Microbes in Human Welfare

2.1 Core concepts

• Microbes are ubiquitous — present in soil, water, air, inside our bodies and those of other animals and plants, and even at sites where no other life-form could survive — deep inside geysers (thermal vents) where temperature may be as high as 100°C, deep in the soil, under metres of snow and in highly acidic environments. The group is diverse: protozoa, bacteria, fungi, microscopic animal and plant viruses, viroids and prions (proteinaceous infectious agents). Bacteria and many fungi can be grown on nutritive media to form colonies visible to the naked eye (NCERT §8 intro, pp. 149–150).
• Curd production: Micro-organisms such as Lactobacillus and others, commonly called lactic acid bacteria (LAB), grow in milk and convert it to curd. During growth LAB produce acids that coagulate and partially digest milk proteins; a small amount of curd added to fresh milk as inoculum/starter contains millions of LAB; at suitable temperatures they multiply, converting milk to curd, and also improve nutritional quality by increasing vitamin B₁₂. In the stomach, LAB also play a beneficial role in checking disease-causing microbes (NCERT §8.1, p. 151).
• Dosa, idli, bread: The dough used for foods such as dosa and idli is fermented by bacteria; the puffed-up appearance is due to CO₂ production. Bread dough is fermented using baker's yeast — Saccharomyces cerevisiae. A number of traditional drinks and foods are also produced by microbial fermentation. 'Toddy', a traditional drink of southern India, is made by fermenting palm sap; fish, soyabean and bamboo-shoots are also fermented to make foods (NCERT §8.1, p. 151).
• Cheeses: One of the oldest food items in which microbes were used. Different varieties have characteristic texture, flavour and taste from the microbes used. The large holes in 'Swiss cheese' are due to large CO₂ production by a bacterium named Propionibacterium sharmanii. 'Roquefort cheese' is ripened by growing a specific fungus on it, which imparts a particular flavour (NCERT §8.1, p. 151).
• Industrial fermentation vessels are called fermentors. Fermented beverages: Microbes — especially yeasts — have been used from time immemorial to produce wine, beer, whisky, brandy and rum. The same yeast, Saccharomyces cerevisiae (called brewer's yeast for this purpose), is used to ferment malted cereals and fruit juices to produce ethanol. Depending on the raw material and processing, different alcoholic drinks are obtained. Wine and beer are produced without distillation, while whisky, brandy and rum are produced by distillation of the fermented broth (NCERT §8.2.1, pp. 151–152).
• Antibiotics (Greek anti = "against"; bio = "life" → "against life" for pathogens, but "pro-life" for humans) are chemical substances produced by some microbes that can kill or retard the growth of other (disease-causing) microbes. Penicillin was the first antibiotic — a chance discovery by Alexander Fleming while working on Staphylococci, who observed a mould around which Staphylococci could not grow; he named the substance after the mould Penicillium notatum. Its full potential as an effective antibiotic was established later by Ernest Chain and Howard Florey; the antibiotic was extensively used to treat American soldiers wounded in World War II. Fleming, Chain and Florey were awarded the Nobel Prize in 1945 for this discovery. Antibiotics improved capacity to treat plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog) (NCERT §8.2.2, pp. 152–153).
• Chemicals, enzymes and bioactive molecules: Microbes commercialise the production of organic acids, alcohols and enzymes. Aspergillus niger (a fungus) → citric acid; Acetobacter aceti (a bacterium) → acetic acid; Clostridium butylicum (a bacterium) → butyric acid; Lactobacillus (a bacterium) → lactic acid; Saccharomyces cerevisiae → ethanol. Lipases are used in detergent formulations to remove oily stains; pectinases and proteases clarify bottled fruit juices; Streptokinase produced by Streptococcus (modified by genetic engineering) is a "clot buster" for myocardial infarction patients. Cyclosporin A from the fungus Trichoderma polysporum is an immunosuppressive agent for organ-transplant patients; Statins produced by the yeast Monascus purpureus are blood-cholesterol-lowering agents acting by competitively inhibiting the enzyme responsible for cholesterol synthesis (NCERT §8.2.3, p. 153).
• Sewage treatment is necessary because untreated municipal waste-water (sewage) — containing large amounts of organic matter and pathogenic microbes — cannot be discharged directly into natural water bodies. Treatment in sewage treatment plants (STPs) uses heterotrophic microbes naturally present in sewage and proceeds in two stages.
• Primary treatment physically removes particles by filtration and sedimentation — first sequential filtration removes floating debris, then sedimentation removes the grit (soil and small pebbles); all settled solids form primary sludge, while the supernatant is the effluent passed to secondary treatment (NCERT §8.3, pp. 153–154).
• Secondary (biological) treatment: The primary effluent enters large aeration tanks where it is constantly agitated mechanically and air is pumped in. This grows aerobic microbes into flocs — masses of bacteria associated with fungal filaments forming mesh-like structures. The microbes consume the major part of the organic matter, significantly reducing the BOD of the effluent. BOD (biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria; BOD measures the rate of uptake of oxygen by microorganisms in a sample and is an indirect measure of organic matter — the greater the BOD of waste water, the more its polluting potential. After significant BOD reduction the effluent is passed to a settling tank where the flocs sediment as activated sludge; a small part is recycled into the aeration tank as inoculum; the rest is pumped into large anaerobic sludge digesters where anaerobic bacteria digest the bacteria and fungi in the sludge producing a mixture of methane, hydrogen sulphide and carbon dioxide — these gases form biogas, used as an inflammable source of energy. The treated effluent is generally released into natural water bodies (NCERT §8.3, p. 154).
• Ganga Action Plan and Yamuna Action Plan were initiated by the Ministry of Environment and Forests to build sewage treatment plants so that only treated sewage is discharged into these rivers (NCERT §8.3, p. 155).
• Biogas is a mixture of gases (predominantly methane) produced by microbial activity and usable as fuel. The type of gas depends on the microbes and substrates. Certain bacteria that grow anaerobically on cellulosic material produce large amounts of methane along with CO₂ and H₂ — these are collectively called methanogens, and a common bacterium is Methanobacterium. They occur in anaerobic sludge during sewage treatment and also in the rumen of cattle (a part of stomach) — there they help digest cellulose and play an important role in cattle nutrition. Cattle dung (gobar) is rich in these bacteria, so dung can be used to generate biogas, commonly called gobar gas. A biogas plant consists of a concrete tank (10–15 ft deep) into which a slurry of dung is fed; a floating cover rises as gas is produced; the gas is supplied through a pipe to nearby houses, and the spent slurry is removed through another outlet for use as fertiliser. The biogas technology was developed in India mainly by IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission) (NCERT §8.4, pp. 155–156).
• Biocontrol agents: Biocontrol is the use of biological methods to control plant diseases and pests, in place of toxic insecticides, pesticides and weedicides. The organic farmer believes biodiversity furthers health and seeks to keep pests at manageable levels rather than eradicate them. Familiar examples — the Ladybird beetle controls aphids; Dragonflies control mosquitoes. Bacillus thuringiensis (Bt) is a microbial biocontrol agent against butterfly caterpillars: it is available as dried spores in sachets, mixed with water and sprayed onto plants (brassicas, fruit trees); the larvae ingest the spores and the toxin is released in the larval gut, killing the caterpillar while leaving other insects unharmed. B. thuringiensis toxin genes have been engineered into plants — Bt-cotton is one example. Trichoderma species are free-living root-ecosystem fungi that biocontrol several plant pathogens. Baculoviruses in the genus Nucleopolyhedrovirus are species-specific, narrow-spectrum insect pathogens with no negative impact on plants, mammals, birds, fish or non-target insects — ideal for Integrated Pest Management (IPM) programmes and ecologically sensitive areas (NCERT §8.5, pp. 156–157).
• Biofertilisers are organisms that enrich the nutrient quality of the soil. Main sources are bacteria, fungi and cyanobacteria. Rhizobium forms symbiotic root nodules on leguminous plants and fixes atmospheric nitrogen into organic forms used by the plant; other bacteria — Azospirillum and Azotobacter — fix atmospheric nitrogen while free-living in the soil, enriching soil nitrogen. Fungi form symbiotic associations with plants called mycorrhiza; many members of the genus Glomus form mycorrhiza — the fungal symbiont absorbs phosphorus from soil and passes it to the plant, and the plant also benefits from resistance to root pathogens, tolerance to salinity and drought, and overall growth. Cyanobacteria — Anabaena, Nostoc, Oscillatoria — are autotrophic microbes that fix atmospheric nitrogen and, in paddy fields, serve as important biofertilisers; blue-green algae also add organic matter and increase soil fertility (NCERT §8.6, pp. 157–158).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 8.1 (p. 150): Bacteria shapes — rod, spherical, rod-shaped with flagella.
• Figure 8.2 (p. 150): Viruses — bacteriophage (head, collar, tail plate, pins, prongs), adenovirus (respiratory infections), rod-shaped TMV.
• Figure 8.3 (p. 150): Bacterial and fungal colonies on petri dishes.
• Figures 8.4 and 8.5 (p. 152): Fermentors and fermentation plant.
• Figure 8.6 (p. 154): Secondary (biological) sewage treatment — aeration tank.
• Figure 8.7 (p. 155): Aerial view of a sewage treatment plant.
• Figure 8.8 (p. 156): Typical biogas plant — dung + water inlet, digester, floating gas-holder containing CH₄ + CO₂, sludge outlet.
• Process flow to memorise (Sewage): Filtration → grit sedimentation → primary sludge + effluent → aeration tank (flocs grow, BOD drops) → settling tank → activated sludge (part recycled) → anaerobic digester → biogas + treated effluent to river.

2.4 Common confusions / NTA trap points

• Penicillium notatum vs Penicillium chrysogenum / Saccharomyces — NCERT explicitly says Fleming named penicillin after Penicillium notatum; don't pick chrysogenum in a NCERT-grounded MCQ.
• Propionibacterium sharmanii (Swiss cheese) vs Roquefort (a fungus, unnamed in NCERT) — students often swap; only Swiss cheese has the named bacterium.
• Cyclosporin A from Trichoderma polysporum (a fungus, immunosuppressant) vs Trichoderma (free-living biocontrol fungus). Same genus, different roles — NCERT mentions both.
• Statins source = Monascus purpureus (a yeast) — not a bacterium, not Saccharomyces.
• Free-living N-fixers (Azospirillum, Azotobacter) vs symbiotic (Rhizobium) vs cyanobacterial N-fixers (Anabaena, Nostoc, Oscillatoria) — NTA loves these matching questions.
• Methanogens like Methanobacterium are anaerobic, found both in sludge digesters and the cattle rumen — not aerobic flocs.
• Bt toxin is released in the larval gut, not on plant surface — and Bt kills caterpillars (lepidopteran larvae), not all insects.
• Wine/beer = not distilled; whisky/brandy/rum = distilled.
• Aspergillus niger (citric acid) ≠ Acetobacter aceti (acetic acid) — same product family, different organisms.
• BOD direction — higher BOD = MORE polluting, not less. Distractors often invert this.
• Streptokinase = clot buster from Streptococcus; do not confuse with streptomycin (antibiotic).

2.5 Key microbes → product / process (NCERT-cited)