CBSE Class 12 Biology Revision Notes Chapter 8: Microbes in Human Welfare

Microbes support human welfare through food production, medicine, industry, waste treatment, energy generation and agriculture. The chapter explains how microscopic organisms and their products are used in everyday life.

Microbes occur in soil, water, air and inside living organisms. They also survive in extreme environments such as hot springs, deep soil, snow-covered regions and highly acidic habitats.

These CBSE Class 12 Biology Revision Notes Chapter 8 follow the current 2026–27 chapter sequence. They explain important microbes, products and biological processes through concise notes, tables and comparisons.

Key Takeaways

• Vitamin B12: Lactic acid bacteria improve the nutritional quality of curd by increasing this vitamin.
• 1945: Alexander Fleming, Ernest Chain and Howard Florey received the Nobel Prize for Penicillin.
• BOD: A higher biochemical oxygen demand indicates greater organic pollution in water.
• Methane: This is the main combustible gas present in biogas.

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Microbes in Human Welfare Class 12 Notes Overview

The Microbes in Human Welfare Class 12 Notes explain how microorganisms benefit households, industries, ecosystems and agriculture.

The chapter moves from familiar food products to large-scale microbial applications.

Access Class 12 Biology Chapter 8 Microbes in Human Welfare Notes in 30 Minutes

Use this order for rapid revision:

1. Learn the main groups of useful microbes.
2. Revise LAB, curd and fermented dough.
3. Recall the role of Saccharomyces cerevisiae.
4. Learn the discovery and use of Penicillin.
5. Match industrial products with their microbial sources.
6. Compare primary and secondary sewage treatment.
7. Revise BOD, flocs and activated sludge.
8. Learn how methanogens produce biogas.
9. Memorise microbial biocontrol examples.
10. Finish with bacterial, fungal and cyanobacterial biofertilisers.

Microorganisms and Their Uses in Human Welfare

Microbes are major components of biological systems.

They include:

• Bacteria
• Fungi
• Protozoans
• Microscopic plants and animals
• Viruses
• Viroids
• Prions

Some bacteria and fungi can be grown on nutrient media. They form visible colonies that help scientists study microorganisms.

Although several microbes cause disease, many have important uses in food, medicine, industry and environmental management.

Microbes in Household Products Class 12 Notes

The section on microbes in household products explains how fermentation changes milk, dough and other raw materials.

Different microbes produce specific flavours, textures, gases and nutrients.

Curd and Lactic Acid Bacteria

Microorganisms such as Lactobacillus are collectively called lactic acid bacteria, or LAB.

During curd formation, LAB:

• Grow in milk
• Convert milk into curd
• Produce acids
• Coagulate milk proteins
• Partially digest milk proteins
• Increase vitamin B12 content

A small quantity of curd acts as an inoculum or starter.

It contains millions of LAB. At a suitable temperature, they multiply and convert fresh milk into curd.

The lactic acid bacteria in the human stomach also help control disease-causing microorganisms.

Fermented Dough for Idli and Dosa

Bacteria ferment the dough used for idli and dosa.

The dough becomes puffed because microbes release carbon dioxide.

The gas becomes trapped in the dough and increases its volume.

Bread and Baker’s Yeast

Bread dough is fermented using baker’s yeast.

The scientific name of baker’s yeast is Saccharomyces cerevisiae.

Yeast ferments sugars and releases:

• Carbon dioxide
• Ethanol

Carbon dioxide makes the dough rise and gives bread a soft texture.

Traditional Fermented Foods and Drinks

Microbes are used to prepare several traditional foods and beverages.

Examples include:

• Toddy from fermented palm sap
• Fermented fish
• Fermented soybean
• Fermented bamboo shoots
• Idli
• Dosa
• Bread

Toddy is a traditional drink prepared in parts of southern India.

Microbes Used in Cheese Production

Cheese is one of the oldest foods produced with microorganisms.

Different microbes create characteristic taste, flavour and texture.

Microbes in Industrial Products Class 12 Biology

The microbes in industrial products section covers beverages, antibiotics, chemicals, enzymes and medically useful molecules.

Industrial production requires microorganisms to be grown under controlled conditions.

Fermenters

Large vessels used for the commercial growth of microbes are called fermenters.

Fermenters maintain suitable conditions such as:

• Temperature
• Nutrients
• pH
• Aeration
• Mixing

They allow microorganisms to produce substances on a large scale.

Fermented Beverages

Yeasts have been used to produce alcoholic beverages for thousands of years.

Brewer’s yeast, Saccharomyces cerevisiae, ferments malted cereals and fruit juices to produce ethanol.

The final beverage depends on:

• Raw material
• Fermentation process
• Presence or absence of distillation

Antibiotics and Penicillin Discovery

Antibiotics are substances produced by certain microbes.

They kill or slow the growth of disease-causing microorganisms.

The Penicillin discovery was a major development in medicine.

Alexander Fleming observed that Staphylococcus bacteria did not grow around a mould contaminating one of his culture plates.

The mould was Penicillium notatum.

Fleming named the antibacterial substance Penicillin.

Ernest Chain and Howard Florey later established its effectiveness as an antibiotic.

Penicillin was widely used to treat wounded soldiers during the Second World War.

Fleming, Chain and Florey received the Nobel Prize in 1945.

Antibiotics help treat bacterial diseases such as:

• Plague
• Whooping cough
• Diphtheria
• Leprosy

Organic Acids and Alcohol Produced by Microbes

Several industrial products from microbes are commercially important.

Microbial Enzymes

Microbes produce enzymes used in industries and households.

Lipases

Lipases are added to detergent formulations.

They break down fats and help remove oily stains from clothes.

Pectinases and Proteases

Pectinases and proteases clarify bottled fruit juices.

They remove suspended material, making the juice clearer.

Streptokinase

Streptokinase is produced by Streptococcus.

It is modified through genetic engineering and used as a clot buster.

It helps remove blood clots from the vessels of patients who have suffered myocardial infarction.

Bioactive Molecules from Microbes

Microorganisms also produce substances used in medical treatment.

Statins reduce cholesterol by competitively inhibiting an enzyme involved in cholesterol synthesis.

Microbes in Sewage Treatment

The section on microbes in sewage treatment explains how microorganisms reduce organic pollutants before wastewater enters rivers or streams.

Municipal wastewater containing human excreta is called sewage.

Sewage contains:

• Organic matter
• Pathogenic microbes
• Suspended particles
• Dissolved waste

Treatment occurs in two major stages.

Primary and Secondary Sewage Treatment

The difference between primary and secondary sewage treatment lies in the method used.

Primary Sewage Treatment

Primary treatment removes particles through filtration and sedimentation.

The main steps are:

1. Floating debris is removed by filtration.
2. Soil and small pebbles are removed by sedimentation.
3. Settled solids form primary sludge.
4. The liquid above the sludge forms primary effluent.
5. Primary effluent moves to secondary treatment.

Secondary or Biological Sewage Treatment

Primary effluent enters large aeration tanks.

Inside these tanks:

• Mechanical agitators mix the water.
• Air is pumped into the effluent.
• Aerobic microbes multiply rapidly.
• Microbes form flocs.
• Organic matter is consumed.
• BOD decreases.

The treated effluent then moves into a settling tank.

BOD in Sewage Treatment

Biochemical oxygen demand, or BOD, is the amount of oxygen required by microbes to oxidise the organic matter in one litre of water.

The BOD in sewage treatment indicates the level of organic pollution.

• High BOD means more organic matter.
• High BOD indicates greater pollution.
• Lower BOD means the sewage has been treated effectively.

The BOD test measures the rate at which microorganisms consume oxygen in a water sample.

Flocs

Flocs are mesh-like masses formed by:

• Aerobic bacteria
• Fungal filaments

These microbes consume organic matter in the aeration tank.

Activated Sludge

After secondary treatment, flocs settle at the bottom of the settling tank.

This sediment is called activated sludge.

A small portion of activated sludge is returned to the aeration tank as inoculum.

The remaining sludge enters anaerobic sludge digesters.

Anaerobic Sludge Digesters

Anaerobic bacteria digest the microbial biomass in activated sludge.

During digestion, they produce:

• Methane
• Carbon dioxide
• Hydrogen sulphide

These gases form biogas.

The treated effluent is generally released into natural water bodies.

Microbes in Biogas Production

The microbes in biogas production section explains how anaerobic bacteria convert cellulosic waste into a usable fuel.

Biogas is a mixture of gases produced by microbial activity.

It contains mainly:

• Methane
• Carbon dioxide
• Hydrogen

Methanogens

Methanogens are anaerobic bacteria that grow on cellulosic material.

They produce large quantities of methane.

An important example is Methanobacterium.

Methanogens occur in:

• Anaerobic sludge
• Cattle rumen
• Cattle dung

Inside the rumen, they help break down cellulose and support cattle nutrition.

Cattle dung is rich in methanogens and is used to produce gobar gas.

Structure and Working of a Biogas Plant

A biogas plant contains a concrete tank about 10 to 15 feet deep.

Its main components include:

• Digester tank
• Dung slurry
• Floating gas holder
• Gas outlet pipe
• Spent slurry outlet

The working process is:

1. Cattle dung and water form a slurry.
2. The slurry enters the digester.
3. Methanogens decompose organic matter.
4. Biogas collects under the floating cover.
5. The cover rises as gas accumulates.
6. Gas passes through a pipe to nearby houses.
7. Spent slurry leaves through another outlet.
8. The spent slurry is used as fertiliser.

Biogas is commonly used for:

• Cooking
• Lighting

IARI and KVIC contributed significantly to the development of biogas technology in India.

Microbes as Biocontrol Agents

Microbes as biocontrol agents reduce dependence on chemical pesticides.

Biocontrol uses natural organisms or biological processes to manage pests and plant diseases.

Chemical pesticides may:

• Harm humans
• Kill beneficial organisms
• Pollute soil
• Contaminate groundwater
• Remain on crops

Biological control maintains pests at manageable levels instead of removing every organism from the ecosystem.

Natural Predators

Some animals naturally control pests.

Bacillus thuringiensis

Bacillus thuringiensis, commonly called Bt, controls butterfly caterpillars.

Bt spores are:

1. Mixed with water.
2. Sprayed on vulnerable plants.
3. Consumed by insect larvae.
4. Activated inside the larval gut.
5. Released as a toxin.
6. Responsible for killing the larvae.

The bacterial toxin generally leaves other insects unharmed.

Bt toxin genes have also been introduced into plants through genetic engineering.

Bt cotton is an important example.

Trichoderma

Trichoderma species are free-living fungi commonly found in root ecosystems.

They act as biological control agents against several plant pathogens.

Baculoviruses

Baculoviruses infect insects and other arthropods.

Most baculoviruses used in biological control belong to the genus Nucleopolyhedrovirus.

They are useful because they:

• Have narrow-spectrum action
• Are species-specific
• Do not harm plants
• Do not harm mammals or birds
• Do not harm fish
• Usually spare beneficial insects

They are valuable in integrated pest management programmes.

Microbes as Biofertilisers

The section on microbes as biofertilisers explains how microorganisms naturally improve soil nutrient quality.

Biofertilisers reduce dependence on chemical fertilisers and support organic farming.

The main sources are:

• Bacteria
• Fungi
• Cyanobacteria

Rhizobium Biofertiliser and Other Nitrogen-Fixing Bacteria

The Rhizobium biofertiliser forms a symbiotic association with leguminous roots.

Rhizobium bacteria live in root nodules and convert atmospheric nitrogen into organic forms.

The plant uses these nitrogen compounds as nutrients.

Free-living nitrogen-fixing bacteria include:

• Azospirillum
• Azotobacter

These bacteria enrich soil nitrogen without living inside root nodules.

Mycorrhiza

Mycorrhiza is a symbiotic association between fungi and plant roots.

Many fungi belonging to the genus Glomus form mycorrhizae.

The fungus:

• Absorbs phosphorus from soil
• Transfers phosphorus to the plant
• Improves resistance to root pathogens
• Increases drought tolerance
• Increases salinity tolerance
• Supports plant growth

The fungus receives food from the plant.

Cyanobacteria Biofertiliser

A cyanobacteria biofertiliser fixes atmospheric nitrogen and adds organic matter to soil.

Examples include:

• Anabaena
• Nostoc
• Oscillatoria

Cyanobacteria are especially useful in paddy fields.

They:

• Improve soil nitrogen
• Add organic material
• Increase soil fertility
• Reduce chemical fertiliser use

Microbes in Human Welfare Quick Revision Tables

The following tables summarise the main concepts from the Class 12 Biology Chapter 8 Notes.

Household Products and Microbes

Industrial Products from Microbes

Sewage Treatment Terms

Biocontrol Agents

Biofertilisers

Useful Links for Class 12 Biology