A complete, exam-focused guide covering microbes in food, industry, sewage treatment, biogas production, biocontrol agents, and biofertilisers, under Microbes in Human Welfare chapter, with NCERT-aligned explanations for Class 12 Biology.
Introduction to Microbes in Human Welfare
When most people hear the word “microbe,” they think of disease. But microorganisms are far more than pathogens- they are essential partners in human civilization, quietly working in our food, medicine cabinets, sewage plants, farms, and energy systems. This chapter explores the beneficial roles of microbes, moving beyond the disease-causing microorganisms studied in the previous chapter to reveal how bacteria, fungi, viruses, and cyanobacteria contribute to human welfare every single day.
Microbes are found virtually everywhere, in soil, water, and air, inside our bodies, and even in extreme environments such as thermal vents reaching 100°C, deep under snow, and highly acidic habitats. This remarkable diversity includes protozoa, bacteria, fungi, viruses, viroids, and prions (proteinaceous infectious particles). Many of these, particularly bacteria and fungi, can be cultured on nutritive media to form visible colonies, a technique fundamental to microbiological research.
1. Microbes in Household Products
Long before “biotechnology” became a buzzword, households across the world were already harnessing microbes for everyday food production.
Curd Production
- Lactic acid bacteria (LAB), such as Lactobacillus, grow in milk and convert it into curd.
- During fermentation, LAB produce acids that coagulate and partially digest milk proteins.
- A small amount of curd added to fresh milk (called the inoculum or starter) contains millions of LAB that multiply at suitable temperatures, converting the milk into curd.
- Beyond taste and texture, this process also increases vitamin B₁₂ content, improving the nutritional quality of the final product.
- LAB present in curd also play a beneficial role inside the human gut, helping check the growth of disease-causing microbes.
Fermented Doughs and Traditional Foods
| Food/Drink | Microbe Involved | Process |
| Dosa and idli batter | Bacteria | Fermentation producing CO₂ gas, giving the batter its puffed-up texture |
| Bread | Saccharomyces cerevisiae (baker’s yeast) | Dough fermentation |
| Toddy (South Indian drink) | Natural yeasts | Fermentation of palm sap |
| Fermented fish, soybean, bamboo shoots | Various microbes | Traditional preservation and flavor development |
Cheese
Cheese is one of the oldest microbially-produced foods, with different varieties owing their distinct flavor, texture, and taste to the specific microbes used:
- Swiss cheese gets its characteristic large holes from CO₂ gas produced by the bacterium Propionibacterium sharmanii.
- Roquefort cheese is ripened by growing a specific fungus on it, imparting its distinctive flavor.
2. Microbes in Industrial Products
Beyond the kitchen, microbes are cultivated on a massive industrial scale in large vessels called fermentors to produce beverages, antibiotics, chemicals, and enzymes.
2.1 Fermented Beverages
- The same yeast used in bread-making, Saccharomyces cerevisiae (also called brewer’s yeast), ferments malted cereals and fruit juices to produce ethanol.
- The type of alcoholic beverage depends on the raw material and processing method:
| Beverage | Distillation Required? |
| Wine, Beer | No (produced without distillation) |
| Whisky, Brandy, Rum | Yes (produced by distilling the fermented broth) |
2.2 Antibiotics
Antibiotics are chemical substances produced by microbes that kill or inhibit the growth of other, disease-causing microbes. The term itself comes from Greek: anti (against) + bio (life)- “against life,” referring specifically to disease-causing organisms, while being pro-life for humans.
The Discovery of Penicillin:
- Alexander Fleming made a chance discovery in 1928 while working with Staphylococci bacteria; he noticed a mould growing on an unwashed culture plate, around which the bacteria could not grow.
- He identified the responsible chemical and named it Penicillin, after the mould Penicillium notatum.
- The antibiotic’s full therapeutic potential was later established by Ernest Chain and Howard Florey.
- Penicillin was extensively used to treat wounded soldiers during World War II.
- Fleming, Chain, and Florey were jointly awarded the Nobel Prize in 1945 for this discovery.
Antibiotics have since transformed the treatment of once-deadly diseases such as plague, whooping cough (kali khansi), diphtheria (gal ghotu), and leprosy (kusht rog).
2.3 Chemicals, Enzymes, and Other Bioactive Molecules
Microbes are widely used for the commercial production of organic acids, alcohols, enzymes, and pharmaceutical compounds:
| Product | Microbe | Type |
| Citric acid | Aspergillus niger | Fungus |
| Acetic acid | Acetobacter aceti | Bacterium |
| Butyric acid | Clostridium butylicum | Bacterium |
| Lactic acid | Lactobacillus | Bacterium |
| Ethanol | Saccharomyces cerevisiae | Yeast |
Enzymes produced by microbes:
- Lipases– used in detergents to remove oily stains from laundry.
- Pectinases and proteases– used to clarify bottled fruit juices (which is why store-bought juice looks clearer than homemade juice).
- Streptokinase– produced by Streptococcus and modified through genetic engineering, used as a “clot buster” to dissolve blood clots in patients who have suffered a myocardial infarction (heart attack).
Other important bioactive molecules:
- Cyclosporin A– an immunosuppressive agent used in organ-transplant patients, produced by the fungus Trichoderma polysporum.
- Statins– cholesterol-lowering drugs produced by the yeast Monascus purpureus; they work by competitively inhibiting the enzyme responsible for cholesterol synthesis.
3. Microbes in Sewage Treatment
Every day, cities generate massive volumes of wastewater, largely composed of human excreta collectively called sewage. Since sewage contains high levels of organic matter and often pathogenic microbes, it cannot be discharged directly into rivers or streams. Instead, it undergoes treatment at Sewage Treatment Plants (STPs) using naturally occurring heterotrophic microbes.
Stage 1: Primary Treatment (Physical Removal)
This stage involves purely physical processes:
- Sequential filtration removes floating debris.
- Sedimentation removes grit (soil and small pebbles).
- Settled solids form primary sludge; the remaining liquid is called the effluent, which proceeds to secondary treatment.
Stage 2: Secondary (Biological) Treatment
- The primary effluent is passed into large aeration tanks, where it is mechanically agitated and pumped with air.
- This promotes the vigorous growth of aerobic microbes, which form flocs– masses of bacteria intertwined with fungal filaments in mesh-like structures.
- These microbes consume most of the organic matter in the effluent, significantly reducing the BOD (Biochemical Oxygen Demand).
What is BOD? BOD measures the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. It is an indirect measure of the organic pollution load in water- the higher the BOD, the more polluted the water.
- Once BOD drops sufficiently, the effluent moves to a settling tank, where bacterial flocs sediment out as activated sludge.
- A portion of the activated sludge is recycled back into the aeration tank as inoculum; the rest is pumped into anaerobic sludge digesters.
- In these digesters, anaerobic bacteria further digest the sludge, releasing a mixture of gases, methane, hydrogen sulphide, and carbon dioxide, collectively known as biogas, which can be used as a fuel source.
- The treated effluent from secondary treatment is finally released into natural water bodies like rivers.
Environmental Significance
Rising urbanisation has outpaced the construction of sewage treatment infrastructure, resulting in untreated sewage being discharged into rivers, a major cause of water pollution and waterborne diseases in India. In response, the Ministry of Environment and Forests launched the Ganga Action Plan and Yamuna Action Plan to build more sewage treatment plants and reduce pollution in these major rivers.
4. Microbes in Production of Biogas
Biogas is a mixture of gases, predominantly methane, produced through microbial activity on organic matter, and used as a renewable fuel source.
Methanogens
- Certain anaerobic bacteria that grow on cellulosic material produce large amounts of methane along with CO₂ and H₂. These bacteria are collectively known as methanogens.
- A common example is Methanobacterium.
- Methanogens are naturally found in:
- Anaerobic sludge during sewage treatment.
- The rumen (a chamber of the stomach) of cattle, where they help break down cellulose from plant-based feed, a task human digestive systems cannot perform.
- Because of this, cattle dung (gobar) is naturally rich in methanogens, making it an excellent raw material for biogas (commonly called gobar gas).
Structure and Function of a Biogas Plant
A typical biogas plant consists of:
- A concrete tank (10–15 feet deep) where a slurry of dung and bio-waste is fed.
- A floating gas-holder cover over the slurry that rises as gas accumulates inside.
- An outlet pipe that supplies biogas directly to nearby homes for cooking and lighting.
- A separate outlet for removing the spent slurry, which can be repurposed as an organic fertiliser.
Biogas technology is particularly suited to rural areas where cattle dung is abundantly available. In India, this technology was pioneered largely through the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).
5. Microbes as Biocontrol Agents
What Is Biocontrol?
Biocontrol refers to using biological methods rather than toxic chemical pesticides and insecticides, to manage plant diseases and pest populations. Conventional chemical approaches, while effective in the short term, pollute soil and groundwater and harm beneficial organisms alongside pests.
Organic farming relies on the principle that biodiversity promotes ecological stability: rather than eradicating pests entirely, the goal is to maintain them at manageable levels through natural checks and balances within a healthy ecosystem.
Natural Predators
- Ladybird beetles– control aphid populations.
- Dragonflies– control mosquito populations.
Microbial Biocontrol Agents
| Biocontrol Agent | Type | Target/Mechanism |
| Bacillus thuringiensis (Bt) | Bacterium | Sprayed as dried spores on crops; produces a toxin that kills caterpillar larvae after ingestion, without harming other insects |
| Trichoderma species | Fungus | Free-living root-associated fungi; act as biocontrol agents against several plant pathogens |
| Baculoviruses (genus Nucleopolyhedrovirus) | Virus | Species-specific, narrow-spectrum insecticidal agents with no known negative effects on plants, mammals, birds, fish, or non-target insects |
Bt and Genetic Engineering: Scientists have introduced Bacillus thuringiensis toxin genes directly into plant genomes using genetic engineering, creating pest-resistant crops. Bt-cotton is a well-known commercial example cultivated across several Indian states.
Because baculoviruses are extremely species-specific and environmentally benign, they are considered ideal for Integrated Pest Management (IPM) programmes, especially in ecologically sensitive areas.
6. Microbes as Biofertilisers
Why Biofertilisers?
Overuse of chemical fertilisers has significantly contributed to environmental pollution, pushing modern agriculture toward organic farming and the use of biofertilisers– living organisms that naturally enrich soil nutrient content.
Bacterial Biofertilisers
- Rhizobium forms a symbiotic association with the root nodules of leguminous plants, fixing atmospheric nitrogen into organic forms usable by the plant.
- Free-living, nitrogen-fixing bacteria such as Azospirillum and Azotobacter also enrich soil nitrogen content without requiring a host plant.
Fungal Biofertilisers: Mycorrhiza
- Many species of the fungal genus Glomus form a symbiotic association with plant roots called mycorrhiza.
- The fungal partner absorbs phosphorus from the soil and transfers it to the plant.
- Plants with mycorrhizal associations show additional benefits, including:
- Resistance to root-borne pathogens
- Increased tolerance to salinity and drought
- Overall improved growth and development
(In return, the fungus receives nutrients and shelter from the plant- a classic example of mutualism.)
Cyanobacteria as Biofertilisers
- Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial habitats, many of which can fix atmospheric nitrogen.
- Common examples include Anabaena, Nostoc, and Oscillatoria.
- In paddy fields, cyanobacteria serve as a particularly important biofertiliser, adding organic matter and boosting soil fertility.
Today, several commercial biofertiliser products are available in the Indian market, helping farmers replenish soil nutrients while reducing dependence on chemical fertilisers.
Quick Revision Summary
| Concept | Key Fact |
| Curd formation | Lactic acid bacteria (LAB), e.g., Lactobacillus |
| Bread fermentation | Saccharomyces cerevisiae (baker’s/brewer’s yeast) |
| Swiss cheese holes | CO₂ from Propionibacterium sharmanii |
| First antibiotic | Penicillin, discovered by Alexander Fleming (1928); Nobel Prize 1945 |
| Citric acid producer | Aspergillus niger |
| Clot-buster enzyme | Streptokinase (from Streptococcus) |
| Immunosuppressant | Cyclosporin A (from Trichoderma polysporum) |
| Cholesterol-lowering drug | Statins (from Monascus purpureus) |
| BOD | Biochemical Oxygen Demand- indicates organic pollution level in water |
| Sewage treatment stages | Primary (physical) → Secondary (biological, aerobic + anaerobic) |
| Methanogens | Anaerobic bacteria producing methane; e.g., Methanobacterium |
| Biogas source | Cattle dung (gobar) rich in methanogens from the rumen |
| Bt as biocontrol | Bacillus thuringiensis– used in Bt-cotton |
| Fungal biocontrol | Trichoderma |
| Viral biocontrol | Baculoviruses (Nucleopolyhedrovirus) |
| Nitrogen-fixing symbiont | Rhizobium (with legumes) |
| Free-living nitrogen fixers | Azospirillum, Azotobacter |
| Mycorrhiza | Symbiosis between Glomus fungi and plant roots; supplies phosphorus |
| Nitrogen-fixing cyanobacteria | Anabaena, Nostoc, Oscillatoria (used in paddy fields) |
Frequently Asked Questions (FAQs)
Q1. Which bacteria are responsible for converting milk into curd? Lactic acid bacteria (LAB), such as species of Lactobacillus, convert milk into curd by producing acids that coagulate milk proteins. This process also increases the curd’s vitamin B₁₂ content.
Q2. Who discovered Penicillin, and how? Alexander Fleming discovered Penicillin in 1928 by chance, after noticing that a mould growing on an unwashed culture plate prevented Staphylococci bacteria from growing nearby. Ernest Chain and Howard Florey later developed it into a usable therapeutic antibiotic.
Q3. What is BOD, and why does it matter in sewage treatment? BOD (Biochemical Oxygen Demand) measures the oxygen required to break down organic matter in a water sample. A higher BOD indicates more organic pollution and greater environmental harm if released untreated.
Q4. How is biogas produced from cattle dung? Cattle dung contains methanogenic bacteria from the animal’s rumen. When dung is fed into a biogas plant as slurry, these bacteria anaerobically digest the organic matter, releasing methane, CO₂, and hydrogen sulphide as biogas — a usable fuel for cooking and lighting.
Q5. What is Bt-cotton, and how does it work? Bt-cotton is a genetically modified cotton crop containing toxin genes from the bacterium Bacillus thuringiensis. The toxin is only activated in the gut of insect larvae that feed on the plant, killing pests while leaving other organisms unharmed.
Q6. What is mycorrhiza, and why is it beneficial to plants? Mycorrhiza is a symbiotic association between certain fungi (such as Glomus) and plant roots. The fungus absorbs phosphorus from the soil and supplies it to the plant, while also improving the plant’s resistance to pathogens and tolerance to drought and salinity.
Q7. Why are biofertilisers preferred over chemical fertilisers? Biofertilisers, such as Rhizobium, Azotobacter, and nitrogen-fixing cyanobacteria, naturally enrich soil nutrients without causing the environmental pollution associated with long-term chemical fertiliser overuse, making them central to sustainable and organic farming.
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