Microbes in Human Welfare | ISC Class 12 Biology Notes

This note covers useful microbes, household food processing, fermented beverages, penicillin, industrial acids and enzymes, sewage treatment, biogas production, biological pest control, integrated pest management and biofertilisers.

What makes microbes useful in human welfare?

Microbes and their products

Microbes, or micro-organisms, include microscopic forms such as bacteria, fungi and protozoa. They occur in soil, water, air and within plants and animals. Although some cause disease, many contribute to food production, treatment of waste, energy generation and agriculture.

A useful distinction is between using a microbe itself and using a substance it produces. Growing bacteria convert milk into curd. In contrast, an antibiotic is a substance produced by some microbes that can kill or retard the growth of disease-causing microbes. Both are applications of microbial activity.

Metabolism means the chemical activities occurring within an organism. Different microbes produce different products during growth and metabolism. Their products also depend on the materials they use. Gas production in dough and production of methane, a combustible gas, involve different microbial activities. Methane predominates in biogas, a gas mixture produced by microbes that may be used as fuel.

How can microscopic organisms be studied?

Bacteria and many fungi can grow on nutritive media, materials supplying nutrients for growth. They form visible colonies, or masses of microbial growth. Such cultures, meaning microbes grown on a suitable medium, help in the study of micro-organisms.

The illustration shows many pale bacterial colonies in one petri dish and a green fungal colony in another. The fungal colony is labelled. The visible colony represents microbial growth, rather than a single enlarged microbial cell.

See Fig. 8.3 in your NCERT textbook

The main applications can be organised by their outcome: food is transformed, useful chemicals are produced, organic waste, material derived from living organisms, is broken down; pests are controlled; or soil nutrients are enriched. Identifying both the organism and its particular role prevents confusion between these applications.

How do microbes produce curd, bread and cheese?

Curd formation

Lactic acid bacteria (LAB) are bacteria that produce lactic acid; Lactobacillus is an example. They grow in milk and convert it into curd. The acids they produce coagulate, or cause the setting of, milk proteins and partially digest these proteins.

An inoculum is a small quantity containing microbes that starts a fresh culture. A little curd used as a starter contains millions of LAB. At suitable temperatures these bacteria multiply in fresh milk, bringing about curd formation.

1. Add a small quantity of curd containing LAB to fresh milk as the inoculum.
2. At a suitable temperature, the bacteria multiply in the milk.
3. During growth, the bacteria produce acids that coagulate and partially digest milk proteins.
4. The milk becomes curd, whose nutritional quality improves through an increase in vitamin B₁₂.

LAB also help check disease-causing microbes in the stomach. This role is separate from their action on milk proteins. Curd formation therefore illustrates both a food-processing use and a benefit associated with these bacteria in the body.

Fermented dough and cheese

Fermentation is microbial conversion of substances such as sugars into products such as acids, alcohol or gases. Bacteria ferment the dough used for idli and dosa. The dough becomes puffed up because of carbon dioxide, written CO₂, a gas released during this activity.

Bread dough is fermented using baker's yeast, Saccharomyces cerevisiae, a fungus. Different microbes also give cheeses their particular texture, flavour and taste. Thus, the identity of the microbe matters as well as the food material on which it grows.

Microbes are also used to ferment fish, soyabean and bamboo shoots to make foods. These examples show that microbial food processing includes more than milk and cereal dough. Different starting materials and microbial activities produce different foods.

How are fermented beverages produced on an industrial scale?

Fermentors and brewer's yeast

A fermentor is a very large vessel used to grow microbes for industrial production. Industrial processes use microbes to make products such as alcoholic beverages and antibiotics. The scale is much larger than the microbial growth involved in preparing household foods.

Saccharomyces cerevisiae, used in bread-making, is also called brewer's yeast when used in beverage production. It ferments malted cereals and fruit juices to produce ethanol, the alcohol present in these beverages. Malted cereals are grains that have been allowed to germinate as part of processing.

The final type of alcoholic drink depends on the raw material and on subsequent processing. The important processing distinction is whether the fermented liquid undergoes distillation, separation using vaporisation followed by condensation. The fermented liquid is also called the fermented broth.

Which beverages are distilled?

Fermentation and distillation describe different operations. Yeast produces ethanol during fermentation. Distillation is a later processing step used for whisky, brandy and rum. Calling a beverage undistilled therefore does not mean that it was produced without microbial fermentation.

These are photographs. Figure 8.4 shows a row of large metal vessels supported above the floor. Figure 8.5 shows equipment and pipework inside a fermentation plant. They illustrate industrial production equipment.

See Figs. 8.4 and 8.5 in your NCERT textbook

The same yeast species can therefore serve different purposes. In bread-making it is associated with fermented dough; in beverage production it supplies ethanol from suitable raw materials. A complete explanation connects the organism, its starting material, its product and any further processing.

What are antibiotics, and how was penicillin discovered?

Meaning and action of antibiotics

Definition: Antibiotics are chemical substances produced by some microbes that can kill or retard the growth of other disease-causing microbes. Retarding growth means slowing it down. An antibiotic is the substance produced, rather than the producing organism itself.

Penicillin was the first antibiotic discovered, and its discovery occurred by chance. Alexander Fleming was studying Staphylococci, a group of bacteria, when he noticed a mould, a fungus, growing in an unwashed culture plate. Staphylococci did not grow around the mould.

From observation to effective use

1. Fleming observed that Staphylococci could not grow around a mould in a culture plate.
2. He established that a chemical produced by the mould caused this effect.
3. He named the chemical penicillin after the mould Penicillium notatum.
4. Ernest Chain and Howard Florey later established its full potential as an effective antibiotic.

Penicillin was extensively used to treat American soldiers wounded in the Second World War. Fleming, Chain and Florey received the Nobel Prize in 1945 for this discovery. Their roles distinguish the original observation from the later establishment of the antibiotic's effectiveness.

Other antibiotics were subsequently purified from other microbes. Antibiotics greatly improved the ability to treat diseases such as plague, whooping cough, diphtheria and leprosy. The usefulness of these substances demonstrates how products of some microbes can help control disease caused by others.

Note: Keep the producer and product distinct: Penicillium notatum is the mould, while penicillin is the antibiotic. Fleming made the initial discovery; Chain and Florey established its full potential as an effective antibiotic later.

Which industrial chemicals, enzymes and medicines come from microbes?

Organic acids and alcohol

Organic acids are carbon-containing acids such as citric, acetic, butyric and lactic acids. Microbes are grown commercially to produce these substances. Remember each acid together with its microbial producer, rather than assuming that one organism produces all industrial acids.

Enzymes and their applications

Enzymes are biological catalysts, substances that speed up chemical reactions. Microbial enzymes have uses in cleaning, food processing and medicine. Their functions differ: removing an oily stain, clarifying fruit juice and removing a blood clot require different enzyme activities.

Genetic engineering means deliberate modification of genetic material. Myocardial infarction is damage involving death of heart muscle because its blood supply is interrupted. Streptokinase is associated with removing blood clots in patients who have undergone myocardial infarction leading to a heart attack.

Cyclosporin A and statins

A bioactive molecule produces an effect in a living system. Cyclosporin A is produced by the fungus Trichoderma polysporum. It acts as an immunosuppressive agent, meaning that it suppresses immune responses, and is used in organ-transplant patients.

Statins produced by the yeast Monascus purpureus are used as blood-cholesterol-lowering agents. Cholesterol is a lipid substance in the body. Statins competitively inhibit the enzyme responsible for its synthesis, meaning that they compete for the enzyme's active site and reduce its activity.

These applications should remain separate: penicillin acts against disease-causing microbes, streptokinase helps remove clots, cyclosporin A suppresses immune responses and statins lower blood cholesterol. The fact that all have microbial origins does not give them the same biological function.

What happens during primary sewage treatment?

Why sewage needs treatment

Sewage is municipal wastewater in which human excreta form a major component. It contains large amounts of organic matter and microbes, many of which are pathogenic, meaning disease-causing. Direct release into rivers and streams can therefore pollute these water bodies.

Sewage is treated in sewage treatment plants (STPs) before disposal to make it less polluting. Treatment has two main stages: primary treatment removes particles physically, while secondary treatment uses biological activity. The liquid leaving the primary stage enters the secondary stage.

Physical separation of particles

1. Pass sewage through sequential filtration, a series of filtering steps that removes floating debris.
2. Allow grit, including soil and small pebbles, to settle by sedimentation, the settling of suspended material.
3. Collect the solids that settle as primary sludge.
4. Transfer the supernatant, the liquid above the settled solids, as primary effluent for secondary treatment.

Effluent means liquid flowing out of a treatment stage. Primary effluent is therefore different from primary sludge: the effluent is liquid, while sludge consists of settled material. The two terms identify different outputs of the same separation process.

Primary treatment removes large and small particles through filtration and sedimentation. It does not replace the subsequent treatment of organic matter by microbes. Remembering the order of these operations explains why the primary settling tank is followed by biological treatment.

Untreated sewage is often discharged into rivers when treatment capacity is insufficient. This contributes to river pollution and an increase in water-borne diseases, diseases spread through contaminated water. Treating sewage before release addresses both organic pollution and the problems associated with its microbial contents.

How do microbes carry out secondary sewage treatment?

Aeration, flocs and oxygen demand

Secondary treatment, also called biological treatment, uses heterotrophic microbes, organisms that obtain nourishment from organic substances. These microbes occur naturally in sewage. Primary effluent enters large aeration tanks, where it is mechanically agitated and air is pumped into it.

Aerobic microbes grow in the presence of oxygen. Aeration encourages their vigorous growth into flocs, masses of bacteria associated with fungal filaments that form mesh-like structures. While growing, these microbes consume the major part of the organic matter in the effluent.

Definition: Biochemical oxygen demand (BOD) is the amount of oxygen that would be consumed if bacteria oxidised all the organic matter in one litre of water. Oxidation here refers to the bacterial breakdown of organic matter using oxygen.

The BOD test measures oxygen uptake by micro-organisms and provides an indirect measure of organic matter. A greater BOD means a greater polluting potential. Microbial consumption of organic matter during secondary treatment significantly reduces the BOD of the effluent.

What happens after the BOD falls?

1. Maintain agitation and aeration so useful aerobic microbes grow and consume the major part of the organic matter.
2. Once BOD has fallen significantly, transfer the effluent to a settling tank.
3. Allow flocs to settle; the resulting sediment is called activated sludge.
4. Return a small part of the activated sludge to the aeration tank as an inoculum.
5. Send the remaining major part to anaerobic sludge digesters, tanks where bacteria digest sludge without oxygen.
6. During digestion, bacteria break down bacteria and fungi in the sludge and produce gases including methane, hydrogen sulphide and carbon dioxide.

Anaerobic means occurring without oxygen. The gases formed in sludge digestion constitute biogas, a combustible gas mixture that can provide energy. The secondary effluent is generally released into natural water bodies such as rivers and streams.

The photograph shows a treatment tank with turbulent, foamy water and a raised structure with railings across it. It illustrates a treatment installation; it does not show microscopic flocs or label individual bacteria.

See Fig. 8.6 in your NCERT textbook

A lower BOD after treatment indicates reduced organic pollution. It must not be confused with an increase in the amount of organic matter. Likewise, activated sludge is the settled microbial material, not the clarified liquid leaving the settling tank.

How do methanogens produce biogas in a biogas plant?

Microbes and the composition of biogas

Biogas is a mixture of gases produced by microbial activity that contains predominantly methane, written CH₄. It may be used as fuel. The exact types of gases formed depend on the microbes and the organic materials on which they grow.

Methanogens are methane-producing microbes that grow anaerobically. Methanobacterium is a common example. During growth on material containing cellulose, a structural carbohydrate in plant material, they produce large amounts of methane along with carbon dioxide and hydrogen, written H₂.

The gas mixture associated with anaerobic sewage-sludge digestion also includes hydrogen sulphide, written H₂S. Do not describe biogas as pure methane or assign a fixed percentage composition. Its defining feature here is the predominance of methane in a microbial gas mixture.

Methanogens occur in anaerobic sewage sludge and in the rumen, a part of the stomach of cattle. They help break down cellulose there and contribute to cattle nutrition. Cattle dung, commonly called gobar, is rich in these microbes and can generate gobar gas.

Construction and operation

A biogas plant has a concrete tank 10 to 15 feet deep. Bio-wastes are collected in it, and a slurry of dung is fed into the tank. A slurry is a mixture of solid material with liquid; here, dung is mixed with water.

1. Feed dung slurry into the concrete tank containing bio-wastes.
2. Microbial activity under anaerobic conditions produces methane-rich biogas.
3. The floating cover over the slurry rises as gas is produced in the tank.
4. Carry biogas through an outlet pipe to nearby houses for use in cooking and lighting.
5. Remove the spent slurry, the material remaining after digestion, through another outlet; it may be used as fertiliser.

The drawing labels dung and water entering a chamber on the left, a central digester and a gas-holder above it. A pipe carries gas upwards and to the right. A chamber on the right is labelled sludge.

See Fig. 8.8 in your NCERT textbook

Biogas plants are more often built in rural areas because cattle dung is available there in large quantities. The process connects waste use, fuel production and a residue that may serve as fertiliser. Gas production and removal of spent slurry are distinct outputs of the plant.

How do biocontrol agents reduce dependence on chemical pesticides?

Biological control and natural predators

Biocontrol means using biological methods to control plant diseases and pests. Pests are organisms that damage crops or otherwise cause harm. Chemical pesticides control pests; insecticides target insects, while weedicides control unwanted plants called weeds.

Chemical pesticides are toxic and extremely harmful to humans and animals. They pollute soil, groundwater, fruits, vegetables and crop plants. Weedicides also contribute to soil pollution. Conventional chemical methods often kill useful and harmful organisms indiscriminately, weakening the natural relationships that help regulate pests.

Biological farming relies on understanding these relationships. Predators capture and feed on other organisms. Parasites depend on other organisms, their hosts, for nourishment. Beneficial predators and parasitic insects need their prey or hosts, so eradication of pests would also remove organisms on which these beneficial forms depend.

The aim is to keep pests at manageable levels within a living system. This requires knowledge of the organisms in the field, their life cycles, feeding patterns and preferred habitats. Ladybirds help control aphids, small plant-feeding insects, while dragonflies help control mosquitoes.

Microbial agents and their targets

Bacillus thuringiensis (Bt) is a bacterium used against butterfly caterpillars. Its dried spores, resistant microbial stages, are supplied in sachets. The spores are mixed with water and sprayed on vulnerable plants such as brassicas, members of the cabbage group, and fruit trees.

1. Mix the dried Bt spores with water to prepare the spray.
2. Spray the mixture on susceptible plants, including brassicas and fruit trees.
3. Insect larvae, the immature feeding stages, eat the treated plant material.
4. Toxin, a poisonous substance, is released in the larval gut and kills the larvae.

This bacterial disease kills the caterpillars while leaving other insects unharmed. Bt toxin genes, units of hereditary information responsible for toxin production, have also been introduced into plants. Such plants resist attack by insect pests; Bt cotton is an example.

Trichoderma species are free-living fungi very common in root ecosystems, the communities of organisms and surroundings associated with roots. They are effective biocontrol agents against several plant pathogens. This role differs from production of cyclosporin A by Trichoderma polysporum.

Baculoviruses are viruses that attack insects and other arthropods, animals with jointed appendages. The majority used in biological control belong to the genus Nucleopolyhedrovirus. A genus is a group of related species. These viruses are useful for species-specific, narrow-spectrum insecticidal applications, meaning a restricted range of targets.

What does integrated pest management mean?

Integrated pest management (IPM) combines suitable pest-control methods while conserving beneficial organisms. Baculoviruses are especially useful when beneficial insects need protection within such a programme or when an ecologically sensitive area is treated.

These viruses have been shown to have no negative impacts on plants, mammals, birds, fish or even non-target insects. This makes target specificity valuable: controlling a pest need not remove the organisms that help control other pests. Biocontrol will greatly reduce dependence on toxic chemicals and pesticides.

How do biofertilisers improve soil nutrient quality?

Nitrogen-fixing bacteria

Definition: Biofertilisers are organisms that enrich the nutrient quality of soil. Their main sources are bacteria, fungi and cyanobacteria. They support nutrient enrichment, whereas biocontrol agents are used to control pests and plant diseases.

Nitrogen fixation converts atmospheric nitrogen into forms that organisms can use. Rhizobium bacteria form nodules, specialised swellings, on the roots of leguminous plants, members of the pea family. This is a symbiotic association, a close relationship between different organisms.

Rhizobium fixes atmospheric nitrogen into organic forms that the plant uses as nutrients. Azospirillum and Azotobacter can fix atmospheric nitrogen while free-living in soil. Their activity enriches soil nitrogen without the root-nodule association described for Rhizobium.

Fungal associations and cyanobacteria

Mycorrhiza is a symbiotic association between a fungus and a plant's roots. Many members of the fungal genus Glomus form mycorrhiza. The fungal partner absorbs phosphorus, a mineral nutrient, from soil and passes it to the plant.

Plants with these associations also show resistance to root-borne pathogens, tolerance to salinity, the presence of salts, and drought, a shortage of available water. They show an overall increase in growth and development. Mycorrhizal nutrient support should not be described as bacterial nitrogen fixation.

Cyanobacteria, also called blue-green algae, are autotrophic microbes, meaning that they make their own organic food. They are widely distributed in water and on land. Many can fix atmospheric nitrogen; examples include Anabaena, Nostoc and Oscillatoria.

In paddy fields, where rice is grown, cyanobacteria serve as important biofertilisers. They also add organic matter to the soil and increase its fertility. Their value therefore includes both nitrogen enrichment and the addition of organic material.

Overuse of chemical fertilisers contributes to environmental pollution. Biofertilisers help replenish soil nutrients and reduce dependence on chemical fertilisers. Their roles are specific: a nitrogen-fixing bacterium, a phosphorus-absorbing fungal partner and a cyanobacterium do not all improve soil by an identical mechanism.

Glossary

• Inoculum — A small quantity containing microbes introduced to start growth in fresh material.
• Fermentation — Microbial conversion of substances such as sugars into acids, alcohol or gases.
• Fermentor — A large vessel used to grow microbes for production on an industrial scale.
• Antibiotic — A microbial chemical product that kills or retards growth of other disease-causing microbes.
• Effluent — Liquid flowing out of a treatment stage, such as the primary settling tank.
• Flocs — Masses of bacteria associated with fungal filaments, forming mesh-like structures during secondary treatment.
• Biochemical oxygen demand — Oxygen consumed if bacteria oxidise all organic matter in one litre of water.
• Activated sludge — Sediment formed when bacterial flocs settle after aeration during secondary sewage treatment.
• Anaerobic digestion — Microbial breakdown of material without oxygen, as occurs in sewage-sludge digesters.
• Methanogens — Microbes that grow anaerobically and produce methane during the breakdown of organic material.
• Biocontrol — Use of biological methods to control plant diseases and harmful pests.
• Integrated pest management — Combining suitable pest-control methods while conserving organisms that contribute to natural pest regulation.
• Biofertilisers — Organisms that enrich soil nutrient quality, including bacteria, fungi and cyanobacteria.
• Mycorrhiza — A symbiotic association between a fungus and plant roots that supports nutrient uptake.

Common errors and misconceptions

• Misconception: All microbes are harmful. Correct: Many microbes support food production, sewage treatment, energy generation, pest control and soil nutrient enrichment.
• Misconception: Yeast is responsible for every fermented food. Correct: LAB produce curd, bacteria ferment idli and dosa dough, and baker's yeast ferments bread dough.
• Misconception: All alcoholic beverages undergo distillation. Correct: Wine and beer are produced without distillation; whisky, brandy and rum are produced by distilling fermented broth.
• Misconception: Primary sludge and activated sludge mean the same thing. Correct: Primary sludge contains solids settled during primary treatment; activated sludge is settled microbial flocs after aeration.
• Misconception: A high BOD indicates clean water. Correct: A higher BOD indicates more organic matter and greater polluting potential; biological treatment significantly reduces BOD.
• Misconception: Biogas is pure methane. Correct: Biogas is a mixture containing predominantly methane, together with other gases produced by microbial activity.
• Misconception: All biofertilisers fix nitrogen. Correct: Rhizobium and several other microbes fix nitrogen, while the mycorrhizal fungal partner absorbs phosphorus and supplies it to the plant.
• Misconception: Biocontrol aims to eradicate every organism called a pest. Correct: Biological farming keeps pests at manageable levels while maintaining beneficial predators and other ecological relationships.

Exam-style questions with model answers

Q1. Name the bacterial group that converts milk into curd and explain its action on milk proteins. [2 marks]

1. Lactic acid bacteria, including Lactobacillus, grow in milk and bring about its conversion into curd.
2. During growth they produce acids that coagulate and partially digest the milk proteins.

Q2. Distinguish wine and beer from whisky, brandy and rum by processing. Name the yeast used and explain what it produces from malted cereals or fruit juices. [3 marks]

1. Saccharomyces cerevisiae, called brewer's yeast in beverage production, ferments malted cereals and fruit juices to produce ethanol, the alcohol in these drinks.
2. Wine and beer are produced without distillation, although their production involves microbial fermentation of suitable starting materials.
3. Whisky, brandy and rum are produced by distillation of the fermented broth, so distillation follows the yeast's production of ethanol.

Q3. Explain the discovery and development of penicillin in four points: Fleming's observation, the cause he identified, the source and naming of the antibiotic, and the later contribution of Chain and Florey. [4 marks]

1. While studying Staphylococci, Alexander Fleming observed a mould in an unwashed culture plate; the bacteria could not grow around it.
2. He found that a chemical produced by the mould was responsible for preventing bacterial growth around the mould.
3. He named the chemical penicillin after the producing mould, Penicillium notatum; penicillin was the first antibiotic discovered.
4. Ernest Chain and Howard Florey later established its full potential as an effective antibiotic. Fleming, Chain and Florey received the Nobel Prize in 1945.

Q4. Explain secondary sewage treatment from the entry of primary effluent into an aeration tank to the handling of activated sludge. Include the effect on biochemical oxygen demand and gas production. [5 marks]

1. Primary effluent enters large aeration tanks, where mechanical agitation and pumped air encourage vigorous growth of useful aerobic microbes into flocs, masses of bacteria associated with fungal filaments.
2. Growing microbes consume the major part of the organic matter. This significantly reduces biochemical oxygen demand, the oxygen needed for bacterial oxidation of organic matter in water.
3. After BOD falls significantly, the effluent enters a settling tank. Flocs settle and form the sediment called activated sludge.
4. A small part of this activated sludge is returned to the aeration tank as an inoculum to support microbial growth.
5. The remaining major part enters anaerobic sludge digesters. Bacteria digest the bacteria and fungi in the sludge, producing gases including methane, hydrogen sulphide and carbon dioxide.

Q5. Three water samples, A, B and C, have BOD values of 20, 8 and 400 milligrams of oxygen per litre, respectively. BOD means biochemical oxygen demand; greater BOD indicates greater organic pollution. The samples are river water, untreated sewage and treated secondary effluent. Assume the river is relatively clean and has lower BOD than the secondary effluent. Assign each sample and justify all three assignments. [3 marks]

1. C is untreated sewage because its BOD, 400 milligrams of oxygen per litre, is the highest, indicating the greatest organic pollution among the samples.
2. B is river water because its BOD, 8 milligrams of oxygen per litre, is the lowest, consistent with the stated relatively clean river.
3. A is secondary effluent because its BOD, 20 milligrams of oxygen per litre, lies between the clean river water and the untreated sewage.

Q6. Describe biogas production in a dung-fed plant in five points: microbial source, conditions and gas, gas-holder movement, delivery and use of gas, and disposal or use of spent slurry. [5 marks]

1. Cattle dung is rich in methanogens, including Methanobacterium, which also occur in the rumen of cattle and help break down cellulose there.
2. Dung slurry is fed into a concrete tank. Anaerobic microbial activity produces biogas, a mixture containing predominantly methane together with other gases.
3. A floating cover is placed over the slurry. It rises as microbial activity produces gas within the tank and gas accumulates beneath the cover.
4. An outlet connected to a pipe supplies biogas to nearby houses. The gas produced can be used for cooking and lighting.
5. Spent slurry leaves through another outlet and may be used as fertiliser. Thus, the plant provides both a fuel and a potentially useful residue.

Q7. Give four biological-control examples, stating the role or target of each: ladybird, dragonfly, Trichoderma and Nucleopolyhedrovirus. Explain why the last is useful when beneficial insects are conserved. [4 marks]

1. Ladybirds are useful predators for controlling aphids, illustrating pest control through natural feeding relationships rather than introduced chemical pesticides.
2. Dragonflies help control mosquitoes. They are an animal biocontrol example, distinct from microbial agents used against plant diseases or insect larvae.
3. Trichoderma species are free-living fungi common in root ecosystems and act as effective biological control agents against several plant pathogens.
4. Nucleopolyhedrovirus provides species-specific, narrow-spectrum insect control. These viruses have been shown to have no negative impacts on non-target insects, making them useful when beneficial insects are conserved.

Q8. Explain four forms of soil enrichment by biofertilisers, covering Rhizobium, free-living nitrogen-fixing bacteria, Glomus mycorrhiza and cyanobacteria. Name the free-living bacteria and state the nutrient contribution in each case. [4 marks]

1. Rhizobium in root nodules of legumes fixes atmospheric nitrogen into organic forms that plants use as nutrients, through a symbiotic association.
2. Azospirillum and Azotobacter can fix atmospheric nitrogen while free-living in soil, enriching its nitrogen content without the Rhizobium root-nodule association.
3. Many Glomus species form mycorrhiza. The fungal partner absorbs phosphorus from soil and passes it to the associated plant.
4. Many cyanobacteria, such as Anabaena, Nostoc and Oscillatoria, fix atmospheric nitrogen. In paddy fields they are important biofertilisers and also add organic matter to soil.

Key takeaways

• Microbial benefits depend on the organism and its activity: food processing, useful chemical production, waste treatment and soil enrichment involve different roles.
• Lactic acid bacteria convert milk into curd, baker's yeast ferments bread dough, and Propionibacterium produces gas responsible for holes in Swiss cheese.
• Wine and beer are undistilled fermented beverages; whisky, brandy and rum are produced by distillation of fermented broth.
• Penicillin, streptokinase, cyclosporin A and statins have different applications despite sharing a microbial origin.
• Primary sewage treatment physically removes particles; secondary treatment uses microbes to consume organic matter and significantly reduce biochemical oxygen demand.
• Methanogens produce methane-rich biogas under anaerobic conditions; dung-fed plants supply gas for cooking and lighting, while spent slurry may be used as fertiliser.
• Biocontrol uses organisms and biological relationships to manage pests, with target-specific agents helping conserve beneficial organisms.
• Biofertilisers enrich soil through processes including nitrogen fixation, fungal phosphorus uptake and the addition of organic matter by cyanobacteria.

Test yourself

Why is a small amount of curd added to fresh milk?

It acts as an inoculum containing millions of lactic acid bacteria, which multiply at suitable temperatures and convert milk into curd.

What causes the large holes in Swiss cheese?

Large amounts of carbon dioxide produced by the bacterium Propionibacterium sharmanii cause the holes.

How do cyclosporin A and statins differ in function?

Cyclosporin A suppresses immune responses in organ-transplant patients. Statins lower blood cholesterol by competitively inhibiting an enzyme involved in its synthesis.

Why is some activated sludge returned to the aeration tank?

A small part is returned as an inoculum, supplying microbes for growth in the aeration tank.

What does a greater biochemical oxygen demand indicate?

It indicates more organic matter available for microbial oxidation and greater polluting potential of the wastewater.

Why is cattle dung useful for biogas production?

Cattle dung is rich in methanogens that produce methane during anaerobic breakdown of organic material.

What happens after caterpillars eat plant material treated with Bt spores?

Toxin is released in the larval gut, killing the larvae. This bacterial disease leaves other insects unharmed.

Which nutrient does the fungal partner supply in a mycorrhizal association?

The fungal partner absorbs phosphorus from soil and passes it to the associated plant.