Microbes in Human Welfare — NEET Notes

The big picture — four arenas where microbes work

Not every microbe is a pathogen. Of the protozoa, bacteria, fungi, viruses, viroids, and prions that occupy every habitat on Earth — from boiling thermal vents to acidic mine drainage to the human gut — only a small minority cause disease. The rest run the planet's nutrient cycles, decompose organic matter, fix nitrogen, and, when we put them to work, fill our kitchens and our pharmacies. NCERT organises this chapter around four arenas in which microbes serve human welfare: the kitchen (household products), the factory (industrial products), the city (sewage and biogas), and the field (biocontrol and biofertilisers).

The names that NEET examines from this chapter are limited in number but very specific. Memorising the source organism for each product is the single highest-yield activity. Begin with this map of the territory:

Microbes in household products

Walk through any kitchen in India and the work of microbes is everywhere — in the curd-pot, the dosa batter on the counter, the bread rising in a tin, the bottle of toddy at the village shop. Most of these transformations follow the same logic: a microbe converts a sugar into something useful (acid, gas, or alcohol) while improving the nutritional or sensory quality of the food.

Curd from milk — Lactobacillus and the LAB

Add a spoonful of curd to warm milk, leave it on the counter, and within a few hours the milk has thickened into curd. The agent is Lactobacillus and its allies — collectively the lactic acid bacteria (LAB). They ferment lactose into lactic acid, which lowers the pH, coagulates the milk proteins (chiefly casein), and partially digests them. The starter culture you add contains millions of LAB; at room temperature they multiply and convert the entire pot. The fermentation also raises the vitamin B₁₂ content of the milk — a fact NEET tested verbatim in 2018. In the human stomach, the same LAB suppress disease-causing microbes, which is why curd is a household remedy for stomach upsets.

Dosa, idli, and the gas-puffed dough

Dosa and idli batters — a slurry of rice and black gram — are fermented overnight by bacteria, predominantly Leuconostoc and Streptococcus species that together act as lactic acid bacteria. The puffed appearance of the dough comes from the CO₂ they release as a by-product. The same gas is what gives bread its soft crumb, but the agent there is different: bread is fermented by baker's yeast (Saccharomyces cerevisiae), a fungus, not a bacterium. NEET asks the bread-yeast pairing almost every year.

Toddy and traditional fermented foods

'Toddy' — a traditional drink of southern India — is made by fermenting the sap from palms. The sap is sweet and rich in sugars; wild yeasts convert it into a mildly alcoholic beverage within hours of tapping. Microbes are also used to ferment fish, soybean, and bamboo shoots into traditional foods. Cheese is one of the oldest microbial products — the large holes in Swiss cheese come from CO₂ released by Propionibacterium sharmanii, while Roquefort cheese is ripened by a specific fungus that gives it its flavour.

Microbes in industrial products — fermented beverages

Industry uses the same biochemistry as your kitchen, only on a different scale. Microbes are grown in huge stainless-steel vessels called fermentors — temperature-controlled, aerated, monitored continuously — where they convert cheap raw materials (molasses, malted cereal, fruit juice) into beverages, antibiotics, organic acids, and enzymes. The same yeast that leavens bread is the workhorse of the alcohol industry.

For all fermented beverages, the organism is Saccharomyces cerevisiae — now called brewer's yeast — and the chemistry is identical: glucose is converted anaerobically to ethanol and CO₂. What differs across products is the raw material and whether the fermented broth is distilled afterwards.

The NCERT line to remember is exact: "Wine and beer are produced without distillation, whereas whisky, brandy and rum are produced by distillation of the fermented broth." NEET has asked this distinction in multiple years.

Antibiotics — the twentieth century's greatest gift

Antibiotics — literally "against life" — are chemicals produced by some microbes that kill or retard the growth of other (disease-causing) microbes. Their discovery transformed medicine. Plague, whooping cough, diphtheria, leprosy — diseases that used to kill millions — became treatable within a generation. NCERT goes so far as to say: "Today, we cannot imagine a world without antibiotics."

Penicillin — a chance discovery

The first antibiotic was found by accident. In 1928, Alexander Fleming, working with Staphylococci bacteria, noticed a mould growing on one of his unwashed culture plates. Around the mould, no bacterial colonies could form. Fleming traced the killing chemical to the mould — Penicillium notatum — and named it penicillin. Its full therapeutic potential, however, took another decade and two more scientists: Ernst Chain and Howard Florey purified the drug and proved its clinical efficacy. By the time American soldiers were wounded in World War II, penicillin was saving lives in field hospitals. Fleming, Chain, and Florey shared the 1945 Nobel Prize in Physiology or Medicine.

"It is not we who discovered penicillin — the mould did. We only happened to be in the way."

Louis Pasteur's heirs — microbes as the original chemists

Pasteur's role here is the meta-discovery: the entire idea that micro-organisms cause fermentation and disease — and can be put to work — comes from his nineteenth-century experiments. Without Pasteur, there is no Fleming.

Streptomycin and the post-penicillin antibiotics

After penicillin, other antibiotics were purified from other microbes. Streptomycin — the first effective drug against tuberculosis — was isolated from the soil bacterium Streptomyces griseus by Selman Waksman in 1943. A whole genus of antibiotics-producing actinomycetes followed; today, soil Streptomyces species are still mined for new antibacterials.

Chemicals, enzymes & bioactive molecules

Microbes are also the cheapest factories for industrial chemicals — organic acids, alcohols, enzymes, and bioactive molecules that pharmacology depends on. The NCERT list is short, finite, and asked verbatim. Memorise the four organic-acid pairings; you will see them on the screen on exam day.

Ethanol and industrial enzymes

For commercial ethanol production, the workhorse is again Saccharomyces cerevisiae. Industrial enzymes from microbes are everywhere: lipases are added to detergents to break down oily stains; pectinases and proteases are used to clarify bottled fruit juices (which is why store-bought juice is clearer than freshly made juice).

Streptokinase — the clot buster

Streptokinase, produced by the bacterium Streptococcus and modified by genetic engineering, is used as a clinical clot buster. It dissolves blood clots from the vessels of patients who have suffered a myocardial infarction. Administered intravenously after a heart attack, it can restore blood flow within hours — a routine cardiology drug today.

Cyclosporin A and statins

Two more bioactive molecules complete the canonical NCERT list, and both are NEET favourites because the source organisms have unfamiliar names:

Microbes in sewage treatment

Every city generates millions of litres of waste water a day — toilet flushes, kitchen sinks, industrial effluents. This sewage is loaded with organic matter and pathogenic microbes. Discharging it into rivers without treatment is what turns waterways into open drains and triggers cholera, typhoid, and dysentery epidemics. The answer is to let microbes themselves clean the water — a method developed more than a century ago and still unmatched by any human technology.

Sewage treatment runs in two stages. The first is mechanical; the second is biological. The difference between them is one of the most frequently asked NEET questions from this chapter.

The secondary tank is where the real chemistry happens. Heterotrophic aerobic microbes — bacteria associated with fungal filaments — form mesh-like structures called flocs. As they consume the organic matter in the effluent, the biochemical oxygen demand (BOD) of the water drops. BOD is defined as the oxygen that bacteria would consume if all the organic matter in one litre of water were oxidised; high BOD means heavy organic pollution. The greater the BOD, the more polluting the water.

India's Ganga Action Plan and Yamuna Action Plan, initiated by the Ministry of Environment and Forests, propose building sewage treatment plants along these rivers so that only treated effluent enters them. The bottleneck is not the science — it is the number of operational STPs relative to the volume of sewage generated by Indian cities.

Microbes in biogas production

The anaerobic sludge digester at a sewage plant produces a flammable gas mixture. That same chemistry — anaerobic microbial digestion of cellulosic material — happens in the rumen of cattle, and can be staged deliberately in a concrete pit called a biogas plant. The product is a mixture of methane, carbon dioxide, hydrogen, and hydrogen sulphide, with methane as the dominant component.

Methanogens are archaea. They live anaerobically wherever cellulose is being broken down — in marshes, in rice paddy soil, in the anaerobic sludge digester, and most importantly for biogas, in the rumen of cattle. The rumen — a specialised compartment of the cattle stomach — is full of these bacteria; they help the cow digest cellulose, which the cow itself cannot. Cattle dung (gobar) is therefore loaded with active methanogens.

A biogas plant is a concrete tank, 10–15 feet deep. A slurry of cattle dung is fed into it. As microbial activity produces gas, a floating cover rises with the gas column. An outlet pipe carries the biogas (the "gobar gas") to nearby homes for cooking and lighting. The spent slurry is drained out as a second outlet and used as fertiliser. The technology was developed in India by the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC) — a fact NCERT explicitly credits.

Microbes as biocontrol agents

Modern industrial agriculture leans heavily on chemical insecticides, weedicides, and fungicides. These chemicals are toxic — they poison soil, contaminate groundwater, and accumulate in fruit, vegetables, and crop plants. They kill useful organisms (pollinators, predators, decomposers) along with the pests, hollowing out the agricultural ecosystem. Biocontrol is the alternative: using natural predators, parasites, and pathogenic microbes to control pests, with the goal of keeping pest populations at manageable levels rather than eradicating them.

NCERT names three microbial biocontrol agents that NEET tests as a cluster:

The Bt example deserves a closer look because it is the bridge to genetic engineering. The dried Bt spores are mixed with water and sprayed on vulnerable crops like brassicas and fruit trees. When a caterpillar eats the leaves, it ingests the spores. In the larva's alkaline midgut, the bacterium releases a crystalline (Cry) protein toxin that perforates the gut cells and kills the larva. Crucially, the toxin is specific to lepidopteran larvae — other insects, including pollinators and predators, are unharmed. The next leap was genetic: scientists cloned the Bt toxin genes into crop plants themselves. Bt cotton, now grown across several Indian states, expresses the Cry protein in its own tissues — caterpillars die when they bite the leaf.

The other two examples follow the same logic of biological specificity. Trichoderma outcompetes pathogenic fungi in the rhizosphere. Nucleopolyhedrovirus (a genus of baculoviruses) infects insects so selectively that an ecologically sensitive area can be sprayed without harming any non-target organism. This is what makes biocontrol valuable in Integrated Pest Management (IPM) programmes.

Microbes as biofertilisers

The chemical fertiliser route — urea, DAP, NPK — has fed the green revolution but at huge environmental cost: groundwater contamination, eutrophication of lakes, soil-microbe death, and runaway costs for the farmer. Biofertilisers are living organisms that enrich soil nutrient quality. The three groups NCERT names are bacteria, fungi, and cyanobacteria — and they work primarily by fixing atmospheric nitrogen or mobilising soil phosphorus.

Rhizobium — the legume symbiont

Rhizobium is the classical example. The bacterium colonises the roots of leguminous plants (peas, beans, soybean, chickpea, groundnut, clover) and induces the formation of root nodules. Inside these nodules, Rhizobium fixes atmospheric N₂ into ammonia using the enzyme nitrogenase. The plant supplies carbon skeletons and an anaerobic environment (the protein leghaemoglobin shields the nitrogenase from O₂); in return, the plant gets a continuous supply of fixed nitrogen — no urea needed. This is why crop rotation with legumes restores soil fertility.

Free-living nitrogen fixers

Not all nitrogen fixers need a plant partner. Azospirillum and Azotobacter live free in the soil and fix atmospheric nitrogen, enriching the soil for crops that follow. They are commercially marketed as biofertilisers and applied as seed coatings or soil amendments.

Mycorrhiza — the fungus-root partnership

Mycorrhiza is a symbiotic association between fungi and plant roots. Many members of the genus Glomus form mycorrhizal associations. The fungus extends a vast hyphal network through the soil and absorbs phosphorus, passing it to the plant; the plant supplies sugars to the fungus. Mycorrhizal plants also show resistance to root pathogens, tolerance to salinity and drought, and overall vigorous growth. This is one of the most ancient symbioses in land-plant evolution.

Cyanobacteria — the paddy-field biofertilisers

Finally, the cyanobacteria — Anabaena, Nostoc, Oscillatoria — are autotrophic microbes that fix atmospheric nitrogen in addition to performing photosynthesis. In waterlogged rice fields, these blue-green algae multiply on the water surface and serve as a major nitrogen source for the rice crop. They also add organic matter to the soil. For paddy farmers, BGA is the cheapest fertiliser available.

The Rhizobium–legume system also connects to the broader nitrogen cycle tested in ecology — atmospheric N₂ becomes ammonia via Rhizobium (nitrogen fixation), then nitrite via Nitrococcus (nitrification step 1), then nitrate via Nitrobacter (nitrification step 2), and finally back to N₂ via Thiobacillus (denitrification). NEET 2021 tested this entire cycle as a matching question.