Microbes in Human Welfare Class 12 Notes

These Microbes in Human Welfare Class 12 notes follow the 2026-27 session and cover NCERT Class 12 Biology Chapter 8 of the rationalised textbook (pages 149-160). The chapter shows the useful side of microbes: the side that makes curd, cheese and antibiotics, cleans sewage, produces biogas, and replaces chemical pesticides and fertilisers.

If you want the full textbook open beside these notes, you can download it from the official NCERT textbook portal.

You get the entire chapter in a form you can revise quickly: a one-screen map of the six sections, tables of every named microbe with textbook page references, a worked example on BOD ranking, common mistake corrections, and a two-minute revision summary. This page is part of the Class 12 Biology notes collection on this site.

Revise in three passes: read the at-a-glance table once, work through the section you are weakest in, then finish with the revision summary and memory devices the night before your test.

Chapter 8 at a glance: the six roles of microbes

Microbes are everywhere — in soil, water, air, inside our bodies, and in places where no other life-form survives: deep inside geysers where temperatures reach about 100°C, deep in the soil, under thick layers of snow, and in highly acidic environments (NCERT, p. 149). They include protozoa, bacteria, fungi, viruses, viroids and prions.

The previous chapter listed the diseases microbes cause (NCERT, p. 151) — revise that side in the Human Health and Disease notes. The message of Chapter 8 is the opposite of that list: not all microbes are harmful (NCERT, p. 151).

The chapter is organised into six sections. Keep this map in view while you revise; each row is a block you can finish in one sitting.

Two cross-links matter for answers: methanogens connect sewage treatment to biogas, and both biocontrol and biofertilisers reduce our dependence on agricultural chemicals.

Household and industrial microbes: products you already use

The first two blocks answer one question: which daily items are actually made by microbes? Household products are fermented at home or in small workshops; industrial products are made in factories. Making anything on an industrial scale needs fermentors — large vessels in which microbes are grown in bulk (NCERT, p. 151).

The star of the household block is lactic acid bacteria (LAB), such as Lactobacillus. When LAB grow in milk, they produce acids that coagulate and partially digest the milk proteins, converting milk to curd; a spoonful of curd added to fresh milk acts as the inoculum or starter (NCERT, p. 151).

Fermentation also improves the nutritional quality of the curd by increasing vitamin B12, and LAB play a beneficial role in the stomach by checking disease-causing microbes.

Complete microbe-to-product table

This table maps every microbe named in the household and industrial sections to its product, with the NCERT page for each (NCERT, pp. 151-153).

Cheese is one of the oldest foods made with microbes; each variety gets its texture, flavour and taste from the specific microbes used (NCERT, p. 151).

Industrial scale: Figures 8.4 and 8.5

Figure 8.4 shows fermentors — the large vessels where an industrial microbe is grown in bulk, for example to make antibiotics or organic acids. Figure 8.5 shows a fermentation plant, the factory-scale version of the same setup (NCERT, p. 152).

The same yeast used for bread — Saccharomyces cerevisiae — is called brewer’s yeast here, because it ferments malted cereals and fruit juices to produce ethanol (NCERT, p. 152). The drink depends on processing: wine and beer are produced without distillation, while whisky, brandy and rum are produced by distilling the fermented broth.

Antibiotics: the chance discovery of penicillin

An antibiotic is a chemical substance produced by some microbes that can kill or retard the growth of other, disease-causing microbes (NCERT, p. 152). Penicillin was the first antibiotic and it was a chance discovery: Alexander Fleming noticed a mould growing in an unwashed culture plate, and around the mould the Staphylococci bacteria could not grow.

The chemical was produced by the mould Penicillium notatum, so Fleming named it penicillin; Ernest Chain and Howard Florey later established its full potential as a medicine. It was used extensively to treat American soldiers wounded in World War II, and Fleming, Chain and Florey shared the Nobel Prize in 1945 (NCERT, p. 152).

Antibiotics transformed the treatment of plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog).

Sewage treatment, BOD and biogas: microbes as cleaners and fuel makers

Sewage is municipal waste water whose major component is human excreta; it contains large amounts of organic matter and many pathogenic microbes (NCERT, p. 153). It cannot be discharged into rivers or streams directly, so it is treated in sewage treatment plants (STPs).

The cleanup is done by the heterotrophic microbes naturally present in the sewage, in the order below (NCERT, p. 154).

1. Primary treatment (physical): floating debris is removed by sequential filtration, then grit (soil and small pebbles) is removed by sedimentation. The solids that settle form the primary sludge; the liquid above is the effluent that moves to the next stage.
2. Secondary or biological treatment: the primary effluent enters large aeration tanks where it is constantly agitated and air is pumped in. Aerobic microbes grow into flocs — mesh-like masses of bacteria associated with fungal filaments.
3. BOD reduction: while growing, the flocs consume most of the organic matter in the effluent, greatly reducing its BOD (biochemical oxygen demand).
4. Settling: the treated effluent passes into a settling tank where the flocs sediment as activated sludge. A small part is pumped back into the aeration tank as inoculum; the rest goes to anaerobic sludge digesters.
5. Anaerobic digestion: anaerobic bacteria digest the bacteria and fungi in the sludge, producing a gas mixture — methane, hydrogen sulphide and carbon dioxide — that forms biogas.

Figure 8.6 shows the aeration stage of this process — open tanks where air is pumped in so that flocs grow and BOD falls. The final effluent is released into natural water bodies like rivers and streams (NCERT, p. 154).

BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria (NCERT, p. 154). The BOD test measures the rate of oxygen uptake by micro-organisms in a water sample, so BOD indirectly measures the organic matter present. The greater the BOD, the greater the polluting potential.

When untreated sewage is discharged into rivers, the rivers get polluted and water-borne diseases increase. The Ministry of Environment and Forests launched the Ganga Action Plan and the Yamuna Action Plan to build more sewage treatment plants, so that only treated sewage is released into these rivers (NCERT, p. 155).

Biogas: methane from methanogens

Methanogens are bacteria that grow anaerobically on cellulosic material and produce large amounts of methane along with \( CO_2 \) and \( H_2 \); Methanobacterium is one common example (NCERT, p. 155). They are found in the anaerobic sludge of sewage plants and in the rumen (a part of the stomach) of cattle, where they help break down cellulose and support cattle nutrition.

Cattle dung — gobar — is rich in these bacteria, so it can be fermented to produce gobar gas.

A typical biogas plant is a concrete tank (10-15 feet deep) into which bio-wastes and a slurry of dung are fed. A floating cover rises as the gas collects, an outlet pipe supplies the biogas to nearby houses, and spent slurry is removed through another outlet to be used as fertiliser (NCERT, pp. 155-156).

The technology was developed in India mainly by the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC). Figure 8.8 shows the complete setup.

Biocontrol agents and biofertilisers: microbes instead of chemicals

Two ideas share one goal: cutting our dependence on agricultural chemicals. Biocontrol uses biological methods to control plant diseases and pests (NCERT, p. 156) instead of toxic insecticides and pesticides. Biofertilisers are organisms that enrich the nutrient quality of the soil (NCERT, p. 158) instead of chemical fertilisers.

The context is organic farming, where pests are kept at manageable levels by the ecosystem’s checks and balances rather than eradicated (NCERT, p. 156).

Biocontrol agents

Bt has taken the story further: genetic engineers have introduced the Bt toxin gene into plants, making them resistant to insect pests. Bt-cotton, cultivated in some states of India, is one such crop (NCERT, p. 157). You will study how these toxin genes are put into plants in the Biotechnology: Principles and Processes notes.

Biofertilisers

Definitions table: terms you should be able to reproduce

These ten terms cover nearly every direct question this chapter can ask. Learn the meaning first, then the example; an answer that opens with a definition and closes with an example is a complete answer.

The chapter’s formula-like ideas: BOD logic and gas patterns

This chapter has no formula boxes, but two relationships keep appearing and exam questions treat them like formulas. Learn them as rules, not as isolated facts.

The BOD rule: greater BOD means more organic matter, so greater polluting potential. BOD is measured by the rate of oxygen uptake by micro-organisms in a water sample (NCERT, p. 154). Whenever a question gives you BOD values, your only job is to rank them.

The gas patterns: the gas produced depends on the microbe and the organic substrate it uses (NCERT, p. 155). One pattern runs through the whole chapter — fermentation of a particular substrate gives a predictable gas.

Worked example: ranking water samples by BOD

Rule used: the greater the BOD of waste water, the greater its polluting potential (NCERT, p. 154).

NCERT Exercise 11 (p. 159) applies the same rule to BOD values of 20 mg/L, 8 mg/L and 400 mg/L — rank them with the same three steps.

Common mistakes in this chapter (and the corrections)

These six errors appear repeatedly in student answers. Learn the corrected version and the reason, so you do not repeat them.

Exam notes: writing answers that start with the definition

• Open with the definition. For any term-based question, write the meaning first, then one NCERT example. For BOD ranking questions, state the rule (‘the greater the BOD, the greater the polluting potential’), then list the samples in order and finish each value with its unit (mg/L).
• Write scientific names correctly. Genus and species both matter: Saccharomyces cerevisiae, Propionibacterium sharmanii, Bacillus thuringiensis. In print, italicise the name; when writing by hand, underline it. Bt is short for Bacillus thuringiensis, so expand it the first time you use it.
• Exercise 6 caution: the chapter text names only one antibiotic-producing fungus — Penicillium notatum. The second species this exercise asks for is not named in the chapter, so take it from your teacher’s notes or class material instead of inventing one.
• Exercise 14 logic: when asked to arrange items in decreasing order of importance for human welfare, justify each position by the scale of its benefit — a life-saving medicine like penicillin outweighs a daily convenience like curd. Give a reason with every item, not just the final order.
• Answer ‘where have you seen this?’ questions with applications: use the real-life application map in the revision summary below — curd, Swiss cheese holes, clear bottled juice, the Ganga Action Plan, gobar gas and Bt-cotton.

Reading the figures: what each diagram shows

Diagram-based questions in this chapter usually ask you to identify a stage or name a part. For each NCERT figure, remember one identifying feature.

Figure 8.4 (Fermentors) and Figure 8.5 (Fermentation Plant)

Shown earlier in this page. Figure 8.4 is the fermentor — the large vessel in which microbes are grown for industrial production. Figure 8.5 is the whole fermentation plant, the large-scale version of the same idea (NCERT, p. 152). If a diagram shows large vessels for growing microbes, the answer is fermentors.

Figure 8.6 (Secondary treatment)

Shown earlier in the sewage section. This is the aeration stage. What to point out: the open tanks into which air is pumped and in which the effluent is agitated; this is where aerobic microbes grow into flocs and BOD falls (NCERT, p. 154).

Figure 8.7 (An aerial view of a sewage plant)

This figure is read from above, and the point is scale — the aerial view shows how large a sewage treatment plant is (NCERT, p. 155). If a question shows the whole plant from the air, the expected observation is its large size.

Figure 8.8 (A typical biogas plant)

Shown earlier in the biogas section. To read it, trace four parts: the concrete tank; the floating cover that rises as gas collects; the outlet pipe that carries biogas to nearby houses; and the second outlet from which spent slurry is removed for use as fertiliser (NCERT, pp. 155-156).

One-glance revision summary: the six sections and their key microbes

If you have two minutes before a test, read this table and the memory devices below it.

Memory devices

The six sections in order spell H-I-S-B-B-B. Remember the sentence ‘How I Study Biology Beats Boredom’ — Household products, Industrial products, Sewage treatment, Biogas, Biocontrol, Biofertilisers.

The four organic-acid producers follow A-C-C-L. Aspergillus (citric acid), Acetobacter (acetic acid), Clostridium (butyric acid), Lactobacillus (lactic acid) (NCERT, p. 153).

Real-life application map

Connect each section to something you have actually seen. This is also how ‘give an example’ answers are built.

The frame statement of the whole chapter (NCERT, pp. 151, 158): not all microbes are harmful — many are essential to human welfare. If a question ever asks ‘are all microbes harmful?’, this is your opening line.

When you finish this chapter, revise other chapters from the Class 12 notes hub, or scan the full CBSE notes library for last-minute revision in other subjects.

FAQs on microbes in human welfare

Why does higher BOD mean more polluted water?

BOD is the oxygen that bacteria would consume to oxidise all the organic matter in one litre of water. More organic matter means more oxygen demand, so a higher BOD signals heavier organic pollution — hence greater polluting potential (NCERT, p. 154).

What is the difference between primary and secondary sewage treatment?

Primary treatment is physical: sequential filtration removes floating debris and sedimentation removes grit, leaving primary sludge and an effluent. Secondary treatment is biological: the effluent is aerated in tanks so aerobic microbes form flocs that consume most of the organic matter and reduce BOD (NCERT, p. 154).

Which microbes are used as biofertilisers in Class 12?

Rhizobium fixes nitrogen in the root nodules of legumes; Azospirillum and Azotobacter fix nitrogen while free-living in the soil; the mycorrhizal fungus Glomus transfers phosphorus to plants; and cyanobacteria such as Anabaena, Nostoc and Oscillatoria are important biofertilisers in paddy fields (NCERT, p. 158).

How does Bacillus thuringiensis kill caterpillars without harming other insects?

Dried spores are sprayed on plants; when a caterpillar larva eats them, a toxin is released in its gut and kills the larva. Because the toxin acts only inside a larva that feeds on the treated plant, other insects remain unharmed (NCERT, p. 157).

Which microbes are used to make cyclosporin A and statins?

Cyclosporin A — an immunosuppressive drug for organ-transplant patients — comes from the fungus Trichoderma polysporum. Statins, which lower blood cholesterol by competitively inhibiting the enzyme for cholesterol synthesis, come from the yeast Monascus purpureus (NCERT, p. 153).

Reference: NCERT Class 12 Biology textbook, chapter 8.