
- In cattle dung ("gobar"), methanogens are abundant.
Biogas Plant Design
- A pit/tank is filled with cattle dung + water slurry.
- The pit is covered by a floating gas holder.
- As gas accumulates, the holder rises.
- A pipe lets gas flow to homes for cooking/light.
- Spent slurry is excellent manure.
Designed in India by Khadi and Village Industries Commission (KVIC) and IARI (Indian Agricultural Research Institute).
5. Microbes as Biocontrol Agents
Biological (rather than chemical) control of pests protects crops and the environment.
- Ladybird beetles control aphids.
- Dragonflies control mosquitoes.
- Bacillus thuringiensis (Bt) — a soil bacterium; its dried spores are sprayed on plants; when caterpillars eat them, the released Bt toxin (Cry proteins) kills them. Bt cotton carries the Bt gene inside its genome (transgenic).
- Trichoderma (fungus) — grows on plant roots; controls soil-borne pathogens.
- Baculoviruses (Nucleopolyhedrovirus, NPV) — species-specific insecticidal viruses; safe for humans, mammals, birds, fish, and beneficial insects — ideal for integrated pest management (IPM).
6. Microbes as Biofertilisers
Biofertilisers enrich soil naturally, reducing chemical fertiliser use.
6.1 Nitrogen-fixing Bacteria
- Symbiotic:
- Rhizobium — lives in the root nodules of legumes, converts atmospheric N₂ to ammonia (biological nitrogen fixation).
- Free-living:
- Azotobacter, Azospirillum — free-living in soil, fix atmospheric N₂.
- Cyanobacteria (blue-green algae):
- Anabaena, Nostoc, Oscillatoria — autotrophic, fix N₂ in paddy fields.
- Azolla — an aquatic fern that harbours Anabaena symbiotically; used in rice paddies.
6.2 Mycorrhiza
- Symbiotic association between fungi and plant roots.
- Glomus forms arbuscular mycorrhizae with many crops.
- Benefits to the plant:
- Enhanced phosphorus absorption.
- Resistance to root pathogens.
- Improved tolerance to drought and salinity.
Key take-aways
- Everyday food, drink, medicine, and industrial products are microbial productions — LAB, yeast, moulds are our silent partners.
- Antibiotics, especially penicillin, are one of humanity's greatest medical achievements; discovered by Fleming, developed by Chain & Florey.
- Sewage treatment and biogas production convert waste into safer water and useful energy through microbial metabolism.
- Bt, Trichoderma, baculoviruses are eco-friendly biocontrol agents; Rhizobium, cyanobacteria, mycorrhiza are biofertilisers — both help sustainable agriculture.