Microbes in household and industrial products, antibiotics, sewage treatment, biogas, biocontrol and biofertilisers — with SVG diagrams, microbe tables, MCQs, A-R and PYQs.
Microbes occur everywhere — soil, water, air, inside organisms, and even at sites like thermal vents (up to 100°C), deep soil, thick snow and highly acidic places. They include protozoa, bacteria, fungi, microscopic algae, viruses, viroids and prions (proteinaceous infectious agents). Bacteria and fungi can be grown on nutritive media to form visible colonies.
| Product | Microbe | Use |
|---|---|---|
| Citric acid | Aspergillus niger (fungus) | Organic acid |
| Acetic acid | Acetobacter aceti (bacterium) | Organic acid (vinegar) |
| Butyric acid | Clostridium butylicum (bacterium) | Organic acid |
| Lactic acid | Lactobacillus (bacterium) | Organic acid |
| Ethanol | Saccharomyces cerevisiae | Commercial alcohol |
| Lipases | Microbes | Detergents — remove oily stains |
| Pectinases, proteases | Microbes | Clarify bottled fruit juices |
| Streptokinase | Streptococcus (modified by genetic engineering) | 'Clot buster' after myocardial infarction |
| Cyclosporin A | Trichoderma polysporum (fungus) | Immunosuppressive in organ transplant |
| Statins | Monascus purpureus (yeast) | Blood-cholesterol lowering; competitively inhibit the enzyme for cholesterol synthesis |
Sewage = municipal waste water (largely human excreta) with lots of organic matter and many pathogens; it cannot be discharged directly into rivers. It is treated by heterotrophic microbes naturally present in it, in two stages:
| Stage | What happens |
|---|---|
| Primary treatment | Physical removal of particles: sequential filtration (floating debris) → sedimentation (grit: soil, pebbles). Settled solids = primary sludge; supernatant = effluent. |
| Secondary (biological) treatment | Primary effluent goes to aeration tanks — constantly agitated, air pumped in. Aerobic microbes grow into flocs (bacteria + fungal filaments in meshes) and consume organic matter → BOD falls. Effluent → settling tank: sediment = activated sludge. Small part back to aeration tank as inoculum; rest → anaerobic sludge digesters → biogas (CH₄, H₂S, CO₂). Final effluent is released into rivers/streams. |
The amount of oxygen consumed if all organic matter in one litre of water were oxidised by bacteria. Higher BOD = more organic matter = greater polluting potential.
Biocontrol = using biological methods to control plant diseases and pests, reducing toxic insecticides, pesticides and weedicides. Organic farmers keep pests at manageable levels through natural checks and balances (biodiversity) rather than eradicating them; integrated pest management (IPM) conserves beneficial insects.
| Agent | Controls | Key points |
|---|---|---|
| Ladybird (beetle) | Aphids | Predator |
| Dragonfly | Mosquitoes | Predator |
| Bacillus thuringiensis (Bt) | Butterfly caterpillars (insect larvae) | Dried spores mixed with water and sprayed on plants; toxin released in the larval gut kills the larvae, other insects unharmed. Bt toxin genes introduced into plants → Bt cotton. |
| Trichoderma (fungus) | Several plant pathogens | Free-living fungus common in root ecosystems. |
| Baculoviruses (Nucleopolyhedrovirus) | Insects and other arthropods | Species-specific, narrow-spectrum; no negative effect on plants, mammals, birds, fish or non-target insects. |
Biofertilisers are organisms that enrich the nutrient quality of soil; main sources are bacteria, fungi and cyanobacteria. They reduce dependence on chemical fertilisers (which pollute) and support organic farming.
Q1. Three water samples A, B, C (river water, untreated sewage, secondary effluent) have BOD 20, 8 and 400 mg/L respectively. Which is most polluted? Assign the labels, assuming river water is relatively clean.
Greater BOD = more organic matter = more polluting.
Sample C (400 mg/L) = untreated sewage.
Sample B (8 mg/L) = river water (cleanest); sample A (20 mg/L) = secondary effluent.
Q2. What is the key difference between primary and secondary sewage treatment?
Physical removal of particles by filtration and sedimentation.
Biological treatment by aerobic microbes (flocs) in aeration tanks, which reduces BOD.
Q3. How does converting milk into curd improve its nutritional quality? Why is a little curd added to fresh milk?
LAB produce acids that coagulate and partially digest milk proteins.
Curd has increased vitamin B₁₂.
A little curd is the inoculum containing millions of LAB, which multiply at a suitable temperature and convert milk into curd.
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
Q. What are antibiotics? Describe the discovery of penicillin.
Antibiotics are chemicals produced by some microbes that kill or retard growth of disease-causing microbes. Alexander Fleming found Staphylococci could not grow around a mould (Penicillium notatum) on his culture plate and named the chemical penicillin; Chain and Florey established its potential. Nobel Prize 1945.
Q. Describe the secondary treatment of sewage. What are flocs and activated sludge?
Primary effluent is aerated in tanks; aerobic microbes form flocs (bacteria with fungal filaments) that consume organic matter and lower BOD. Flocs settle as activated sludge; part returns as inoculum, rest goes to anaerobic digesters producing biogas.
Q. How is biogas produced? Name the microbes involved.
Dung slurry is fed into a digester tank; methanogens (Methanobacterium) digest cellulosic material anaerobically, producing methane, CO₂ and H₂. Gas collects under a floating cover and is piped for cooking and lighting; spent slurry is fertiliser.
Q. What is Bt? How is it useful in pest control?
Bacillus thuringiensis — spores sprayed on plants are eaten by caterpillars; toxin released in the larval gut kills them without harming other insects. Its toxin gene is also put into plants (Bt cotton).
Q. Name microbes that act as biofertilisers and state their roles.
Rhizobium (symbiotic N₂ fixation in legume nodules); Azospirillum, Azotobacter (free-living N₂ fixers); Glomus mycorrhiza (phosphorus uptake); cyanobacteria — Anabaena, Nostoc (paddy fields).