Microbes in Human Welfare

The chapter is organised into six sections that mirror six settings where microbes are put to work. Section 8.1 stays in the kitchen — curd, idli and dosa batter, bread, toddy and cheese, all products of fermentation by lactic acid bacteria or yeast. Section 8.2 scales the same idea up to industry: fermented beverages produced in large fermentors, the discovery and production of antibiotics such as penicillin, and the commercial manufacture of organic acids, enzymes and bioactive molecules like streptokinase, cyclosporin A and statins. Section 8.3 follows municipal sewage through primary (physical) and secondary (biological) treatment, introducing the biochemical oxygen demand (BOD) as the key measure of water pollution. Section 8.4 explains how the same anaerobic bacteria that digest sludge — the methanogens — are harvested deliberately in biogas plants fed on cattle dung. Section 8.5 turns to agriculture and biocontrol: Bacillus thuringiensis, Trichoderma and baculoviruses as alternatives to chemical pesticides. Section 8.6 closes with biofertilisers — Rhizobium, free-living nitrogen fixers, mycorrhiza and cyanobacteria — as alternatives to chemical fertilisers. Two worked derivations trace the fermentation biochemistry behind household products and the full pathway from sewage to biogas, and a third traces exactly how the Bt toxin kills an insect larva, step by step.

Microbes in household products

The most familiar microbial product in an Indian kitchen is curd. Lactobacillus and other lactic acid bacteria (LAB) grow in milk, and during this growth they produce acids that coagulate and partially digest the milk proteins, converting milk to curd. A small amount of curd added to fresh milk as an inoculum or starter carries millions of LAB, which multiply at a suitable temperature and complete the conversion; this also improves the nutritional quality of the product by increasing its vitamin B12 content. LAB are useful inside the body too — in the human stomach, they play a beneficial role in checking disease-causing microbes. The same broad process, fermentation, is behind other everyday foods. Dough used for idli and dosa is fermented by bacteria, and the puffed-up appearance of the batter comes from CO2 gas produced during fermentation. Bread dough is fermented using baker's yeast, Saccharomyces cerevisiae, whose metabolism likewise releases CO2 and causes the dough to rise. Toddy, a traditional drink of parts of southern India, is made by fermenting palm sap, and microbes similarly ferment fish, soyabean and bamboo-shoots into other traditional foods. Cheese is one of the oldest microbially made foods, and different cheeses owe their texture, flavour and taste to the specific microbes used — the large holes in Swiss cheese come from CO2 produced by the bacterium Propionibacterium sharmanii, while Roquefort cheese is ripened by growing a specific fungus on it, giving it its particular flavour.

Microbes in industrial products

Scaling fermentation up from a kitchen to an industrial vessel called a fermentor lets microbes manufacture beverages, antibiotics, chemicals and enzymes at commercial volumes.

Fermented beverages. Yeasts, principally Saccharomyces cerevisiae (the same species used for bread-making, here called brewer's yeast), ferment malted cereals and fruit juices to produce ethanol. Depending on the raw material and whether the fermented broth is distilled, different alcoholic drinks result: wine and beer are produced without distillation, while whisky, brandy and rum are produced by distilling the fermented broth.

Antibiotics. An antibiotic is a chemical substance produced by a microbe that can kill or retard the growth of other, disease-causing microbes — the word literally means "against life," in reference to the target organisms, while being "pro-life" for the human patient. Penicillin, the first antibiotic discovered, was a chance discovery: Alexander Fleming, while working with Staphylococcus bacteria, noticed a mould growing on an unwashed culture plate, around which the Staphylococcus colonies could not grow. The inhibitory chemical came from the mould, which Fleming named Penicillium notatum, and he called the chemical penicillin. Its full potential as a usable drug, however, was established later by Ernest Chain and Howard Florey, and penicillin was extensively used to treat wounded American soldiers in World War II. Fleming, Chain and Florey shared the Nobel Prize in 1945 for this discovery. Antibiotics discovered since have greatly improved humanity's capacity to treat once-deadly diseases such as plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog).

Chemicals, enzymes and other bioactive molecules. Microbes are commercially exploited for organic acids, alcohols and enzymes. Aspergillus niger (a fungus) produces citric acid; Acetobacter aceti (a bacterium) produces acetic acid; Clostridium butylicum (a bacterium) produces butyric acid; and Lactobacillus (a bacterium) produces lactic acid. Saccharomyces cerevisiae is used for commercial ethanol production. Lipases produced by microbes are added to detergents to remove oily stains from laundry, and pectinases and proteases clarify bottled fruit juices, which is why bottled juice looks clearer than juice made at home. Streptokinase, produced by Streptococcus and modified by genetic engineering, is used as a clot buster to dissolve blood clots after myocardial infarction (heart attack). Cyclosporin A, an immunosuppressive agent for organ-transplant patients, is produced by the fungus Trichoderma polysporum. Statins, produced by the yeast Monascus purpureus, are commercialised as blood-cholesterol-lowering agents, acting by competitively inhibiting the enzyme the body uses to synthesise cholesterol.

Microbes in sewage treatment

Cities and towns generate enormous quantities of waste water every day, called sewage, whose major component is human excreta along with large amounts of organic matter and microbes, many of them pathogenic. Sewage cannot be discharged directly into rivers and streams — it must first be treated in a sewage treatment plant (STP) by the heterotrophic microbes naturally present in it, in two stages.

Primary treatment is physical: it removes particles, large and small, from the sewage by filtration and sedimentation. Floating debris is removed first by sequential filtration, and then grit (soil and small pebbles) is removed by sedimentation. All the solids that settle out form the primary sludge, and the liquid that remains on top — the supernatant — is called the effluent, which passes on to secondary treatment.

Secondary (biological) treatment passes the primary effluent into large aeration tanks, mechanically agitated and constantly supplied with pumped-in air. This favours vigorous growth of aerobic microbes, which form flocs — masses of bacteria associated with fungal filaments, forming mesh-like structures. As these microbes grow, they consume most of the organic matter in the effluent, which significantly reduces the effluent's biochemical oxygen demand (BOD): 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 oxygen uptake by micro-organisms in a water sample and thus indirectly measures the organic matter present — the greater a sample's BOD, the greater its polluting potential — and sewage is treated until BOD falls significantly. Once BOD has fallen enough, the effluent passes into a settling tank where the bacterial flocs sediment out as activated sludge. A small part is pumped back into the aeration tank as inoculum for the next batch, while the larger remaining part goes to anaerobic sludge digesters, where anaerobic bacteria digest the sludge's bacteria and fungi, producing a mixture of methane, hydrogen sulphide and carbon dioxide — together, biogas, which is inflammable and usable as an energy source. The effluent from secondary treatment is generally released into a river or other natural water body. This microbial method has treated millions of gallons of waste water daily, in almost every part of the world, for more than a century — no engineered technology has yet rivalled it. But rapid urbanisation has outpaced the building of new treatment plants, so untreated sewage is often discharged directly into rivers, worsening pollution and water-borne disease. India's Ministry of Environment and Forests has run the Ganga Action Plan and the Yamuna Action Plan to build more sewage treatment plants so that only treated sewage reaches these rivers.

Microbes in production of biogas

Biogas is a mixture of gases, predominantly methane, produced by microbial activity and usable as fuel. Different microbes produce different gaseous end-products depending on the substrate: in dough fermentation, cheese-making and beverage production, the main gas produced is CO2. Certain bacteria, however, grow anaerobically on cellulosic material and produce large amounts of methane along with CO2 and H2. These bacteria are collectively called methanogens, and Methanobacterium is one common example. Methanogens are commonly found in the anaerobic sludge of sewage treatment plants, and they are also present in the rumen (a chamber of the stomach) of cattle, where they help break down the cellulose in the cattle's food and thereby play an important role in ruminant nutrition (humans, notably, cannot digest cellulose in their food, lacking such microbes and the enzyme cellulase). Because of this, cattle excreta — dung, or gobar — is rich in methanogens, and this makes dung suitable raw material for generating gobar gas, the common Indian term for biogas. A biogas plant consists of a concrete tank, 10 to 15 feet deep, into which a slurry of dung is fed. A floating cover sits over the slurry and rises as gas is produced by microbial activity in the tank. An outlet pipe carries the biogas to nearby houses for use in cooking and lighting, and a separate outlet removes the spent slurry, which can be used as a fertiliser. Because cattle dung is abundantly available in rural areas, biogas plants are more commonly built there. In India, this technology was developed largely through the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

Microbes as biocontrol agents

Biocontrol refers to using biological methods, rather than toxic chemical insecticides and pesticides, to control plant diseases and pests. Conventional chemical control kills useful and harmful organisms indiscriminately and pollutes soil, groundwater, fruits, vegetables and crop plants. Organic farming instead works from the belief that biodiversity itself furthers ecosystem health, keeping pest populations at manageable levels through a complex web of natural checks and balances — predators, parasites and competitors — rather than eradicating pests outright, since the predatory and parasitic species that keep pests in check depend on those very pests as food or hosts. Familiar macroscopic examples of biocontrol include the ladybird beetle, which controls aphids, and dragonflies, which control mosquitoes. Among microbial biocontrol agents, the bacterium Bacillus thuringiensis (commonly abbreviated Bt) controls butterfly and moth caterpillars. Bt is sold in sachets as dried spores, which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees; insect larvae eat the sprayed leaves, and in the alkaline gut of the larva the Bt toxin is released and kills the larva, while other insects remain unharmed. Genetic engineering has since allowed scientists to introduce the Bt toxin gene directly into plants, producing insect-resistant crops such as Bt cotton, cultivated in several Indian states (this genetic-engineering application is developed further in the biotechnology chapter that follows). Trichoderma is a genus of free-living fungi very common in plant root ecosystems and an effective biocontrol agent against several plant pathogens; it is being developed for use in treating plant disease. Baculoviruses are pathogens of insects and other arthropods; most baculoviruses used as biocontrol agents belong to the genus Nucleopolyhedrovirus. They are excellent candidates for species-specific, narrow-spectrum insecticidal use, with no demonstrated negative impact on plants, mammals, birds, fish or non-target insects — a valuable property when beneficial insects must be conserved as part of an integrated pest management (IPM) programme, or when treating an ecologically sensitive area.

Microbes as biofertilisers

Chemical fertilisers, used to meet growing agricultural demand, have themselves become a significant source of environmental pollution, driving interest in organic farming and the use of biofertilisers — organisms that enrich the nutrient quality of soil. The main biological sources of biofertilisers are bacteria, fungi and cyanobacteria. Rhizobium forms a symbiotic association with the roots of leguminous plants, producing the characteristic root nodules; within these nodules, Rhizobium fixes atmospheric nitrogen into organic nitrogenous forms the plant can use. Other bacteria fix atmospheric nitrogen while free-living in soil, without needing a plant symbiosis — examples include Azospirillum and Azotobacter, both of which enrich the nitrogen content of soil. Certain fungi also form symbiotic associations with plant roots, called mycorrhiza; many members of the genus Glomus form mycorrhizal associations. In this partnership, the fungal symbiont absorbs phosphorus from the soil and passes it on to the plant, and plants with such associations also gain resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in growth and development. Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial habitats; many, including Anabaena, Nostoc and Oscillatoria, can fix atmospheric nitrogen. In paddy fields specifically, cyanobacteria serve as an important biofertiliser, and blue-green algae generally also add organic matter to the soil, increasing its fertility. A number of commercial biofertiliser products are available to Indian farmers today to replenish soil nutrients and reduce dependence on chemical fertilisers.

Board level - lactic acid bacteria in food and their uses

Name a food in which lactic acid bacteria are naturally used, describe how they act on it, and state two beneficial applications of LAB besides that food product.

Step 1. Identify the food. Lactic acid bacteria, chiefly Lactobacillus, are used to convert milk into curd, and a starter culture of curd (rich in LAB) is added to fresh milk to begin the process.

Step 2. Describe the action. The LAB multiply in the milk at a suitable temperature and, during their growth, produce acids that coagulate and partially digest the milk proteins, turning liquid milk into the semi-solid, sour product we call curd; this also raises the vitamin B12 content of the product compared with the starting milk.

Step 3. State two further applications. First, in the human digestive tract, LAB play a beneficial role by checking the growth of disease-causing microbes. Second, lactic acid bacteria of the Lactobacillus type are also used industrially as producers of lactic acid itself, an organic acid with its own commercial uses. Answer: curd, formed when LAB (for example Lactobacillus) ferment milk sugar to acids that coagulate milk protein, also raising vitamin B12; LAB additionally protect the gut against pathogens and are cultured industrially to manufacture lactic acid.

Board level - identifying the microbe behind an industrial product

Match each product to the microbe that produces it and name the type of organism (bacterium or fungus) in each case: (a) citric acid, (b) Swiss cheese's large holes, (c) streptokinase, (d) statins.

Step 1. Citric acid is produced industrially by Aspergillus niger, a fungus, grown under controlled fermentation conditions.

Step 2. The large holes characteristic of Swiss cheese are produced by a large amount of CO2 gas generated by Propionibacterium sharmanii, a bacterium, during the cheese's ripening.

Step 3. Streptokinase, the clot-buster used after myocardial infarction, is produced by the bacterium Streptococcus and then modified by genetic engineering for medical use.

Step 4. Statins, the cholesterol-lowering agents that act by competitively inhibiting the body's cholesterol-synthesising enzyme, are produced by the yeast Monascus purpureus, a fungus. Answer: (a) Aspergillus niger, fungus; (b) Propionibacterium sharmanii, bacterium; (c) Streptococcus, bacterium; (d) Monascus purpureus, fungus (a yeast).

Competitive level (NEET) - ranking three water samples by BOD

Three water samples — river water, untreated sewage, and secondary effluent from a sewage treatment plant — were tested for BOD but their labels A, B and C were lost. The recorded values were A = 20 mg/L, B = 8 mg/L and C = 400 mg/L. Given that river water is relatively clean, identify which label corresponds to which sample, and rank all three by polluting potential.

Step 1. Recall the meaning of BOD. BOD is the amount of oxygen microbes would need to oxidise all the organic matter in one litre of a water sample; a higher BOD means more organic matter is present and awaiting microbial breakdown, and therefore a greater polluting potential once that sample is released into a water body.

Step 2. Rank the numeric values from lowest to highest polluting potential: B (8 mg/L) is lowest, A (20 mg/L) is intermediate, and C (400 mg/L) is by far the highest — roughly twenty times A and fifty times B.

Step 3. Assign identities using the biology of each source. River water, stated to be relatively clean, should carry the least organic load, so river water corresponds to B (8 mg/L). Untreated sewage carries the heaviest, completely unprocessed organic and microbial load, so untreated sewage corresponds to C (400 mg/L), the extreme outlier. Secondary effluent has already passed through aerobic microbial treatment that consumed most of its organic matter and lowered its BOD substantially, but it is not chemically pure like river water, so it takes the intermediate value: secondary effluent corresponds to A (20 mg/L). Answer: B (8 mg/L) is river water, A (20 mg/L) is treated secondary effluent, and C (400 mg/L) is untreated sewage — the most polluted sample is C, and the order of decreasing pollution is C greater than A greater than B. Timing note: about 90 seconds. The whole question collapses to one rule — higher BOD, more organic matter, more pollution — applied to three numbers that differ by an order of magnitude or more, so no calculation is needed, only correct biological reasoning about which source should carry the least and the most untreated organic matter.

• Confusing primary and secondary sewage treatment. Primary treatment is purely physical (filtration and sedimentation, producing primary sludge); only secondary treatment is biological and is the stage that actually lowers BOD via microbial flocs. A question asking "which stage reduces BOD" is asking about secondary treatment specifically.
• Reading BOD backwards. A higher BOD means more organic pollution, not less oxygen pollution or more purity — students sometimes assume a "demand" for oxygen sounds like a good, healthy sign, when in fact it signals unconsumed organic matter.
• Treating Fleming as the sole discoverer of usable penicillin. Fleming discovered the antibiotic effect by chance; it was Ernest Chain and Howard Florey who developed it into a therapeutically usable drug. All three shared the 1945 Nobel Prize — mentioning only Fleming is an incomplete answer on a full-marks question.
• Mixing up which yeast does what. Saccharomyces cerevisiae is the single species behind both bread-making and (as brewer's yeast) beverage fermentation — students sometimes invent two different yeast species for these two uses.
• Assuming Bt is a chemical pesticide. Bacillus thuringiensis is a bacterium sprayed as dried spores; its toxin is only activated inside a target insect's alkaline gut, which is exactly why it does not harm other organisms the way a broad-spectrum chemical insecticide would.
• Confusing biocontrol with biofertiliser. Biocontrol agents (Bt, Trichoderma, baculoviruses) manage pests and pathogens; biofertilisers (Rhizobium, Azotobacter, mycorrhiza, cyanobacteria) enrich soil nutrients. They serve different purposes even though both replace a chemical input (pesticide versus fertiliser respectively).
• Forgetting that methanogens link sewage treatment and biogas. The same broad group of anaerobic bacteria that digest activated sludge in a sewage plant's anaerobic digester also live in the cattle rumen and in cattle dung, which is why a rural gobar gas plant and an urban sewage plant's sludge digester run on the same underlying microbiology.

• Explain how lactic acid bacteria convert milk to curd and state the biochemical basis of the coagulation.
• Name the microbes responsible for at least four household or industrial fermented products, including cheese varieties, and describe each microbe's role.
• Describe the discovery of penicillin, correctly crediting Fleming's chance discovery and Chain and Florey's development of it into a usable drug.
• List at least four bioactive molecules or industrial chemicals produced by named microbes (citric acid, streptokinase, cyclosporin A, statins) and their uses.
• Distinguish primary from secondary sewage treatment, and explain how BOD is defined, measured and used to judge water quality.
• Trace the fate of activated sludge from the settling tank through partial recycling to anaerobic digestion and biogas production.
• Explain what methanogens are, name one example, and describe both their role in the cattle rumen and their use in a rural biogas plant.
• Compare Bacillus thuringiensis, Trichoderma and baculoviruses as biocontrol agents, and explain the mechanism by which the Bt toxin kills a target insect larva.
• Distinguish biocontrol agents from biofertilisers, and name the specific bacteria, fungi and cyanobacteria used as biofertilisers along with what each one supplies to the soil.

Ask the tutor: Want to work through how the Bt toxin gene is engineered into cotton, or trace the exact biochemistry of one fermentation pathway (curd, dosa batter or bread) step by step?