I would carry a sample of curd (dahi) from home. Reasons:

• Curd is rich in living Lactobacillus bacteria — easy to find dense rod-shaped cells under the microscope.
• Sample preparation is simple: spread a thin smear on a slide, stain with methylene blue, observe under 100× oil immersion.
• Sample is safe to handle — no pathogens; just food-grade bacteria.
• You can also demonstrate the process by comparing fresh milk (very few bacteria) with curd (millions).

Alternatives: yoghurt, idli batter, kombucha, bread dough.

1. Bread dough rising: Saccharomyces cerevisiae ferments sugar → CO₂ + ethanol. CO₂ bubbles cause the dough to puff up.
2. Idli/dosa batter rising: Leuconostoc, Streptococcus + yeast ferment urad/rice → CO₂ traps in batter, makes idlis soft.
3. Swiss-cheese holes: Propionibacterium sharmanii produces CO₂ during ripening; CO₂ trapped in cheese paste forms holes.
4. Biogas plant: Methanogens digest organic matter → CH₄ + CO₂ + H₂S = biogas.
5. Sewage sludge digester: Same process — gases bubble up.

Lactic acid bacteria (LAB) — mainly Lactobacillus, Lactococcus, Streptococcus — are found in:

• Curd / yoghurt
• Cheese (during initial fermentation)
• Idli / dosa batter (with Leuconostoc)
• Fermented pickles, sauerkraut, kimchi
• Buttermilk

Useful applications:

1. Convert milk → curd, increasing vitamin B12 and digestibility.
2. Check growth of disease-causing microbes in our stomach (probiotic effect).
3. Preserve food by lowering pH (pickling, fermentation).
4. Industrial production of lactic acid (food, biodegradable plastics).
5. Used in cheese ripening.

• Wheat: Bhatura (yeast-leavened), kulcha, naan, bread, pizza dough — all rise due to Saccharomyces cerevisiae.
• Rice: Idli, dosa, dhokla (with chickpea), appam, neer dosa — fermented batters with Leuconostoc, Streptococcus + yeast.
• Bengal gram (chana/besan): Dhokla (with rice/semolina), khaman, papdi — fermented with LAB.
• Mixed: Kanji (fermented carrot drink), kulhad lassi, mithai, certain pickles, vadi/badi (sun-fermented).

Microbes have provided antibiotics — chemical substances produced by one microbe that kill or inhibit other microbes, especially disease-causing bacteria.

• Penicillin from Penicillium notatum — discovered by Fleming (1928); developed by Chain & Florey for WWII soldiers; saved millions from wound infection deaths.
• Streptomycin from Streptomyces griseus — TB treatment.
• Tetracycline, erythromycin, gentamicin, kanamycin — all from soil microbes.

Before antibiotics, diseases like pneumonia, syphilis, plague and tuberculosis were major killers. Antibiotics added ~20 years to average human life expectancy in the 20th century and made surgery, cancer chemotherapy, and organ transplantation possible.

1. Penicillium notatum / P. chrysogenum → Penicillin.
2. Cephalosporium acremonium → Cephalosporin.

(Many bacteria of the genus Streptomyces are also important antibiotic producers — streptomycin, tetracycline, neomycin etc.)

Sewage = the municipal waste-water generated daily from kitchens, bathrooms, industries and storm drains. It contains large amounts of organic matter and pathogenic microbes.

Harmful effects if untreated:

• Spreads waterborne diseases — typhoid, cholera, dysentery, hepatitis-A.
• High organic load → low dissolved O₂ in water bodies → fish kill, dead zones.
• Eutrophication of rivers/lakes by N and P.
• Foul odour, breeding ground for mosquitoes and houseflies.
• Contaminates groundwater used for drinking.

Primary alone is insufficient; secondary is essential before discharge.

Yes. Microbes generate energy by producing biofuels.

• Biogas: Methanogens (e.g., Methanobacterium) digest organic matter anaerobically → methane + CO₂. Used as cooking fuel, electricity, vehicle fuel (Bio-CNG).
• Bioethanol: Yeast (Saccharomyces cerevisiae) ferments sugar/starch → ethanol, blended with petrol (E5, E10).
• Hydrogen: Certain microalgae and bacteria produce H₂ gas via biophotolysis.
• Microbial fuel cells (MFCs): Electroactive bacteria transfer electrons to an electrode while metabolising organic matter — generates electricity directly from wastewater.

Replacing chemical fertilisers (biofertilisers):

• Rhizobium — fixes N₂ in legume root nodules (peas, gram, soybean).
• Azotobacter, Azospirillum — free-living N-fixers for non-legume crops.
• Anabaena, Nostoc, Oscillatoria — cyanobacterial N + carbon fixers for paddy.
• Glomus (mycorrhiza) — absorbs P, water, micronutrients.

Replacing chemical pesticides (biocontrol):

• Bacillus thuringiensis — Cry toxin against insect larvae; foundation of Bt cotton.
• Trichoderma — controls soil-borne fungi (Fusarium, Pythium).
• Baculoviruses (NPV) — species-specific viral kills only the target pest.
• Pheromone traps + predator release also part of IPM.

Benefits: Lower chemical residues, healthier soil, sustainable productivity, biodiversity preservation.

Increasing BOD: Distilled water < Pond water < River water (polluted river).

• Distilled water — pure H₂O, no microbes, no organic matter. BOD ≈ 0.
• Pond water — some microbes and algae; modest organic matter. BOD low to moderate.
• River water — particularly downstream of cities — receives sewage and effluents; high microbial load. BOD high.

Note: The order assumes the river is in/near a city; an unpolluted Himalayan stream could have lower BOD than a stagnant pond.