Microbes in Human Welfare | CBSE Class 12 Biology Notes

This note covers NCERT Class 12 Biology Chapter 8, Microbes in Human Welfare: microbes in household products, fermented beverages, antibiotics and other industrial products, sewage treatment, biogas, biocontrol agents and biofertilisers. Most questions from this chapter pair an organism with its product or use, so the organisms are gathered into tables that can be learnt as pairs.

What are microbes, and where are they found?

Besides macroscopic plants and animals, microbes are the major components of biological systems on this earth. They are present everywhere: in soil, water and air, and inside our bodies and those of other animals and plants. They are present even at sites where no other life-form could possibly exist.

• Deep inside geysers (thermal vents), where the temperature may be as high as 100°C.
• Deep in the soil.
• Under layers of snow several metres thick.
• In highly acidic environments.

Microbes are diverse: protozoa, bacteria, fungi, microscopic animal and plant viruses, viroids, and also prions, which are proteinaceous infectious agents.

Microbes like bacteria and many fungi can be grown on nutritive media to form colonies that can be seen with the naked eye. Such cultures are useful in studies on micro-organisms.

Figure 8.1 shows bacteria: (a) rod-shaped and (b) spherical bacteria, each magnified 1500X, and (c) a rod-shaped bacterium with long wavy flagella, magnified 50,000X. Figure 8.2 shows viruses magnified about 1,00,000 to 1,50,000X: (a) a bacteriophage, with its head, collar, tail, plate, pins and prongs labelled; (b) an adenovirus, which causes respiratory infections; and (c) the rod-shaped tobacco mosaic virus. Figure 8.3 shows (a) many small bacterial colonies in a petri dish and (b) a single round fungal colony in a petri dish.

See Figs. 8.1, 8.2 and 8.3 in your NCERT textbook

Microbes cause a large number of diseases in human beings, animals and plants. That does not mean all microbes are harmful. Several microbes are useful to man in diverse ways, and this chapter is about those uses.

How are microbes used in household products?

Curd

Micro-organisms such as Lactobacillus and others, commonly called lactic acid bacteria (LAB), grow in milk and convert it to curd.

1. A small amount of curd is added to fresh milk as the inoculum or starter. It contains millions of LAB.
2. At suitable temperatures the LAB multiply.
3. During growth, the LAB produce acids that coagulate and partially digest the milk proteins, converting the milk to curd.
4. The process also improves the nutritional quality of the milk by increasing vitamin B₁₂.

In our stomach too, the LAB play a very beneficial role in checking disease-causing microbes.

Dough, cheese and traditional foods

Cheese is one of the oldest food items in which microbes were used. Different varieties of cheese are known by their characteristic texture, flavour and taste, and the specificity comes from the microbes used.

Note: The same yeast, Saccharomyces cerevisiae, is used for three different purposes in this chapter. It is called baker's yeast when it ferments bread dough, brewer's yeast when it ferments beverages, and it is also used for the commercial production of ethanol.

How are fermented beverages made?

Production of microbial products on an industrial scale requires growing microbes in very large vessels called fermentors.

Microbes, especially yeasts, have been used from time immemorial for the production of beverages like wine, beer, whisky, brandy and rum. The yeast Saccharomyces cerevisiae, the same one used for bread-making and here commonly called brewer's yeast, is used to ferment malted cereals and fruit juices to produce ethanol.

Depending on the type of raw material used for fermentation and the type of processing, with or without distillation, different types of alcoholic drinks are obtained.

Figure 8.4 is a photograph of fermentors, the very large vessels in which microbes are grown on an industrial scale. Figure 8.5 is a photograph of a fermentation plant.

See Figs. 8.4 and 8.5 in your NCERT textbook

What are antibiotics, and how was penicillin discovered?

Definition: Antibiotics are chemical substances produced by some microbes that can kill or retard the growth of other (disease-causing) microbes.

"Anti" is a Greek word meaning "against", and "bio" means "life". Together they mean "against life", in the context of disease-causing organisms. With reference to human beings, antibiotics are "pro life". Antibiotics produced by microbes are regarded as one of the most significant discoveries of the twentieth century.

The discovery of penicillin

Penicillin was the first antibiotic to be discovered, and it was a chance discovery.

1. Alexander Fleming, while working on Staphylococci bacteria, observed a mould growing in one of his unwashed culture plates, around which Staphylococci could not grow.
2. He found that this was due to a chemical produced by the mould, and he named it penicillin after the mould Penicillium notatum.
3. Its full potential as an effective antibiotic was established much later by Ernest Chain and Howard Florey.
4. The antibiotic was extensively used to treat American soldiers wounded in World War II.
5. Fleming, Chain and Florey were awarded the Nobel Prize in 1945 for this discovery.

After penicillin, other antibiotics were purified from other microbes. Antibiotics have greatly improved our capacity to treat deadly diseases such as plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog), which used to kill millions all over the globe.

Which chemicals, enzymes and bioactive molecules come from microbes?

Microbes are used for the commercial and industrial production of certain chemicals, like organic acids, alcohols and enzymes, and of other bioactive molecules.

Note: Three pairs are easy to mix up. Cyclosporin A comes from the fungus Trichoderma polysporum, and Trichoderma also appears later as a biocontrol agent. Statins come from Monascus purpureus. Streptokinase comes from Streptococcus, which is a bacterium, not a fungus.

How is sewage treated?

Large quantities of waste water are generated every day in cities and towns. A major component of this waste water is human excreta. This municipal waste water is called sewage. It contains large amounts of organic matter and microbes, many of which are pathogenic. It cannot be discharged directly into natural water bodies like rivers and streams, so before disposal it is treated in sewage treatment plants (STPs) to make it less polluting.

The treatment is done by the heterotrophic microbes naturally present in the sewage, in two stages.

Primary treatment

Primary treatment basically involves the physical removal of particles, large and small, from the sewage through filtration and sedimentation.

1. Floating debris is removed by sequential filtration.
2. The grit (soil and small pebbles) is removed by sedimentation.
3. All solids that settle form the primary sludge, and the supernatant forms the effluent.
4. The effluent from the primary settling tank is taken for secondary treatment.

Secondary treatment, or biological treatment

1. The primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped into it.
2. This allows vigorous growth of useful aerobic microbes into flocs: masses of bacteria associated with fungal filaments to form mesh-like structures.
3. While growing, these microbes consume the major part of the organic matter in the effluent, which significantly reduces its BOD. The sewage water is treated till the BOD is reduced.
4. The effluent is then passed into a settling tank, where the bacterial flocs are allowed to sediment. This sediment is called activated sludge.
5. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.
6. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. There, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge, producing a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas.
7. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

Figure 8.6 is a photograph of a large aeration tank in which the water is churned and foaming, as the effluent is agitated and air is pumped in. Figure 8.7 is an aerial photograph of a sewage treatment plant.

See Figs. 8.6 and 8.7 in your NCERT textbook

This methodology has been practised for more than a century, in almost all parts of the world, and till date no man-made technology has been able to rival the microbial treatment of sewage. Because of increasing urbanisation, sewage is being produced in much larger quantities than ever before, but the number of sewage treatment plants has not increased enough. Untreated sewage is often discharged directly into rivers, leading to their pollution and an increase in water-borne diseases. The Ministry of Environment and Forests has initiated the Ganga Action Plan and the Yamuna Action Plan, under which a large number of sewage treatment plants are proposed so that only treated sewage is discharged into the rivers.

What is BOD, and what does it tell us?

Definition: BOD (biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.

The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water. Indirectly, therefore, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, the more is its polluting potential.

Worked example: labelling three water samples by BOD

1. Three samples, river water, untreated sewage and the secondary effluent from a sewage treatment plant, were labelled A, B and C without noting which was which. Their BOD values were A = 20 mg/L, B = 8 mg/L and C = 400 mg/L. Assume the river water is relatively clean.
2. BOD rises with the organic matter present, so the most polluted sample has the highest BOD. C, at 400 mg/L, is the most polluted: it is the untreated sewage.
3. Secondary treatment consumes the major part of the organic matter, so the secondary effluent has a much lower BOD than untreated sewage, but still more organic matter than clean river water. That is A, at 20 mg/L.
4. The relatively clean river water has the lowest BOD: B, at 8 mg/L.

How is biogas produced?

Definition: Biogas is a mixture of gases, containing predominantly methane, produced by microbial activity, which may be used as fuel.

The type of gas produced depends on the microbes and the organic substrates they use. In the fermentation of dough, cheese making and the production of beverages, the main gas produced is CO₂. However, certain bacteria that grow anaerobically on cellulosic material produce a large amount of methane along with CO₂ and H₂. These bacteria are collectively called methanogens, and one common example is Methanobacterium.

The biogas plant

1. The plant consists of a concrete tank, 10 to 15 feet deep, in which bio-wastes are collected and a slurry of dung is fed.
2. A floating cover is placed over the slurry. It keeps on rising as gas is produced in the tank by microbial activity.
3. An outlet connected to a pipe supplies the biogas to nearby houses, where it is used for cooking and lighting.
4. The spent slurry is removed through another outlet and may be used as fertiliser.

A deep tank, the digester, is sunk into the ground. A pit on the left, marked dung and water, feeds the slurry into the lower part of the digester through a sloping pipe. A gas-holder, the floating cover, sits in the mouth of the digester over the slurry and contains CH₄ + CO₂ and other gases. A pipe marked gas leads away from the top of the gas-holder. On the right, another pipe carries the spent slurry out of the digester into a pit marked sludge.

See Fig. 8.8 in your NCERT textbook

Cattle dung is available in large quantities in rural areas, so biogas plants are more often built in rural areas. The technology of biogas production was developed in India mainly due to the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

How are microbes used as biocontrol agents?

Definition: Biocontrol refers to the use of biological methods for controlling plant diseases and pests.

These problems have increasingly been tackled with chemicals: insecticides and pesticides. These chemicals are toxic and extremely harmful to human beings and animals alike, and have been polluting our soil, ground water, fruits, vegetables and crop plants. Soil is also polluted by the weedicides used to remove weeds.

The organic farmer's approach

• A key belief of the organic farmer is that biodiversity furthers health: the more variety a landscape has, the more sustainable it is.
• The insects sometimes called pests are not eradicated, but are kept at manageable levels by a complex system of checks and balances within a living ecosystem.
• Eradicating pests is seen as not only possible but also undesirable, because without them the beneficial predatory and parasitic insects that depend on them as food or hosts could not survive.
• An important part of the approach is to become familiar with the various life forms in the field, predators as well as pests, with their life cycles, patterns of feeding and preferred habitats.

How Bt is used

1. Bacillus thuringiensis is available in sachets as dried spores.
2. The spores are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees.
3. The insect larvae eat them, and in the gut of the larvae the toxin is released and the larvae get killed.
4. The bacterial disease kills the caterpillars but leaves other insects unharmed.

Through genetic engineering, scientists have introduced B. thuringiensis toxin genes into plants, which are then resistant to attack by insect pests. Bt-cotton is one such example, cultivated in some states of India.

Baculoviruses have been shown to have no negative impacts on plants, mammals, birds, fish or even non-target insects. This is especially desirable when beneficial insects are being conserved to aid an overall integrated pest management (IPM) programme, or when an ecologically sensitive area is being treated.

How do biofertilisers enrich the soil?

Definition: Biofertilisers are organisms that enrich the nutrient quality of the soil. Their main sources are bacteria, fungi and cyanobacteria.

The use of chemical fertilisers to meet the ever-increasing demand for agricultural produce has contributed significantly to environmental pollution, and there is large pressure to switch to organic farming and the use of biofertilisers.

Plants with mycorrhizal associations show other benefits too: resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development. Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments. A number of biofertilisers are now available commercially in India, and farmers use them regularly to replenish soil nutrients and reduce dependence on chemical fertilisers.

Short answers to the NCERT exercise

Glossary

• Prions — Proteinaceous infectious agents, one of the diverse kinds of microbes along with viruses and viroids.
• Lactic acid bacteria — Bacteria such as Lactobacillus that grow in milk, produce acids and convert it to curd.
• Inoculum — A small amount of a culture, such as curd added to fresh milk, that starts a fermentation.
• Fermentors — The very large vessels in which microbes are grown for production on an industrial scale.
• Antibiotics — Chemical substances produced by some microbes that kill or retard the growth of other, disease-causing microbes.
• Streptokinase — An enzyme from the bacterium Streptococcus, used as a clot buster in patients who have had a heart attack.
• Sewage — Municipal waste water, of which human excreta is a major component, containing organic matter and many pathogenic microbes.
• Flocs — Masses of bacteria associated with fungal filaments to form mesh-like structures during secondary treatment.
• Activated sludge — The sediment of bacterial flocs in the settling tank, part of which is used as inoculum.
• BOD — The amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
• Methanogens — Bacteria that grow anaerobically on cellulosic material and produce methane along with CO₂ and H₂.
• Biocontrol — The use of biological methods for controlling plant diseases and pests.
• Mycorrhiza — A symbiotic association between a fungus and a plant, in which the fungus passes phosphorus to the plant.
• Biofertilisers — Organisms, mainly bacteria, fungi and cyanobacteria, that enrich the nutrient quality of the soil.

Common errors and misconceptions

• Misconception: All microbes are harmful. Correct: Many microbes cause disease, but several are useful to man, in foods, industry, sewage treatment, biogas, biocontrol and biofertilisers.
• Misconception: The holes in Swiss cheese are made by a fungus. Correct: They are made by the large amount of CO₂ produced by the bacterium Propionibacterium sharmanii. A fungus ripens Roquefort cheese.
• Misconception: Whisky and beer are both produced by distillation. Correct: Wine and beer are produced without distillation. Whisky, brandy and rum are produced by distillation of the fermented broth.
• Misconception: Fleming established penicillin as an effective antibiotic. Correct: Fleming discovered it. Its full potential as an effective antibiotic was established later by Ernest Chain and Howard Florey.
• Misconception: Primary treatment of sewage uses microbes. Correct: Primary treatment is physical, by filtration and sedimentation. Microbes act in secondary (biological) treatment.
• Misconception: A low BOD means water is highly polluted. Correct: The greater the BOD, the more organic matter and the more polluting the water. Clean water has a low BOD.
• Misconception: Biogas is mostly carbon dioxide. Correct: Biogas contains predominantly methane, along with gases such as CO₂.
• Misconception: Bt kills all the insects on a sprayed plant. Correct: The bacterial disease kills the caterpillars that eat the spores but leaves other insects unharmed.

Exam-style questions with model answers

Q1. Name the microbes that produce cyclosporin A and statins, and state one use of each. [2 marks]

1. Cyclosporin A is produced by the fungus Trichoderma polysporum and is used as an immunosuppressive agent in organ-transplant patients.
2. Statins are produced by the yeast Monascus purpureus and are used as blood-cholesterol lowering agents.

Q2. What is BOD? What does a high BOD indicate? [2 marks]

1. BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
2. BOD is an indirect measure of the organic matter in water, so a high BOD indicates a large amount of organic matter and a greater polluting potential.

Q3. How do lactic acid bacteria convert milk into curd, and how do they benefit us? [3 marks]

1. A small amount of curd added to milk as inoculum contains millions of LAB, such as Lactobacillus, which multiply at suitable temperatures.
2. During growth they produce acids that coagulate and partially digest the milk proteins, converting the milk to curd.
3. Curd has improved nutritional quality because of an increase in vitamin B₁₂, and in our stomach LAB check disease-causing microbes.

Q4. Describe the discovery of penicillin. [3 marks]

1. Alexander Fleming, while working on Staphylococci bacteria, observed a mould growing in an unwashed culture plate, around which Staphylococci could not grow.
2. He found that this was due to a chemical produced by the mould, and named it penicillin after the mould Penicillium notatum.
3. Ernest Chain and Howard Florey later established its full potential as an effective antibiotic, and it was used to treat soldiers wounded in World War II. Fleming, Chain and Florey were awarded the Nobel Prize in 1945.

Q5. How do biofertilisers enrich the fertility of the soil? Give examples. [3 marks]

1. Nitrogen-fixing bacteria add nitrogen: Rhizobium in the root nodules of legumes, and free-living Azospirillum and Azotobacter in the soil.
2. Mycorrhizal fungi such as Glomus absorb phosphorus from the soil and pass it to the plant, which also gains resistance to root-borne pathogens and tolerance to salinity and drought.
3. Cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix atmospheric nitrogen and add organic matter to the soil; they are important in paddy fields.

Q6. How are microbes used in biocontrol? Give three examples. [3 marks]

1. Bacillus thuringiensis spores are sprayed on plants such as brassicas and fruit trees; caterpillars that eat them are killed by the toxin released in their gut, while other insects are unharmed.
2. The fungus Trichoderma, common in root ecosystems, is an effective biocontrol agent of several plant pathogens.
3. Baculoviruses of the genus Nucleopolyhedrovirus attack insects and other arthropods with narrow, species-specific action and no negative impact on plants, mammals, birds, fish or non-target insects, which suits integrated pest management.

Q7. Describe the treatment of sewage in a sewage treatment plant. [5 marks]

1. Primary treatment is physical. Floating debris is removed by sequential filtration, and grit is removed by sedimentation. The settled solids form the primary sludge, and the supernatant is the effluent.
2. In secondary (biological) treatment, the primary effluent is passed into aeration tanks, where it is agitated and air is pumped in, so that aerobic microbes grow into flocs.
3. The microbes consume the major part of the organic matter, which significantly reduces the BOD of the effluent.
4. The effluent then passes into a settling tank, where the flocs sediment as activated sludge. A small part is returned to the aeration tank as inoculum.
5. The rest of the sludge goes to anaerobic sludge digesters, where anaerobic bacteria digest it and produce biogas. The effluent from secondary treatment is released into rivers and streams.

Q8. What are methanogens? Explain how biogas is produced in a biogas plant. [5 marks]

1. Methanogens are bacteria, such as Methanobacterium, that grow anaerobically on cellulosic material and produce a large amount of methane along with CO₂ and H₂. They occur in the anaerobic sludge of sewage treatment and in the rumen of cattle, so cattle dung is rich in them.
2. A biogas plant has a concrete tank, 10 to 15 feet deep, in which bio-wastes are collected and a slurry of dung is fed.
3. A floating cover over the slurry keeps rising as gas is produced by microbial activity.
4. An outlet connected to a pipe supplies the biogas to nearby houses for cooking and lighting, and the spent slurry is removed through another outlet and may be used as fertiliser.
5. The technology was developed in India mainly through the efforts of IARI and KVIC, and the plants are built mostly in rural areas, where cattle dung is available in large quantities.

Key takeaways

• Microbes include protozoa, bacteria, fungi, viruses, viroids and prions, and are found even in thermal vents at up to 100°C and in highly acidic places.
• Lactic acid bacteria turn milk into curd and increase vitamin B₁₂; yeast ferments bread dough, and bacteria ferment idli and dosa dough.
• Saccharomyces cerevisiae is used for bread, for beverages and for ethanol; wine and beer are not distilled, but whisky, brandy and rum are.
• Penicillin was discovered by Fleming from Penicillium notatum, and Chain and Florey established it; all three shared the 1945 Nobel Prize.
• Learn the product-microbe pairs: citric acid and Aspergillus niger, cyclosporin A and Trichoderma polysporum, statins and Monascus purpureus, streptokinase and Streptococcus.
• Primary sewage treatment is physical; secondary treatment is biological, with flocs lowering the BOD and forming activated sludge.
• Methanogens such as Methanobacterium produce biogas, mostly methane, from cattle dung in plants developed by IARI and KVIC.
• Biocontrol uses ladybirds, dragonflies, Bt, Trichoderma and baculoviruses; biofertilisers include Rhizobium, Glomus and cyanobacteria.

Test yourself

What makes idli and dosa dough look puffed up?

The dough is fermented by bacteria, and the puffed-up appearance is due to the production of CO₂ gas.

Which bacterium makes the large holes in Swiss cheese?

The large holes in Swiss cheese are due to a large amount of CO₂ produced by the bacterium Propionibacterium sharmanii.

Which microbe produces citric acid?

Citric acid is produced by Aspergillus niger, which is a fungus.

Why are bottled fruit juices clearer than juices made at home?

Bottled juices are clarified by the use of the enzymes pectinases and proteases, which are produced by microbes.

What is activated sludge?

Activated sludge is the sediment of bacterial flocs formed in the settling tank after secondary treatment; part of it is reused as inoculum.

Which gases are produced in anaerobic sludge digesters?

Anaerobic bacteria in the digesters produce a mixture of methane, hydrogen sulphide and carbon dioxide, which forms biogas.

Which insects control aphids and mosquitoes?

The ladybird, a beetle with red and black markings, controls aphids, and dragonflies control mosquitoes.

Which fungus forms mycorrhiza, and what does it provide to the plant?

Many members of the genus Glomus form mycorrhiza; the fungus absorbs phosphorus from the soil and passes it to the plant.