Microbes in Human WelfareNEET Zoology · Class 12 · NCERT Chapter 8

Introduction

Microbes are everywhere. They live in soil, water, air, deep in the oceans and inside the bodies of animals and plants. The word "microbe" covers many different types of organisms: bacteria, fungi (yeasts and moulds), protozoa, algae, viruses, viroids and prions.

Most people think of microbes only as agents of disease. But the reality is different. Microbes produce your food, clean your water, protect your crops and even provide your medicines. This chapter is about the good side of microbiology.

Expect about 1 question from this chapter in NEET 2027. The most reliable scoring areas are: knowing the exact microbe-product pair (Lactobacillus and curd, Penicillium notatum and penicillin, Aspergillus niger and citric acid), understanding sewage BOD and flocs, and matching biocontrol agents and biofertilisers to their targets.

Microbes in Household Products

Long before people understood what microbes were, they used them to make food. Many traditional foods are made by microbial fermentation.

Curd, Dough and Cheese

• Curd: made by Lactobacillus (also called lactic acid bacteria or LAB). LAB convert lactose in milk to lactic acid. The acid lowers the pH and coagulates milk proteins to form curd. A small amount of old curd added to fresh milk provides the starter culture (inoculum). LAB also produce vitamin B12 and keep harmful microbes in check in the gut.
• Dough fermentation: when you make idli, dosa or bread, Saccharomyces cerevisiae (baker's yeast) ferments the sugars in the batter or dough. It produces CO2 and ethanol. The trapped CO2 makes the dough rise and gives bread its spongy texture. The ethanol evaporates during baking or steaming.
• Toddy: a traditional fermented drink made from the sap of palms. Natural yeasts and bacteria carry out the fermentation.
• Swiss cheese: ripened using Propionibacterium sharmanii. This bacterium produces large amounts of CO2 during ripening. The CO2 gets trapped inside the cheese block, forming the large characteristic holes.
• Roquefort cheese: ripened by growing the fungus Penicillium roqueforti on it. This gives the cheese its distinctive blue-green veins and tangy flavour. Note: this is a different organism from the antibiotic-producing Penicillium notatum.

Microbes in Industrial Products

Industry uses microbes on a large scale to make beverages, antibiotics, organic acids, enzymes and other useful molecules.

Fermented Beverages

Alcoholic beverages are produced by the fermentation of sugars by Saccharomyces cerevisiae (brewer's yeast or baker's yeast):

• Wine: made from fermented grape juice. Not distilled. Alcohol content about 8 to 15%.
• Beer: made from fermented malted barley (grain). Not distilled. Alcohol content about 4 to 8%.
• Whisky: fermented grain mash, then distilled to concentrate alcohol.
• Rum: fermented molasses (a byproduct of sugar refining), then distilled.
• Brandy: distilled from fermented fruit juice, most often grape.

The key distinction for NEET: wine and beer are not distilled, so they have lower alcohol content. Whisky, rum and brandy are distilled and have higher alcohol content.

Antibiotics

An antibiotic is a chemical produced by a microorganism that kills or stops the growth of other microorganisms. The most important antibiotic discovery in history is penicillin.

• Discovery of penicillin: Alexander Fleming discovered in 1928 that the mould Penicillium notatum was killing bacteria on his culture plates. The mould was secreting a substance that killed bacteria. Fleming named it penicillin.
• Development as medicine: Ernst Chain and Howard Florey developed penicillin into a usable medicine in the 1940s. All three shared the Nobel Prize in Physiology or Medicine in 1945.
• Penicillin is the first antibiotic used in medicine and one of the most important discoveries in the history of medicine.

Chemicals, Enzymes and Bioactive Molecules

Microbes produce many industrial chemicals and bioactive molecules:

• Organic acids:
  • Citric acid from Aspergillus niger (a mould). Used as a food preservative and flavouring in beverages and candy.
  • Acetic acid (vinegar) from Acetobacter aceti. Acetobacter oxidises ethanol to acetic acid.
  • Butyric acid from Clostridium butylicum (an anaerobic bacterium).
  • Lactic acid from Lactobacillus. Used in food, cosmetics and pharmaceuticals.
• Industrial enzymes: microbes produce enzymes used in food, detergent and textile industries.
  • Lipase: breaks down fats. Used in detergents and dairy.
  • Pectinase: breaks down pectin. Used in fruit juice clarification.
  • Protease: breaks down proteins. Used in detergents and meat tenderisers.
• Streptokinase (clot buster): produced by Streptococcus. Used in heart attack and stroke patients to dissolve blood clots. It activates plasminogen to plasmin, which breaks down the fibrin network in clots.
• Cyclosporin A (immunosuppressant): produced by the fungus Trichoderma polysporum. Used to prevent rejection of transplanted organs by suppressing the immune response. It is a cyclic peptide.
• Statins (blood cholesterol lowering): produced by the yeast Monascus purpureus. Statins inhibit HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis in the liver. They are among the most prescribed drugs in the world.

Microbes in Sewage Treatment

Sewage is the wastewater produced by homes, industries and commercial establishments. It contains large amounts of organic matter, pathogens and nutrients. Releasing untreated sewage into rivers or the sea would cause serious pollution.

BOD (Biological Oxygen Demand) is the amount of oxygen that aerobic bacteria need to break down the organic matter in a given volume of water. High BOD means a lot of organic pollutants. Clean water has low BOD. Sewage treatment aims to reduce BOD before releasing the effluent.

• Primary treatment (physical): large floating objects are removed by bar screens. Grit and sand settle out. In settling tanks, suspended organic solids sink to the bottom as primary sludge and scum floats to the top. BOD is reduced only partially (about 20 to 30%).
• Secondary treatment (biological): primary effluent is pumped into a large aeration tank. Air is bubbled in to keep it aerobic. Bacteria and fungi form mesh-like clumps called flocs. The microbes in the flocs use oxygen to break down organic matter, dramatically reducing the BOD (by about 90%). The effluent then passes to a settling tank where the flocs settle as activated sludge. Some activated sludge is pumped back into the aeration tank as an inoculum. The remaining excess sludge goes to anaerobic digesters.
• Anaerobic sludge digestion: the activated sludge is fed into anaerobic (airless) digesters. Methanogens (like Methanobacterium) break down the organic matter and produce biogas (mainly methane, CH4, and CO2). The biogas is collected and used as fuel. The remaining digested sludge can be used as manure.

Microbes in Production of Biogas

Biogas is a mixture of gases produced by the microbial breakdown of organic material in the absence of oxygen (anaerobic conditions). Its main component is methane (CH4), which is combustible and used as a fuel. Other gases include CO2, hydrogen sulphide and small amounts of water vapour.

• Methanogens: the microbes responsible for methane production. They are archaea. Key example: Methanobacterium. They use hydrogen and CO2 to make methane. They are strict anaerobes.
• Biogas plant (gobar gas plant): a tank where cattle dung (gobar) mixed with water is fed in. The dung slurry is digested anaerobically by methanogens. Biogas collects in a gas collector above the slurry. The spent slurry is removed from the other end and used as manure. Biogas is used for cooking and lighting in rural areas.
• Biogas technology is particularly important in rural India. It converts agricultural waste into clean fuel and manure.

Microbes as Biocontrol Agents

Biocontrol (biological control) means using living organisms to control pests and plant diseases instead of chemical pesticides. It is safer for the environment, leaves no toxic residue and often targets only the pest species.

• Bacillus thuringiensis (Bt): a soil bacterium that produces protein crystals (Bt toxin) toxic to lepidopteran larvae (caterpillars). Farmers spray dried spore preparations on crops. When caterpillars eat the leaves, the Bt toxin is activated in their alkaline gut and kills them. The toxin is harmless to plants, mammals and birds. Bt genes are also inserted into crop plants (transgenic Bt crops like Bt cotton) to make them pest-resistant.
• Trichoderma: a free-living fungus in soil that is parasitic on plant pathogenic fungi. Used to protect plant roots from soil-borne fungal diseases. Applied as a soil treatment.
• Baculoviruses (NPV): viruses that infect insects. They are species-specific and do not affect non-target organisms, plants or mammals. This makes them ideal for integrated pest management (IPM) because they leave no toxic residue.
• Predatory insects: ladybird beetles feed on aphids (greenfly, blackfly); dragonfly larvae in water feed on mosquito larvae; adult dragonflies catch adult mosquitoes.

Microbes as Biofertilisers

Biofertilisers are living microorganisms that improve plant nutrition by enriching the soil. They are a sustainable alternative to chemical fertilisers.

• Rhizobium (symbiotic nitrogen fixation): lives in root nodulesof leguminous plants (beans, peas, peanuts, soybean). It converts atmospheric N2 into ammonia using the enzyme nitrogenase. The plant supplies carbohydrates to the bacterium. This enriches the soil with fixed nitrogen.
• Mycorrhiza (Glomus): a symbiotic fungus associated with the roots of most vascular plants. Fungal hyphae extend far beyond the root surface, greatly increasing the absorptive area for phosphate and water. The plant supplies sugars to the fungus. Mycorrhizal plants also show greater resistance to root pathogens.
• Free-living cyanobacteria: Nostoc, Oscillatoria and Anabaena fix atmospheric nitrogen in soil and paddy fields. Anabaena lives symbiotically inside the leaves of the floating fern Azolla. Azolla-Anabaena is an important biofertiliser for rice cultivation in Asia.
• Azospirillum and Azotobacter: free-living nitrogen-fixing bacteria that colonise the roots of cereal crops (wheat, maize) and enrich the soil. Unlike Rhizobium, they do not form root nodules.

Worked NEET Problems

Summary Cheat Sheet

• Curd: Lactobacillus (LAB) converts lactose to lactic acid; coagulates milk proteins; also produces vitamin B12.
• Bread, idli, dosa: Saccharomyces cerevisiae ferments sugars, producing CO2 (makes it spongy) and ethanol.
• Swiss cheese holes: Propionibacterium sharmanii produces CO2 during ripening.
• Roquefort cheese: ripened by Penicillium roqueforti (NOT the antibiotic-producing species).
• Fermented beverages: wine and beer = not distilled (Saccharomyces cerevisiae); whisky/rum/brandy = distilled.
• Penicillin: from Penicillium notatum; discovered by Fleming (1928); developed by Chain and Florey; Nobel Prize 1945.
• Streptokinase (clot buster): from Streptococcus. Dissolves blood clots.
• Citric acid: Aspergillus niger. Acetic acid (vinegar): Acetobacter aceti. Butyric acid: Clostridium butylicum. Lactic acid: Lactobacillus.
• Cyclosporin A: from Trichoderma polysporum; immunosuppressant used in organ transplants.
• Statins: from Monascus purpureus; lower blood cholesterol by inhibiting HMG-CoA reductase.
• BOD: Biological Oxygen Demand; measure of organic pollutants in water. High BOD = more pollution.
• Sewage treatment: primary (physical, removes solids) then secondary (biological, flocs reduce BOD by 90%) then anaerobic digestion (methanogens produce biogas).
• Flocs: mesh-like clumps of bacteria and fungi in the aeration tank; key to BOD reduction.
• Activated sludge: settled flocs. Some is returned to aeration tank; rest goes to digesters.
• Biogas: mainly methane (CH4) + CO2. Produced by methanogens (Methanobacterium) in anaerobic conditions. Gobar gas plant uses cattle dung.
• Bacillus thuringiensis (Bt): kills caterpillars (lepidopteran larvae) using protein crystals. Harmless to non-target organisms.
• Trichoderma: free-living fungus that controls soil-borne plant pathogens. Different strain (T. polysporum) makes cyclosporin A.
• Baculovirus (NPV): insect-specific virus; species-specific; no toxic residue.
• Ladybird: controls aphids. Dragonfly: controls mosquitoes (larvae in water, adults in air).
• Rhizobium: symbiotic; in root nodules of legumes; fixes N2; needs legume host.
• Mycorrhiza (Glomus): symbiotic fungus; increases phosphate absorption; improves pathogen resistance.
• Cyanobacteria (Anabaena, Nostoc, Oscillatoria): free-living and symbiotic N-fixers. Azolla-Anabaena = rice field biofertiliser.
• Azospirillum and Azotobacter: free-living N-fixers for cereal crops; no root nodules.

Next: use the interactive learning widgets to explore the microbe-product explorer, walk through the three stages of sewage treatment with BOD changes and compare all biocontrol agents and biofertilisers, or work through the 14+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.