Microbes In Human Welfare — Complete NCERT Guide with Diagrams

Microbes Do More Work For Us Than We Realise

When we hear “microbes,” most students think of disease. That’s the NCERT Chapter 8 trap — your brain files bacteria under “pathogens” and forgets that the curd on your plate, the antibiotic in your medicine cabinet, and the biogas keeping rural kitchens running all exist because of microorganisms.

This chapter carries solid weightage in NEET — typically 2-3 questions per year — and board exams love asking one-mark definitions from here. The good news: once you understand the logic behind each application, the facts stick naturally. We don’t need to memorise; we need to understand what problem microbes are solving in each context.

The chapter breaks into four neat blocks: food processing, industrial products, sewage treatment, and biocontrol/biofertilisers. Work through each block as a separate story, and you’ll never confuse lactobacillus with rhizobium again.

Key Terms & Definitions

Fermentation — the metabolic process where microorganisms break down organic molecules (usually sugars) in the absence of oxygen, producing useful byproducts. The curd-making process is fermentation.

Antibiotics — chemical substances produced by microorganisms that kill or inhibit the growth of other microorganisms. Penicillin (from Penicillium notatum) is the textbook example.

Biocontrol — using living organisms (bacteria, fungi, insects) to control pests and diseases, replacing chemical pesticides.

Biofertilisers — organisms that enrich the nutrient quality of soil. They fix atmospheric nitrogen or solubilise minerals, reducing the need for synthetic fertilisers.

BOD (Biochemical Oxygen Demand) — the amount of oxygen required by bacteria to break down organic matter in a given volume of water. High BOD = highly polluted water. This is a frequently tested definition.

Flocs — masses of bacteria associated with fungal filaments that form during sewage treatment. They help remove organic matter from effluent.

Mycorrhiza — symbiotic association between fungi and plant roots. The fungi increase the absorptive surface area of roots.

Core Concepts

1. Microbes in Food Processing

The table below summarises the organisms and their products — this is the most direct source of NEET one-mark questions from this topic.

Product	Microorganism	Key point
Curd/Dahi	Lactobacillus	Lowers pH by producing lactic acid
Bread	Saccharomyces cerevisiae	CO₂ from fermentation makes bread porous
Toddy	Natural fermentation of sap	—
Wine/Beer	Saccharomyces cerevisiae	Does not distill; uses only fermentation
Whisky/Brandy	Saccharomyces cerevisiae	Fermentation + distillation
Cheese	Various bacteria/fungi	Propionibacterium makes Swiss cheese holes; Penicillium roquefortii gives Roquefort its flavour
Idli/Dosa	Bacteria + yeast mix	Fermentation produces CO₂ for puffiness

NEET 2022 asked about Lactobacillus specifically — know that it is added to milk and its role is to convert lactose to lactic acid, which lowers pH and causes milk proteins to coagulate.

2. Microbes in Industrial Products

Beverages and fermented foods use the same organisms as food processing. The distinction examiners test is fermented vs. distilled:

Wine and beer = fermentation only (low alcohol content)
Whisky, brandy, rum = fermentation + distillation

Antibiotics — this is where students often lose marks on one-line answer questions. Learn these pairs cold:

Penicillin → Penicillium notatum (discovered by Alexander Fleming, 1928)
Streptomycin → Streptomyces griseus
Chloramphenicol → Streptomyces venezuelae
Tetracycline → Streptomyces sp.

Fleming’s observation was accidental — he noticed the mould Penicillium contaminating a culture plate and killing surrounding bacteria. Ernest Chain and Howard Florey developed it into a usable drug. NEET has tested the “who discovered” question multiple times.

Chemicals, Enzymes & Other Bioactive Molecules:

Aspergillus niger — produces citric acid (used in soft drinks and confectionery)

Acetobacter aceti — produces acetic acid (vinegar)

Clostridium butylicum — produces butyric acid

Saccharomyces cerevisiae — produces ethanol

Streptokinase (from Streptococcus) is used as a “clot buster” — it removes clots from blood vessels of heart patients. This is a high-value NEET fact.

Cyclosporin A (from Trichoderma polysporum) is an immunosuppressant used in organ transplant patients.

Statins (from Monascus purpureus) lower blood cholesterol by competitive inhibition of enzymes that synthesise cholesterol.

3. Microbes in Sewage Treatment

Sewage treatment is the most systematically asked subtopic in board exams. Learn it as a two-stage process.

Primary Treatment (Physical)

Sewage is passed through filters to remove large floating matter
Sedimentation separates grit and sand
Produces primary sludge (settled solids) + clarified sewage

Secondary Treatment (Biological — the important part)

The clarified sewage is pumped into large aeration tanks. This is where microbes do the actual work:

Aerobic microbes (natural inhabitants of sewage) form flocs — masses of bacteria associated with fungal filaments
The flocs consume organic matter, significantly reducing BOD
The effluent is then passed into a settling tank
The bacterial mass (activated sludge) settles — a small portion is pumped back to the aeration tank as inoculum; the rest is pumped to large anaerobic sludge digesters

In the sludge digesters: Anaerobic bacteria digest the sludge and produce biogas (methane + CO₂ + H₂S). This biogas can be used as fuel.

After secondary treatment, the BOD is reduced significantly — the effluent can be released into rivers/streams.

CBSE board exams frequently ask: “What is BOD? What does a high BOD indicate?” — two separate marks. BOD is the oxygen needed by bacteria to decompose organic matter. High BOD = high organic pollution = less oxygen available for aquatic life.

4. Microbes as Biocontrol Agents

Chemical pesticides are effective but harmful to non-target organisms and cause soil/water pollution. Biocontrol offers a sustainable alternative.

Bacillus thuringiensis (Bt):

Produces proteins toxic to insect larvae (butterflies, mosquitoes, flies)
Bt spores available commercially; farmers mix with water and spray on crops
The toxin is activated in the alkaline insect gut and kills larvae
This is also the basis of Bt cotton and Bt brinjal (genetic engineering connection)

Trichoderma (a free-living fungus in soil):

Controls several plant pathogens
This is the same organism that produces cyclosporin A — examiners sometimes combine facts

Baculoviruses (Nuclear Polyhedrosis Viruses — NPV):

Species-specific — they attack only the target insect
No side effects on non-target organisms, plants, mammals, or birds
Excellent for integrated pest management programs

Predators as biocontrol:

Ladybird beetle — controls aphids
Dragonflies — control mosquitoes

5. Microbes as Biofertilisers

Chemical fertilisers are expensive and cause long-term soil damage. Biofertilisers are sustainable alternatives.

Bacteria:

Rhizobium — symbiotic nitrogen fixer, lives in root nodules of legumes
Azospirillum and Azotobacter — free-living nitrogen fixers in soil
Nostoc, Anabaena, Oscillatoria — cyanobacteria that fix atmospheric nitrogen (important in paddy fields)

Fungi (Mycorrhiza):

Association between fungal hyphae and plant roots
Increases phosphorus absorption (key function)
Increases tolerance to drought and salt stress
Examples: Glomus species

Azolla:

A small water fern
Contains the cyanobacterium Anabaena azollae in its leaves
Fixes atmospheric nitrogen
Used as biofertiliser in paddy fields — NEET has tested this directly

The difference between Rhizobium (symbiotic, legumes only) and Azotobacter (free-living, any soil) is a favourite one-mark board question. Know both. Rhizobium is more efficient because it lives in the anaerobic root nodule environment, which protects the nitrogenase enzyme from oxygen.

Solved Examples

Example 1 — CBSE Level

Q: What is the role of Lactobacillus in curd formation?

Solution: Lactobacillus is a lactic acid bacterium (LAB). When added to warm milk, it multiplies rapidly at body temperature (~37°C). It converts lactose (milk sugar) to lactic acid through fermentation. The lactic acid lowers the pH of milk, causing the milk proteins (casein) to coagulate and set the curd. LAB also improves the nutritional quality of milk by increasing Vitamin B₁₂.

Example 2 — NEET Level

Q: In a sewage treatment plant, the secondary treatment involves aerobic decomposition of organic matter. At which step does anaerobic digestion occur, and what is its significance?

Solution: Anaerobic digestion occurs in the sludge digesters — after the secondary treatment, the settled bacterial biomass (sludge) is transferred to large, closed anaerobic tanks. Here, anaerobic bacteria digest the organic matter in the sludge and produce biogas — a mixture of methane (CH₄), carbon dioxide (CO₂), and hydrogen sulphide (H₂S). The significance is twofold: (1) sludge volume is reduced and stabilised, making disposal safer, and (2) biogas can be used as a renewable fuel source (this is why municipal biogas plants often adjoin sewage treatment plants).

Example 3 — NEET Level (Discrimination Question)

Q: Match the correct pairs: (a) Cyclosporin A, (b) Streptokinase, (c) Statins. Organisms: (i) Streptococcus, (ii) Monascus purpureus, (iii) Trichoderma polysporum.

Solution: (a) – (iii): Cyclosporin A is produced by Trichoderma polysporum. It is an immunosuppressant.

(b) – (i): Streptokinase is produced by Streptococcus. It is used as a clot buster.

This type of matching question appeared in NEET 2019. The key trap is confusing Trichoderma — students know it as a biocontrol fungus but forget its industrial product.

Example 4 — Advanced Application (NEET)

Q: Why are Baculoviruses considered better biocontrol agents compared to chemical pesticides?

Solution: Baculoviruses (specifically Nuclear Polyhedrosis Viruses) are narrow-spectrum agents — they infect only specific target insects and have no effect on non-target organisms including plants, mammals, birds, or beneficial insects like bees. Chemical pesticides, by contrast, kill indiscriminately, contaminate soil and water, and accumulate in food chains through biomagnification. NPVs are therefore safe for integrated pest management programs and are part of sustainable agriculture practices.

Exam-Specific Tips

NEET Weightage: This chapter contributes 2-3 questions per year. High-yield areas: organism-product pairs (fermentation), sewage treatment steps, biofertilisers (Rhizobium vs. free-living fixers), and biocontrol agents. One question is typically a direct recall; one or two are application/discrimination.

For CBSE Board Exams:

3-mark questions on sewage treatment (primary + secondary) are consistent across years
Define BOD, explain activated sludge, and name at least two organisms involved
One-mark questions on antibiotics: always state the source organism, not just the antibiotic name
In 5-mark questions, draw a labelled diagram of primary + secondary treatment — it fetches full marks

For NEET:

Organism-product pairs are the #1 source of questions from this chapter — make a single table and revise it 3 times before the exam
Don’t skip the enzyme/bioactive molecule section — cyclosporin A, streptokinase, and statins are frequently tested
Azolla + Anabaena combination for paddy field nitrogen fixation is a high-yield specific fact

Common Mistakes to Avoid

Mistake 1: Confusing fermentation with distillation. Wine and beer involve only fermentation — they have lower alcohol content (5-15%). Whisky, brandy, and rum require distillation after fermentation to concentrate alcohol. NEET has tested this distinction.

Mistake 2: Saying Penicillium is a bacterium. Penicillium notatum is a fungus (mould). It produces the antibiotic penicillin, but it is not itself a bacterium. Similarly, Trichoderma is a fungus, not a bacterium.

Mistake 3: Writing “bacteria fix nitrogen in all plants.” Rhizobium fixes nitrogen only in legumes (pulses, peas, beans) via root nodules. Free-living fixers like Azotobacter and Azospirillum work in the soil associated with the roots of non-leguminous plants, but they do not form nodules.

Mistake 4: Mixing up BOD interpretation. High BOD means more organic pollution (more oxygen is consumed by microbes decomposing waste), which means less dissolved oxygen for aquatic organisms. Students sometimes write it the other way around. High BOD = bad water quality.

Mistake 5: Forgetting what Monascus purpureus is. It is a yeast — a fungus. Students often don’t know what category it falls under. It produces statins. In NEET matches and MCQs, “yeast” or “fungus” as the category is the distinction they test.

Practice Questions

Q1. Name the microorganism used in large holes (or eyes) in Swiss cheese.

Propionibacterium sharmanii (sometimes listed as Propionibacterium freudenreichii). It produces large amounts of CO₂ during fermentation, which creates the characteristic holes. It also imparts a slightly nutty flavour to Swiss cheese.

Q2. What is activated sludge? How is it used in the sewage treatment process?

Activated sludge is the mass of bacteria-rich sludge that settles at the bottom of the settling tank after secondary treatment. A small portion of this sludge is pumped back into the aeration tank as an inoculum to speed up decomposition of incoming organic matter. The remainder is transferred to anaerobic sludge digesters where it is further decomposed by anaerobic bacteria, producing biogas.

Q3. Cyclosporin A is used as an immunosuppressant. Name the organism that produces it and explain why such drugs are needed.

Cyclosporin A is produced by the fungus Trichoderma polysporum. Immunosuppressants are needed in organ transplantation — when a foreign organ is transplanted into a patient’s body, the patient’s immune system recognises the organ as “non-self” and mounts an attack (rejection). Cyclosporin A suppresses this immune response, preventing rejection and allowing the transplanted organ to function.

Q4. What is the difference between Rhizobium and Azotobacter as biofertilisers?

Rhizobium is a symbiotic nitrogen-fixing bacterium that lives in root nodules of leguminous plants (peas, beans, groundnut). It fixes nitrogen only in association with the host plant. Azotobacter is a free-living nitrogen-fixing bacterium found in soil. It fixes nitrogen independently, without requiring a host plant, and benefits a broader range of crops. Rhizobium is generally more efficient because the root nodule provides an anaerobic environment that protects the nitrogenase enzyme from oxygen inhibition.

Q5. A sample of river water shows a very high BOD. What does this indicate, and what are the likely consequences for the river ecosystem?

High BOD indicates high organic pollution — the water contains large amounts of biodegradable organic matter, likely from sewage or industrial discharge. Bacteria will consume large amounts of dissolved oxygen while decomposing this organic matter. This depletes dissolved oxygen levels in the water, leading to the death of fish and other aerobic aquatic organisms. The river effectively becomes anaerobic in affected stretches.

Q6. Why are Baculoviruses described as “narrow-spectrum” biocontrol agents? Is this an advantage or disadvantage?

Baculoviruses (NPVs) infect only specific arthropod species — they have no effect on plants, mammals, birds, or non-target insects. This narrow spectrum is an advantage in biocontrol because: (1) beneficial insects like bees and ladybird beetles are unharmed, (2) there is no risk of toxicity to humans or animals handling the product, and (3) there is no accumulation in food chains. The limitation is that you need a different viral preparation for each pest species, making it more expensive than broad-spectrum chemical pesticides.

Q7. Streptokinase is called a “clot buster.” Justify this name.

Streptokinase is an enzyme produced by Streptococcus. It acts as a plasminogen activator — it activates plasminogen (a blood protein) to form plasmin, which breaks down fibrin, the protein that forms blood clots. When injected into a patient with a blocked coronary artery (heart attack) or blocked blood vessels, streptokinase dissolves the fibrin clot, restoring blood flow. Hence the name “clot buster.”

Q8. What is the role of cyanobacteria in paddy fields? Name two cyanobacteria that are used as biofertilisers.

Cyanobacteria fix atmospheric nitrogen (N₂) into ammonia (NH₃), which is directly usable by plants. In paddy (rice) fields, the flooded, waterlogged conditions favour the growth of cyanobacteria. They enrich the soil with fixed nitrogen, reducing the need for chemical nitrogenous fertilisers. Two commonly used cyanobacteria: Nostoc and Anabaena (also Oscillatoria). Anabaena azollae lives in symbiosis with the fern Azolla, which is specifically used in paddy fields.

Frequently Asked Questions

What is the difference between a biofertiliser and a chemical fertiliser?

A chemical fertiliser (like urea or DAP) directly supplies nutrients in inorganic form. It works faster but causes soil degradation and water pollution over time. A biofertiliser is a living organism (bacteria, fungi, cyanobacteria) that enriches soil by fixing nitrogen, solubilising phosphates, or increasing root absorption. Biofertilisers are sustainable and improve long-term soil health, but their effect is slower and less dramatic.

Why is penicillin considered such an important discovery?

Before penicillin, bacterial infections were often fatal — pneumonia, wound infections, and tuberculosis killed millions. Penicillin (discovered 1928, developed into medicine by the 1940s) was the first effective antibiotic and transformed medicine. It works by inhibiting bacterial cell wall synthesis. Fleming, Chain, and Florey received the Nobel Prize in 1945. NEET expects you to know the discovery story and the source organism.

Is baker’s yeast and brewer’s yeast the same organism?

Both are Saccharomyces cerevisiae, but different strains optimised for different purposes. Baker’s yeast produces CO₂ (for leavening bread) while brewer’s yeast produces ethanol (for fermentation in beer and wine). In board exams, stating Saccharomyces cerevisiae for both is correct.

Why does bread rise when yeast is added?

Saccharomyces cerevisiae ferments the sugars in the dough under anaerobic conditions, producing CO₂ and ethanol. The CO₂ gets trapped in the gluten matrix of the dough, forming bubbles that make the dough rise and give bread its porous texture. The ethanol evaporates during baking.

What is the connection between Trichoderma and biocontrol?

Trichoderma species are free-living fungi found in soil. They are mycoparasites — they parasitise and destroy pathogenic fungi that attack plant roots. This makes them effective biocontrol agents against soil-borne fungal diseases. The same organism (T. polysporum) also produces cyclosporin A — a useful industrial enzyme. NEET occasionally tests both facts together.

How is biogas from sewage treatment plants used?

The biogas produced in sludge digesters (mainly methane + CO₂) is collected and can be used directly as cooking or heating fuel, or fed into gas-powered generators to produce electricity. Many sewage treatment plants are partially self-powered using this biogas — it offsets operational energy costs.

What is the significance of Azolla in sustainable agriculture?

Azolla is a small aquatic fern that harbours the cyanobacterium Anabaena azollae in its leaf cavities. This Anabaena fixes atmospheric nitrogen, which the fern absorbs and uses for growth. When Azolla is incorporated into paddy field water, it acts as a “green manure” — decaying Azolla releases fixed nitrogen into the soil, benefiting the rice crop. It is a particularly important biofertiliser in Asian rice cultivation.