What this chapter is about
Microbes are microscopic organisms including bacteria, fungi, protozoa, and certain algae and viruses. While many people associate microbes with disease, the vast majority are either harmless or positively beneficial to human life. This chapter explores the numerous ways in which microbes contribute to human welfare, from the food we eat to the medicines we use and the environment we depend upon.
A Class 12 student studies this chapter to understand how microorganisms are harnessed in household products, industrial processes, sewage treatment, biogas production, and agriculture. This knowledge connects earlier concepts of microbiology and metabolism to real-world applications in biotechnology and environmental science. After studying this chapter, you should be able to explain how fermentation works, describe the role of microbes in producing antibiotics and other industrial products, and appreciate microbial contributions to sustainable practices like biological pest control and soil fertility.
Key ideas
- Microbes in household products: Fermentation by bacteria and yeasts produces curd, bread, cheese, and alcoholic beverages. Lactobacillus bacteria convert milk lactose into lactic acid, coagulating milk proteins to form curd.
- Industrial fermentation: Large-scale fermenters use specific microbes to produce beverages (beer, wine), organic acids (citric acid, acetic acid), enzymes (lipases, proteases), and bioactive molecules under controlled conditions.
- Antibiotics and their production: Antibiotics are substances produced by certain microbes that inhibit or kill other microorganisms. Penicillium notatum produces penicillin, while Streptomyces species produce streptomycin and other antibiotics used to treat bacterial infections.
- Sewage treatment: Microbes decompose organic matter in sewage through primary and secondary treatment stages. Aerobic bacteria in activated sludge consume organic pollutants, reducing the biological oxygen demand (BOD) of wastewater before safe discharge.
- Biogas production: Methanogenic bacteria anaerobically digest organic waste in biogas plants, producing methane-rich biogas used as fuel. Cattle dung (gobar) is a common substrate in rural India.
- Biofertilisers: Certain microbes enhance soil fertility naturally. Nitrogen-fixing bacteria (Rhizobium in legume root nodules, free-living Azotobacter), cyanobacteria, and mycorrhizal fungi improve nutrient availability for crops.
- Biological control of pests: Microbes like Bacillus thuringiensis (Bt) produce toxins that kill specific insect pests, reducing dependence on chemical pesticides. Baculoviruses target arthropod pests without harming non-target organisms.
Formulas and facts to remember
- Biological Oxygen Demand (BOD): The amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter in water. High BOD indicates high pollution; effective sewage treatment lowers BOD.
- Fermentation: An anaerobic metabolic process where microbes convert sugars into acids, gases, or alcohol. Saccharomyces cerevisiae (brewer's yeast) ferments glucose to ethanol and carbon dioxide.
- Penicillin: The first discovered antibiotic, produced by the fungus Penicillium notatum, effective against many Gram-positive bacteria by inhibiting cell wall synthesis.
- Methanogens: Anaerobic archaebacteria (like Methanobacterium) that produce methane during biogas generation from organic waste.
- Rhizobium: A symbiotic nitrogen-fixing bacterium that forms nodules on legume roots, converting atmospheric nitrogen (N₂) into ammonia (NH₃) usable by plants.
- Mycorrhiza: A mutualistic association between fungi and plant roots that enhances phosphorus uptake and water absorption.
- Bacillus thuringiensis (Bt): A soil bacterium producing crystal proteins (Cry proteins) toxic to certain insect larvae when ingested, widely used as a biopesticide.
- Primary sewage treatment: Physical removal of large debris and suspended solids through screening and sedimentation.
- Secondary sewage treatment: Biological treatment using aerobic microbes in aeration tanks to decompose dissolved organic matter.
Worked examples
Example 1: Calculating BOD reduction in sewage treatment
A sewage treatment plant receives wastewater with an initial BOD of 250 mg/L. After secondary treatment, the effluent has a BOD of 20 mg/L. Calculate the percentage reduction in BOD.
Solution: Initial BOD = 250 mg/L Final BOD = 20 mg/L Reduction in BOD = 250 − 20 = 230 mg/L Percentage reduction = (230 / 250) × 100 = 92%
The treatment plant achieves 92% BOD reduction, indicating highly effective microbial decomposition of organic pollutants.
Example 2: Nitrogen fixation by Rhizobium
A farmer grows chickpea (a legume) in a field. The Rhizobium bacteria in root nodules fix approximately 80 kg of nitrogen per hectare per year. If the farmer cultivates 3 hectares, how much nitrogen is biologically fixed?
Solution: Nitrogen fixed per hectare = 80 kg Total area = 3 hectares Total nitrogen fixed = 80 × 3 = 240 kg
The farmer benefits from 240 kg of biologically fixed nitrogen, reducing the need for synthetic nitrogen fertilisers.
Example 3: Biogas production estimation
A village biogas plant processes 50 kg of cattle dung daily. If each kilogram of dung produces approximately 0.04 m³ of biogas, calculate the daily biogas output.
Solution: Cattle dung processed = 50 kg/day Biogas yield = 0.04 m³/kg Daily biogas production = 50 × 0.04 = 2.0 m³
The plant produces 2.0 cubic metres of biogas daily, sufficient for cooking needs of a small household.
Common mistakes
- Thinking all microbes cause disease → Most microbes are beneficial or neutral; only a small fraction are pathogenic.
- Confusing fermentation with respiration → Fermentation is anaerobic and produces less energy; respiration can be aerobic and yields much more ATP.
- Believing antibiotics work against viruses → Antibiotics target bacterial structures and do not affect viruses.
- Assuming all nitrogen-fixing bacteria are symbiotic → Some, like Azotobacter and Azospirillum, are free-living in soil.
- Mixing up primary and secondary sewage treatment → Primary treatment is physical (sedimentation); secondary treatment is biological (microbial decomposition).
Quick revision
- Lactobacillus converts milk to curd by producing lactic acid through fermentation.
- Penicillin from Penicillium was the first antibiotic; Streptomyces produces streptomycin.
- BOD measures organic pollution; lower BOD after treatment means cleaner water.
- Methanogens produce methane in biogas plants from anaerobic digestion of organic waste.
- Rhizobium fixes atmospheric nitrogen in legume root nodules; Azotobacter is free-living.
- Bacillus thuringiensis (Bt) produces insecticidal Cry proteins used in biological pest control.