Microbes in Human Welfare

Bacteria are found in virtually every habitat on Earth due to their remarkable adaptability: 1. Soil: • Bacteria are the most abundant organisms in soil (~1 billion per gram of fertile soil) • Decomposers: break down dead organic matter → recycle nutrients • Nitrogen-fixing bacteria: Rhizobium (in root nodules), Azotobacter, Anabaena — fix atmospheric N₂ • Nitrifying bacteria: Nitrosomonas, Nitrobacter — convert ammonia to nitrates 2. Water bodies (freshwater and marine): • Oceans contain vast numbers of cyanobacteria (photosynthetic — major O₂ producers) • Deep-sea hydrothermal vents: Chemolithotrophs survive in extreme heat, pressure, no sunlight • Cyanobacteria like Trichodesmium fix nitrogen in oceans 3. Air: • Bacteria exist as aerosols; spores survive in air • Upper atmosphere: bacterial spores found at 77 km altitude 4. Living organisms (as normal flora): • Human gut: 10¹³–10¹⁴ bacteria (gut microbiome) — essential for digestion and immunity • Skin, oral cavity, respiratory tract, urogenital tract • Symbionts in animals and plants (Rhizobium in legume root nodules) 5. Extreme environments: • Hot springs (Thermus aquaticus — source of Taq polymerase) • Polar ice caps (psychrophiles) • Salty environments — Dead Sea, salt flats (halophiles) • Acidic environments (pH < 2) — mine drainage (acidophiles) • Deep subsurface rocks — lithotrophs 6. Food and household items: • On food surfaces, in fermented foods • On commonly touched surfaces Bacteria are found everywhere because they: reproduce rapidly, have minimal nutritional requirements, form resistant endospores, evolve resistance quickly, and exploit every available energy source.

(a) Household food products: • Curd/yoghurt: Lactobacillus acidophilus and other lactic acid bacteria ferment lactose → lactic acid; milk proteins coagulate → curd • Bread: Saccharomyces cerevisiae (baker's yeast) ferments sugar → CO₂ leavens bread • Idli, dosa: Fermentation by bacteria and yeast — puffiness due to CO₂ • Cheese: Various bacteria and fungi — Lactobacillus for curdling, Propionibacterium for Swiss cheese holes and flavor, Penicillium camemberti for Brie • Vinegar: Acetobacter oxidises ethanol → acetic acid • Soy sauce, miso, tempeh: Aspergillus, Rhizopus fermentation of soya beans • Toddy: Fermented palm sap by yeasts (b) Industrial products: • Fermented beverages: Saccharomyces cerevisiae — beer, wine, whisky, brandy • Antibiotics: Penicillin (Penicillium notatum/chrysogenum), Streptomycin (Streptomyces griseus), Tetracycline (Streptomyces) • Organic acids: Citric acid (Aspergillus niger), Gluconic acid (A. niger), Lactic acid (Lactobacillus) • Enzymes: Lipases, proteases, amylases from Aspergillus, Bacillus • Vitamins: Riboflavin (B₂) from Ashbya gossypii; Cyanocobalamin (B₁₂) from Streptomyces • Streptokinase: From Streptococcus — used as clot buster in heart attacks • Cyclosporin A: Trichoderma polysporum — immunosuppressant for organ transplants • Statins: Monascus purpureus — blood cholesterol lowering drugs (c) Sewage treatment: • Primary treatment: Physical removal of solids • Secondary (biological) treatment: Bacteria and fungi naturally present in sewage decompose organic matter; activated sludge (flocs of bacteria) formed; flocs settle in settling tanks • Anaerobic digestion in sludge digesters: bacteria produce biogas (CH₄, CO₂, H₂S); sludge used as manure • Flocs of bacteria reduce BOD (Biological Oxygen Demand); treated water released to water bodies (d) Biogas production: • Anaerobic bacteria decompose animal dung, sewage sludge, crop waste in biogas plants • Methanogen bacteria (e.g., Methanobacterium) convert cellulose, sugars → methane (CH₄) • Biogas composition: ~70% CH₄, CO₂, traces of H₂S • Used for cooking and lighting • By-product: Slurry — used as fertiliser • KVIC (Khadi and Village Industries Commission) promotes biogas plants in rural India (e) Biocontrol agents: • Bacillus thuringiensis (Bt): Produces Cry toxin (endotoxin) — kills insect larvae (caterpillars, beetles, mosquito larvae) by forming pores in their gut; safe for non-target organisms; basis of Bt cotton • Baculoviruses (Nucleopolyhedrovirus): Infect and kill insects — specific, safe for non-target organisms, useful in integrated pest management • Trichoderma: Fungus used to control root pathogens (biological fungicide) • Ladybird beetles, lacewings: Biological control of aphids • Bacillus sphaericus: Mosquito larvicide (f) Biofertilisers: • Rhizobium: Symbiotic nitrogen-fixing bacteria in legume root nodules; 100–300 kg N/ha/year • Azospirillum and Azotobacter: Free-living nitrogen-fixing bacteria in soil • Cyanobacteria (BGA): Anabaena, Nostoc — fix nitrogen in paddy fields; Azolla-Anabaena symbiosis widely used in rice cultivation • Mycorrhizal fungi: Mutualistic fungi on plant roots; increase uptake of phosphorus, water, micronutrients; help establish plants in adverse conditions

Sewage: • Wastewater generated by residential, commercial, and industrial activities • Contains: human excreta, food wastes, detergents, oils, industrial effluents, and various chemicals • Highly polluted — contains large numbers of pathogenic organisms Composition of sewage: • Organic matter (highly biodegradable) • Inorganic salts (nitrogen, phosphorus compounds) • Pathogenic microorganisms (bacteria, viruses, protozoa, helminths) • Industrial pollutants, heavy metals • Detergents, soaps How sewage is harmful: 1. Spread of waterborne diseases: • Bacterial: Cholera (Vibrio cholerae), Typhoid (Salmonella typhi), Dysentery (Shigella) • Viral: Hepatitis A, Poliomyelitis • Protozoal: Amoebiasis (Entamoeba histolytica), Giardiasis • If sewage contaminates drinking water → epidemic outbreaks 2. Eutrophication: • High N and P in sewage → excessive algal growth in water bodies (algal bloom) • Algae die → decomposed by bacteria → massive oxygen consumption • Depletion of dissolved oxygen (DO) → death of fish and aquatic organisms • Water body becomes 'dead' — unable to support life 3. BOD increase: • High Biological Oxygen Demand (BOD) — large amount of organic matter in sewage demands oxygen for decomposition • Depletes DO in water → kills aquatic organisms 4. Soil contamination: • Disposal of untreated sewage on land contaminates soil → gets into groundwater, crops 5. Aesthetic and sensory nuisance: • Foul odour (H₂S from anaerobic decomposition) • Visual pollution 6. Vector breeding: • Stagnant sewage → mosquito breeding → malaria, dengue Proper sewage treatment (primary, secondary, tertiary) is essential to protect public health and environment.

BOD = Biological Oxygen Demand Definition: • The amount of dissolved oxygen (DO) consumed by microorganisms (bacteria) when decomposing organic matter present in water under aerobic conditions at a standard temperature (20°C) over 5 days • Measured in mg of O₂ per litre of water (mg/L or ppm) What BOD signifies: 1. Measure of organic pollution: • High BOD = large amount of organic matter in water = highly polluted water • Low BOD = little organic matter = clean water 2. Oxygen available for aquatic life: • Bacteria decomposing organic matter consume dissolved oxygen • High BOD → bacteria consume O₂ → DO falls • Low DO → fish and other aquatic organisms suffocate and die 3. Indicator of sewage contamination: • Domestic sewage has BOD of 100–400 mg/L • Industrial effluents can have BOD of thousands of mg/L • Clean river water: BOD < 2 mg/L Standard values: • Drinking water: < 1 mg/L • Clean river: 1–2 mg/L • Mildly polluted: 3–5 mg/L • Heavily polluted: > 10 mg/L • Raw domestic sewage: ~200–400 mg/L Relationship with sewage treatment: • Secondary sewage treatment reduces BOD of sewage by 90% before discharge into water bodies • Treated water should have BOD < 10 mg/L before release In short: BOD is an indirect measure of organic pollution — higher BOD means more organic pollution and less oxygen available for aquatic life.

Role of microbes in biogas production: Biogas is a mixture of gases (mainly methane, CH₄ ~55–70%; CO₂ ~30–40%; traces of H₂S) produced by anaerobic microbial decomposition of organic matter. Key microorganisms involved: Step 1 — Hydrolysis: • Hydrolytic bacteria: Break down complex organic molecules • Cellulose-degraders (Clostridium, Ruminococcus): cellulose → glucose • Proteolytic bacteria: proteins → amino acids • Lipolytic bacteria: fats → fatty acids + glycerol Step 2 — Acidogenesis: • Acidogenic bacteria (e.g., Bacteroides, Clostridium): Convert sugars, amino acids, fatty acids → organic acids (acetic acid, propionic acid, butyric acid), alcohols, H₂, CO₂ Step 3 — Acetogenesis: • Acetogenic bacteria: Convert organic acids and alcohols → acetic acid + H₂ + CO₂ Step 4 — Methanogenesis: • Methanogenic archaea (methanogens): Most important microbes for biogas • Examples: Methanobacterium, Methanococcus, Methanosarcina • Convert acetic acid and H₂/CO₂ → methane (CH₄) • Reactions: – CH₃COOH → CH₄ + CO₂ – CO₂ + 4H₂ → CH₄ + 2H₂O • Methanogens are strict anaerobes — require completely oxygen-free conditions Biogas plant design: • Input: Animal dung (mainly cattle dung), crop residue, sewage sludge • Mixed with water → slurry • Fed into anaerobic digester (sealed tank) • Biogas collects in gas holder above • Spent slurry (digestate) — excellent nitrogen-rich fertiliser Applications: • Cooking fuel, lighting, electricity generation • Reduces dependence on fossil fuels; reduces greenhouse gas emissions from dung decomposition

Antibiotics: • Chemical substances produced by microorganisms that kill or inhibit the growth of other microorganisms • First antibiotic: Penicillin — discovered by Alexander Fleming (1928) from Penicillium notatum • Used to treat bacterial infections Examples of antibiotics and their sources: • Penicillin: Penicillium chrysogenum — kills gram-positive bacteria (by inhibiting cell wall synthesis) • Streptomycin: Streptomyces griseus — used for tuberculosis • Erythromycin: Streptomyces erythraea — respiratory and skin infections • Tetracycline: Streptomyces species — broad-spectrum • Chloramphenicol: Streptomyces venezuelae • Cephalosporin: Cephalosporium species Precautions while taking antibiotics: 1. Complete the full course: • Even if you feel better, complete the entire prescribed course • Stopping early allows surviving bacteria to multiply and develop resistance 2. Take only when prescribed: • Don't use antibiotics for viral infections (cold, flu, COVID-19) — they are ineffective against viruses • Self-medication is dangerous 3. Do not share antibiotics: • Different infections may require different antibiotics • Someone else's prescription may not be right for you 4. Take at correct dosage and timing: • Follow the dosage schedule (some need food, some need empty stomach) • Missing doses can reduce effectiveness 5. Do not save antibiotics for later: • Expired or inappropriately stored antibiotics may be ineffective or harmful 6. Inform doctor of allergies: • Penicillin allergy is common; anaphylactic reactions can be life-threatening 7. Be aware of side effects: • Diarrhoea (disruption of gut flora), nausea, rashes • Severe: antibiotic-associated colitis (Clostridium difficile)