Microbes, immunity and modern biotechnology

Microbes are protozoa, bacteria, fungi, and microscopic viruses, viroids and prions, and they exist almost everywhere, including at sites where no other life-form survives: deep inside thermal vents at temperatures near 100°C, under metres of snow, and in highly acidic environments. NCERT's Class 12 biology treats microbes, human immunity, and biotechnology as one connected story, because each explains the other. Microbes make the antibiotics and industrial chemicals below; the immune system decides how the body responds to both real infections and engineered medicine; and biotechnology's biggest applications, Bt cotton, engineered insulin, gene therapy, are microbial tricks that scientists identified first and then industrialised. UPSC's trap in this cluster is rarely the definition, it is the specific mechanism or the microbe-to-product pairing.

Microbes as an industrial workforce

Fermentation by Lactobacillus turns milk into curd, and yeast (Saccharomyces cerevisiae) leavens dough for bread, idli and dosa. Industrially, specific microbes are matched to specific products: citric acid from the fungus Aspergillus niger, acetic acid from Acetobacter aceti, butyric acid from Clostridium butylicum, and lipases and pectinases that clean detergent stains and clarify bottled fruit juice. Three genetically-tuned bioactive molecules are worth knowing by name: streptokinase (from Streptococcus, a clot-buster after a heart attack), cyclosporin A (from the fungus Trichoderma polysporum, an immunosuppressant for organ-transplant patients), and statins (from the yeast Monascus purpureus, cholesterol-lowering agents that inhibit the enzyme that makes cholesterol).

The best-known story is penicillin. Alexander Fleming noticed, by chance, that a mould growing in an unwashed culture plate stopped Staphylococcus from growing nearby, and named the chemical after the mould, Penicillium notatum. Its full therapeutic potential was established later by Ernest Chain and Howard Florey, and it treated wounded soldiers widely in the Second World War. All three shared the 1945 Nobel Prize.

Sewage, biogas and biocontrol

A sewage treatment plant works in two stages. Primary treatment physically removes debris and grit by filtration and sedimentation. Secondary (biological) treatment pumps the effluent into aeration tanks, where aerobic microbes form flocs and consume most of the organic matter, which lowers the BOD (biochemical oxygen demand), the oxygen that bacteria would consume oxidising the organic matter in one litre of water, and therefore a direct measure of pollution load. The settled sludge then goes to anaerobic digesters, where methanogens such as Methanobacterium (the same bacteria found in cattle rumen) break it down and release methane, hydrogen sulphide and carbon dioxide as biogas.

Microbes also replace chemical pesticides as biocontrol agents: Bacillus thuringiensis (Bt) spores kill caterpillar larvae that ingest them, Trichoderma fungi control root pathogens, and baculoviruses (genus Nucleopolyhedrovirus) are narrow-spectrum, insect-specific viral pesticides. As biofertilisers, Rhizobium fixes nitrogen in legume root nodules, Azospirillum and Azotobacter fix it free-living in soil, Glomus fungi form mycorrhiza that pass on phosphorus, and cyanobacteria such as Anabaena and Nostoc enrich paddy fields.

The immune system: innate and acquired

Innate immunity is non-specific and present from birth, built from four barriers: physical (skin, mucus), physiological (stomach acid, saliva, tears), cellular (phagocytic leukocytes, natural killer cells), and cytokine (interferons that protect uninfected cells from a virus). Acquired immunity is pathogen-specific and has memory: a first exposure produces a low-intensity primary response, and a later exposure to the same pathogen produces a stronger, faster secondary (anamnestic) response.

Two lymphocyte types carry this out. B-lymphocytes produce antibodies (H2L2: two light and two heavy peptide chains), the basis of the humoral response. T-lymphocytes do not secrete antibodies themselves but drive cell-mediated immunity (CMI), which is why the body distinguishes self from non-self and rejects a mismatched organ transplant. Active immunity (antibodies made by the host, as in vaccination) is slow but durable; passive immunity (ready-made antibodies given directly, as with snake-antivenom or a mother's colostrum) is immediate but temporary. Modern vaccines can exploit recombinant DNA directly: the hepatitis B vaccine is antigenic protein expressed in yeast, not the pathogen itself.

Genetic engineering: from bacteria to the field and the clinic

Genetic engineering runs on a small toolkit: restriction enzymes ("molecular scissors", the first isolated from the E. coli strain RY13, giving EcoRI its name) cut DNA at specific sites, and DNA ligase joins the cut piece into a plasmid vector for multiplication inside a host bacterium.

Bt cotton applies this directly: cry genes (cryIAc and cryIIAb control bollworm, cryIAb controls corn borer) are cloned from Bacillus thuringiensis into the plant. The toxin sits as an inactive protoxin until an insect ingests it; the insect's alkaline gut activates it, and the toxin punches pores in midgut cells, killing the insect while leaving other species unaffected.

Genetically engineered insulin solved a real problem: insulin extracted from cattle and pigs sometimes triggered allergic reactions. In 1983, Eli Lilly produced human insulin's two chains (A and B) separately in E. coli plasmids and joined them with disulphide bonds, matching the natural hormone exactly. Gene therapy followed the same logic for inherited defects: the first clinical gene therapy, in 1990, treated a child with adenosine deaminase (ADA) deficiency by growing her lymphocytes outside the body and delivering a functional ADA gene into them via a retroviral vector.

A related but distinct technique is molecular identification. DNA fingerprinting, developed by Alec Jeffreys, uses repetitive "satellite" DNA (VNTRs) that varies between individuals, and is used in forensics, paternity testing and biodiversity studies. DNA barcoding is a newer, different tool: a short, standardised gene region (commonly the mitochondrial COI gene in animals) that identifies which species a sample belongs to, even from a partial specimen, rather than telling one individual apart from another.

The exam angle

UPSC's favourite trap in this cluster is pairing confusion, so keep these distinctions sharp:

• Antibiotic vs antibody: an antibiotic is a chemical one microbe makes against another; an antibody is a protein the host's B-cells make.
• Innate vs acquired immunity: innate is non-specific, present at birth; acquired is pathogen-specific, and has memory.
• Humoral vs cell-mediated: B-cells and antibodies (humoral); T-cells drive cell-mediated immunity and graft rejection.
• Active vs passive immunity: active is slow but durable (vaccination); passive is instant but temporary (antitoxin, colostrum).
• DNA fingerprinting vs DNA barcoding: fingerprinting tells individuals apart; barcoding tells species apart.
• Microbe-to-product matching (Aspergillus niger to citric acid, Trichoderma to cyclosporin A, Monascus purpureus to statins) is a recurring statement-matching format; learn the pairs, not the category.

Quick revision points

• Microbes span protozoa, bacteria, fungi, viruses, viroids and prions, surviving even in extreme habitats (thermal vents, deep snow, acid).
• Penicillin: Fleming's chance discovery, named after Penicillium notatum, developed by Chain and Florey, 1945 Nobel to all three.
• Streptokinase (clot-buster), cyclosporin A (immunosuppressant) and statins (cholesterol-lowering) are genetically-derived microbial products.
• BOD measures pollution load; secondary sewage treatment lowers it via aerobic flocs before anaerobic digesters release biogas.
• Biocontrol: Bacillus thuringiensis, Trichoderma, baculoviruses. Biofertilisers: Rhizobium, Azospirillum/Azotobacter, Glomus mycorrhiza, cyanobacteria.
• Innate immunity (four barriers, non-specific) vs acquired immunity (B-cells/humoral, T-cells/cell-mediated, has memory).
• Bt cotton: cry genes activated by alkaline insect gut pH, punch pores in midgut cells.
• Engineered insulin (Eli Lilly, 1983, chains A and B); first gene therapy (1990, ADA deficiency).
• DNA fingerprinting identifies individuals (Alec Jeffreys, VNTR); DNA barcoding identifies species (commonly the COI gene).

Practise the linked questions below to see exactly how UPSC turns these mechanisms and pairings into statement-based traps.