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.

Penicillin's success set off a systematic hunt for other soil microbes capable of the same trick, and it found one in 1943: Selman Waksman's laboratory at Rutgers, screening soil actinomycetes (the Streptomyces group) one by one for antibiotic activity, identified streptomycin in a culture of Streptomyces griseus. Streptomycin became the first antibiotic effective against tuberculosis, at a time when the disease had no reliable cure, a genuine turning point distinct from penicillin's own story. Waksman alone received the 1952 Nobel Prize in Physiology or Medicine for the discovery, a credit later widely disputed because it was his graduate student, Albert Schatz, who actually isolated the organism and ran the decisive experiments; Schatz went on to win a legal settlement recognising him as co-discoverer. The two stories make a useful contrast: Fleming's penicillin discovery was pure accident, while Waksman's programme was a deliberate, systematic screen, and Streptomyces remains the single most productive genus for antibiotics overall, also yielding tetracycline and chloramphenicol.

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.

Vaccines: one principle, several platforms

Vaccination works by giving acquired immunity a rehearsal: an antigen is introduced deliberately so the primary response, and the long-lived memory B and T cells that come with it, are already in place before a real infection arrives, so the fast secondary response does the actual fighting. What differs between vaccines is exactly what gets introduced.

A live attenuated vaccine uses the real pathogen, weakened until it can no longer cause serious disease but can still replicate briefly and provoke a strong, broad response; the oral polio vaccine and the BCG vaccine against tuberculosis are the standard examples. An inactivated (killed) vaccine uses a pathogen destroyed by heat or a chemical so it cannot replicate at all, trading some strength for a wider safety margin; the injectable Salk polio vaccine is the classic case, and Bharat Biotech's Covaxin, built on a whole virus particle inactivated with beta-propiolactone, is India's own recent example. A subunit (or toxoid) vaccine skips the whole pathogen and uses only one specific antigenic piece of it, or a detoxified version of its toxin, which is safer still because nothing in the dose can cause disease; the hepatitis B vaccine above is exactly this category, and the tetanus and diphtheria components of routine childhood immunisation are toxoids.

Two newer platforms carry an antigen's genetic blueprint rather than the antigen itself, and India has fielded a real example of each. A viral vector vaccine engineers a harmless, unrelated virus to carry the target pathogen's gene into human cells, which then manufacture the antigen themselves; the Serum Institute of India's Covishield, built on a chimpanzee adenovirus carrying the SARS-CoV-2 spike gene, belongs here, not in the inactivated category it is sometimes wrongly filed under. An mRNA vaccine goes further still and delivers the genetic instructions directly, as a strand of messenger RNA in a lipid particle, so the body's own cells build the antigen and then break the mRNA down; Gennova Biopharmaceuticals' GEMCOVAC-19, cleared by India's drug regulator in 2022, was the country's first mRNA vaccine and only the world's third to reach approval, built on a self-amplifying platform stable at ordinary refrigerator temperatures.

Monoclonal antibodies: one B-cell's antibody, made at scale

A real infection provokes a polyclonal response: many different B-cell clones activate together, each recognising a different piece (epitope) of the pathogen, so the resulting antiserum is a mixture of many distinct antibodies. That mixture works, but it cannot be manufactured consistently, since no two batches of immune serum are identical. Monoclonal antibodies (MABs) solve that by isolating a single B-cell clone, so every antibody molecule produced is identical and binds exactly one epitope.

The production method is hybridoma technology, worked out by Georges Köhler and César Milstein in 1975 (shared, with Niels Jerne, in the 1984 Nobel Prize in Physiology or Medicine). An animal, typically a mouse, is immunised with the target antigen, and antibody-producing B-lymphocytes are then harvested from its spleen. A B-lymphocyte alone cannot be kept alive indefinitely in culture, so it is fused with an immortal myeloma (cancerous plasma) cell, producing a hybrid cell, the hybridoma, that inherits both properties needed: the B-cell's ability to make one specific antibody, and the myeloma cell's ability to divide without limit. Individual hybridomas are cloned out and screened, and the clone making the wanted antibody is grown at scale for a continuous, identical supply.

MABs now sit in both diagnostic and therapeutic use. A home pregnancy test kit uses a monoclonal antibody to detect human chorionic gonadotropin (hCG) specifically, which is why it gives a sharp positive-or-negative result rather than a variable one. Therapeutically, trastuzumab, marketed as Herceptin, is a monoclonal antibody engineered to bind HER2, a receptor protein overexpressed on certain aggressive breast cancer cells, blocking the growth signal that receptor would otherwise send; it is one of the most established examples of a monoclonal antibody used as a targeted cancer treatment rather than broad chemotherapy.

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.

Bt cotton is India's only commercially cultivated GM crop, but it is not the only real application of the same idea. Golden rice, developed by Ingo Potrykus and Peter Beyer from the early 1990s, carries genes (originally from daffodil, later improved with a maize gene) that let the rice grain's endosperm synthesise beta-carotene, the vitamin A precursor ordinary rice endosperm cannot make at all, aimed at vitamin A deficiency in populations where rice is the dietary staple. The Philippines became the first country to approve Golden Rice for commercial planting, in 2021, but a Philippine Court of Appeals revoked that biosafety permit in April 2024 on a citizen petition (a Writ of Kalikasan) citing unresolved scientific uncertainty, and the case remains before the Philippine Supreme Court, a reminder that a crop clearing the science does not automatically clear the law.

India's own live GM controversy is GM mustard (DMH-11), developed at Delhi University under Deepak Pental using a gene borrowed from a soil bacterium to make mustard, normally self-pollinating, hybridise the way maize and other high-yield hybrid crops already do. The GEAC (Genetic Engineering Appraisal Committee) cleared DMH-11 for environmental release in 2022, but the clearance was immediately challenged in the Supreme Court; a two-judge bench delivered a split verdict in 2024, one judge upholding the GEAC's clearance and the other criticising it for relying only on foreign studies, and both directing the government to frame a national GM policy. The matter now awaits a larger bench, an unsettled, ongoing status worth holding onto precisely because UPSC tends to test whether a candidate knows a controversy is still live rather than assuming it closed.

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.
• Vaccine platform matching: live attenuated (OPV, BCG) uses the weakened real pathogen; inactivated (Salk IPV, Covaxin) uses a killed one; subunit/toxoid (hepatitis B, DPT) uses only an antigen fragment; viral vector (Covishield) and mRNA (GEMCOVAC-19) deliver the antigen's genetic code instead. Covishield and Covaxin are routinely confused with each other; only Covaxin is inactivated, Covishield is viral vector.
• Monoclonal vs polyclonal antibody: a monoclonal antibody comes from one B-cell clone via hybridoma technology and binds a single epitope; a polyclonal response is a real infection's natural mixture from many clones.
• Antibiotic source organism: penicillin comes from a fungus (Penicillium notatum), streptomycin from a bacterium (Streptomyces griseus, an actinomycete); the two are not from the same kingdom, and UPSC has tested that pairing directly.
• 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).
• Streptomycin (Streptomyces griseus, Waksman's lab, 1943): first effective TB antibiotic; Waksman took the 1952 Nobel alone, though Albert Schatz did the actual isolation.
• Vaccine types: live attenuated (OPV, BCG), inactivated (Salk IPV, Covaxin), subunit/toxoid (hepatitis B, DPT), viral vector (Covishield), mRNA (GEMCOVAC-19, India's first, 2022).
• Monoclonal antibodies (Köhler and Milstein, 1975, hybridoma technology): one B-cell clone fused with an immortal myeloma cell, single-epitope antibody; used diagnostically (pregnancy test kits, detecting hCG) and therapeutically (trastuzumab, targeting HER2 in breast cancer).
• Bt cotton: cry genes activated by alkaline insect gut pH, punch pores in midgut cells.
• GM crops beyond Bt cotton: Golden rice (beta-carotene biofortification, Potrykus and Beyer; Philippines approved it in 2021, a court revoked the permit in 2024); GM mustard DMH-11 (Delhi University, GEAC cleared it in 2022, Supreme Court gave a split verdict in 2024, still unresolved).
• 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.