Board exam focus — Microbes in Human Welfare (CBSE & HBSE)

CBSE focuses on beneficial roles of microbes in household products (curd, cheese, bread), industrial products (antibiotics, statins, streptokinase, cyclosporin-A), biogas production, biofertilisers, biological control, and sewage treatment. HBSE emphasises examples, beneficial microbes, BOD, biofertilisers, and biogas.

Microbes in Household and Industrial Products

Microbes and Their Beneficial Roles

Microbes (microorganisms) — bacteria, fungi, protozoa, viruses, and algae — are ubiquitous and play crucial roles in human welfare through their metabolic activities.

Microbes in Household Food Products

1. Curd (Dahi):

• Formed by lactic acid fermentation of milk
• Microbe: Lactobacillus acidophilus (Lactic Acid Bacteria — LAB)
• LAB produce lactic acid from lactose → lowers pH → denatures milk proteins → curd sets
• A small amount of curd added to fresh milk as inoculum (contains LAB)
• LAB are also normal flora of human vagina (acidic environment prevents infection) and gut
• Nutritional benefit: increases Vitamin B12 content; improves digestibility; probiotics

2. Bread:

• Microbe: Saccharomyces cerevisiae (Baker's yeast)
• Fermentation: yeast converts sugars (glucose, maltose) to ethanol + CO2 under anaerobic conditions
• CO2 causes dough to rise (leavening) → light, porous texture after baking
• Ethanol evaporates on baking

3. Idli, Dosa, Dhokla:

• Made from batter of rice + urad dal (black gram)
• Microbes: Leuconostoc mesenteroides (bacterium) and Streptococcus species
• Lactic acid fermentation → batter becomes sour, rises due to CO2 production
• Dhokla: chickpea flour + curd; similar fermentation

4. Cheese:

• Curd (coagulated milk protein — casein) + microorganisms + aging
• Lactobacillus and related LAB for basic fermentation
• Specific characteristics due to different microbes:
• Roquefort cheese (France): Penicillium roqueforti — blue-green veins; nutty flavour; semi-soft
• Camembert cheese (France): Penicillium camemberti — white mould rind; creamy interior
• Swiss cheese: Propionibacterium shermanii — produces propionic acid + CO2 → large characteristic holes ("eyes"); flavour from propionic acid
• Cottage cheese: fresh; not aged; mild

5. Toddy:

• Fermented sap of coconut palm (Cocos nucifera) or palm trees
• Yeast fermentation; traditionally consumed

6. Vinegar:

• Acetobacter aceti oxidises ethanol to acetic acid (in the presence of oxygen)

Industrial Products from Microbes

A. Antibiotics:

Penicillin:

• Discovered by Alexander Fleming (1928): noticed mould (Penicillium notatum) contaminating a Staphylococcus culture plate and lysing the bacteria around it
• Developed into medicine by Howard Florey and Ernst Chain during World War II (Nobel Prize 1945)
• Industrial production uses Penicillium chrysogenum
• Mechanism: inhibits bacterial cell wall synthesis (transpeptidase/penicillin-binding protein inhibition)
• Killed countless soldiers' wound infections; first antibiotic used clinically

Streptomycin: from Streptomyces griseus; first antibiotic effective against tuberculosis; discovered by Waksman (Nobel Prize 1952)

Tetracycline: from Streptomyces species; broad-spectrum antibiotic

Chloramphenicol: from Streptomyces venezuelae; broad-spectrum

Erythromycin: from Streptomyces erythraeus

B. Enzymes:

• Lipase: Aspergillus — detergent industry (removes fat stains)
• Pectinase: Aspergillus — fruit juice clarification (breaks down pectin)
• Protease: Bacillus — detergent industry (removes protein stains)
• Amylase: starch to glucose conversion for food/brewing

C. Bioactive Molecules:

Streptokinase ("clot buster"):

• Produced by Streptococcus species
• Enzyme that activates plasminogen → plasmin → dissolves fibrin clots
• Used clinically to dissolve blood clots in myocardial infarction (heart attack), pulmonary embolism, stroke
• "Clot buster" or thrombolytic agent

Cyclosporin A:

• Produced by the fungus Trichoderma polysporum (soil fungus)
• Immunosuppressant: inhibits T cell activation (calcineurin inhibitor — blocks IL-2 production)
• Critical in organ transplantation to prevent graft rejection
• Also used in autoimmune diseases (rheumatoid arthritis, psoriasis)

Statins (cholesterol-lowering drugs):

• Produced by Monascus purpureus (a yeast) and also Aspergillus terreus
• Mechanism: inhibit HMG-CoA reductase (3-hydroxy-3-methylglutaryl coenzyme A reductase) — rate-limiting enzyme in cholesterol biosynthesis pathway
• Lower LDL cholesterol → reduce risk of cardiovascular disease
• Commercial statins: lovastatin (mevastatin from Penicillium), atorvastatin, simvastatin (semi-synthetic derivatives)

D. Other Industrial Products:

• Organic acids: citric acid (Aspergillus niger — ferments sucrose/molasses); gluconic acid (Aspergillus); lactic acid (LAB); acetic acid (Acetobacter)
• Vitamins: Vitamin B12 (Pseudomonas species); riboflavin/B2 (Ashbya gossypii, Eremothecium ashbyii)
• Ethanol/Biofuel: Saccharomyces cerevisiae ferments sugars to ethanol (gasohol/bioethanol from sugarcane molasses in Brazil)
• Amino acids: glutamic acid (Corynebacterium glutamicum) for MSG (monosodium glutamate)

Biogas, Biofertilisers and Biological Control

Biogas Production

Biogas is a mixture of gases produced by anaerobic decomposition of organic matter by microorganisms. It is a renewable source of energy.

Composition of Biogas:

• Methane (CH4): ~55-75% (main combustible component)
• Carbon dioxide (CO2): ~25-45%
• Hydrogen sulphide (H2S): small amounts
• Water vapour

Microorganisms in Biogas Production:

Methanogens: strictly anaerobic archaebacteria that produce methane from CO2 and H2 or from acetate.

• Methanobacterium, Methanobacillus, Methanococcus
• They are present in gut of cattle (ruminants), termites, marshes, swamps, and sewage sludge
• Use acetate: CH3COOH → CH4 + CO2
• Use CO2 + H2: CO2 + 4H2 → CH4 + 2H2O

Biogas Plant: Two main designs used in India:

1. KVIC (Khadi and Village Industries Commission) Floating Drum Type:

• An underground cylindrical digester tank made of concrete
• Filled with slurry of dung and water (1:1 ratio)
• A cylindrical floating drum (made of steel) sits on top of the slurry
• As gas is produced, the drum rises (fills with biogas)
• Gas outlet pipe from top of drum
• Effluent (spent slurry) outlet pipe at bottom
• Inlet for adding fresh dung slurry

2. Deenbandhu (Fixed Dome Type):

• Cheaper to construct; no moving parts
• Underground dome-shaped digester
• Fixed concrete dome on top (no floating drum)
• Gas pressure builds inside as gas is produced → slurry displaced to overflow tank

Process:

1. Anaerobic bacteria first break down complex organic material (cellulose, proteins, fats) — hydrolysis and acidogenesis
2. Acetogenic bacteria produce acetate and H2
3. Methanogens convert these to methane

Benefits of biogas plants:

• Clean, renewable fuel for cooking and lighting (rural India)
• Reduces dependence on firewood (prevents deforestation)
• Effluent slurry is an excellent organic fertiliser (nitrogen, phosphorus)
• Helps in waste management

Biofertilisers

Biofertilisers are organisms (bacteria, cyanobacteria, fungi) that enrich the nutrient quality of soil, particularly by fixing atmospheric nitrogen or solubilising phosphorus.

Nitrogen-Fixing Biofertilisers:

1. Rhizobium (Symbiotic):

• Gram-negative bacterium; genus Rhizobium and related genera (Mesorhizobium, Bradyrhizobium, Sinorhizobium)
• Forms root nodules on legume plants (pea, bean, soybean, groundnut, clover, alfalfa)
• Inside nodule, bacteria fix N2 using the enzyme nitrogenase (contains Mo, Fe): N2 + 8H+ + 8e- → 2NH3 + H2
• Nitrogenase is oxygen-sensitive; leghaemoglobin (pink pigment in nodules) maintains low oxygen level
• Host plant provides carbon (sugars); bacteria provide fixed nitrogen (ammonia) to plant
• Species-specific: Rhizobium leguminosarum (pea, bean), Bradyrhizobium japonicum (soybean)
• Application: seed inoculation with Rhizobium culture before planting

2. Azospirillum (Associative):

• Azospirillum lipoferum, A. brasilense
• Associative symbiosis with roots of grasses and cereals (wheat, maize, sorghum)
• Fixes N2 in the rhizosphere; also produces auxins promoting root growth

3. Azotobacter (Free-living):

• Azotobacter chroococcum, A. vinelandii
• Free-living, aerobic soil bacterium
• Fixes N2 in soil; also produces vitamins and growth regulators
• Used as soil inoculant for non-leguminous crops

4. Cyanobacteria (Blue-Green Algae — BGA):

• Anabaena, Nostoc, Oscillatoria, Tolypothrix
• Heterocysts: specialised cells that fix N2 (microaerobic environment inside heterocyst)
• Free-living or symbiotic in soil and water
• Anabaena azollae: symbiotic with water fern Azolla pinnata (aquatic fern)
• Azolla-Anabaena association used as biofertiliser in rice fields (paddy fields) since ancient times in China and Vietnam — adds 30-60 kg N/ha/year

Phosphate-Solubilising Bacteria:

• Bacillus megaterium, Pseudomonas striata
• Solubilise insoluble phosphates (Ca3(PO4)2 etc.) by producing organic acids and phosphatases
• Make phosphorus available for plant uptake

Mycorrhiza (Fungal Biofertilisers):

• Mutualistic association between fungi and plant roots
• Glomus species — Vesicular Arbuscular Mycorrhiza (VAM) — most common type
• Fungal hyphae extend far into soil → large surface area → absorb phosphorus, water, and micronutrients (Zn, Cu) more efficiently than roots alone
• Transfer these to the plant; in return, plant provides carbon (photosynthates — sugars) to fungus
• Also increases resistance to drought, soil pathogens
• Found in majority of plant species; essential in nutrient-poor soils (tropical forests)
• Ectomycorrhiza (hyphae form mantle around root) vs Endomycorrhiza (hyphae penetrate root cells — VAM)

Biological Control

Biological control (biocontrol) uses natural enemies (predators, parasites, pathogens) to regulate pest populations, reducing the need for chemical pesticides.

Bacillus thuringiensis (Bt):

• Soil bacterium; forms spores containing crystalline (Cry) proteins (delta-endotoxins = Bt toxin)
• Cry proteins are protoxins (inactive in the crystalline form in spores)
• When ingested by insect larvae (especially Lepidoptera — caterpillars; also Diptera — mosquito larvae; Coleoptera — beetles), the alkaline midgut activates Cry proteins (by proteolytic cleavage)
• Active Cry toxin binds specific glycoprotein receptors on midgut epithelial cells → pores form in cell membrane → cell lyses → insect dies
• Highly specific to insects (binding sites on insect receptors only — not present in mammals, birds, fish) → safe to non-target organisms
• Used: Bt sprays on vegetable crops (cabbage, cotton, brinjal); Bt genes incorporated into plants (Bt cotton, Bt brinjal)

Baculoviruses:

• Nuclear Polyhedrosis Virus (NPV) — Nucleopolyhedrovirus (genus)
• Very narrow host range (species-specific) → minimal impact on non-target organisms, beneficial insects, birds, mammals
• Used against crop pests; especially useful in integrated pest management (IPM)
• Example: Spodoptera NPV against spodoptera moths

Other Biocontrol Agents:

• Trichoderma spp. (fungi): antagonistic to soil-borne fungal pathogens (Fusarium, Pythium, Rhizoctonia); produce antibiotics; compete for nutrients
• Predatory insects: Cryptolaemus montrouzieri (ladybird beetle) against mealybugs; Chrysoperla carnea (green lacewing) against aphids
• Parasitoid wasps: Trichogramma against lepidopteran egg pests; Aphidius parasitises aphids

Sewage Treatment and Microbes

Sewage Treatment

Sewage is the waste-water generated in homes, cities, factories, and hospitals. It contains: suspended solids, dissolved organic matter, pathogenic microorganisms (bacteria, viruses, protozoa, helminths), and in industrial sewage, heavy metals and toxic chemicals.

Releasing untreated sewage into rivers/lakes causes: waterborne diseases, eutrophication, and fish kills. Sewage treatment makes the water safe for release into natural water bodies.

Primary Treatment (Physical): Removes large particulate matter physically.

1. Screening (Bar screens): removes large floating objects (rags, paper, sticks, plastic)
2. Grit removal: heavy particles (sand, gravel) settle in grit chambers
3. Sedimentation (Primary settling tanks): suspended solids settle as primary sludge at the bottom; oils/fats float (skimmed off); supernatant (primary effluent) passed to secondary treatment
4. Reduces suspended solids by ~50-70%; BOD by ~30%

Secondary Treatment (Biological — Key Stage): Uses microorganisms (bacteria, protozoa) to biodegrade dissolved organic matter.

Activated Sludge Process (most common in urban areas):

1. Primary effluent flows into large aeration tanks
2. Air (or oxygen) is pumped in — supplies O2 for aerobic microbial decomposition
3. Aerobic bacteria and other microbes break down organic matter → CO2 + H2O + microbial biomass
4. Microbes aggregate into flocs (flocculated masses, clumps)
5. Flocs settle in secondary settling tanks
6. Supernatant (secondary effluent) = greatly reduced BOD; passed to tertiary treatment or chlorination
7. Settled sludge (activated sludge): a portion recirculated to aeration tank (seeding); rest → anaerobic sludge digester
8. Anaerobic digester: methanogens decompose the remaining organic matter → biogas (methane + CO2); used as energy at the plant

Biochemical Oxygen Demand (BOD): BOD = amount of dissolved oxygen (mg/L) consumed by aerobic microorganisms to decompose organic matter in a water sample at a standard temperature (20°C) over 5 days.

High BOD = highly polluted water (more organic matter → more O2 consumed by bacteria → less O2 available for fish → fish kill)

Fresh river water: BOD ~1-2 mg/L Treated sewage effluent target: BOD < 30 mg/L Raw sewage: BOD 100-300 mg/L

Trickling Filter: Alternative to activated sludge. Sewage is sprayed/trickled over a bed of rocks or plastic media coated with a biofilm of bacteria and algae. Microbes degrade organic matter as sewage trickles through.

Tertiary Treatment (optional, for advanced purification):

• Sand filtration
• Activated carbon adsorption
• Disinfection: chlorination, UV irradiation, ozonation → kills pathogens
• Removes: nitrates, phosphates, heavy metals
• Effluent after tertiary treatment can be safely discharged into rivers/lakes

Role of Microbes in Sewage Treatment Summary:

• Aerobic bacteria (aeration tank): degrade organic matter
• Methanogens (anaerobic digester): produce biogas from sludge
• Protozoa: graze on bacteria → clarifies effluent
• Algae (oxidation ponds): produce O2 for aerobic bacteria

Significance:

• Prevents water pollution
• Produces biogas (energy recovery)
• Produces compost (from sludge)
• Protects aquatic ecosystems and public health

Frequently asked questions

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Do these notes follow CBSE and HBSE?

Yes. The Microbes in Human Welfare notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Microbes in Human Welfare.