Microbes in Human Welfare: Class 12 Biology · Chapter 8

A single gram of fertile garden soil can hold a hundred million to a billion bacteria, along with fungi, protozoa and viruses. Organisms this small, which you can study only with a microscope, are called microbes (सूक्ष्मजीव). They live in air, water and soil, on and inside plants and animals, and in your own gut.

Their range of homes is far wider than that of any plant or animal. Microbes have been found in acidic hot springs with a pH close to 0, in Antarctic ice, in rock kilometres below the surface and around sea-floor vents hotter than 100 °C; the record holder multiplies at about 122 °C. Wherever there is liquid water, some microbe has usually found a way to live in it.

The word microbe covers very different things (Fig. 8.1). Bacteria, protozoa, yeasts, moulds and microscopic algae are made of cells. Viruses are far smaller and are not cells at all: each is a protein coat around DNA or RNA, and it can multiply only inside a host cell. Viroids are smaller still, bare loops of RNA that infect plants, and prions are misfolded proteins that cause brain diseases such as mad cow disease.

You cannot see a single bacterium, but you can see millions of them together. Spread a few cells on a jelly-like nutrient medium in a petri dish, keep it warm, and each cell divides again and again into a spot called a colony, visible to the naked eye within a day or two. Growing colonies like these is how microbiologists identify microbes, count them and test what kills them.

Ask most people about microbes and they think of germs, the pathogens you studied in Chapter 7. Pathogens, though, are a small minority. We depend on microbes for food, medicines and fuel, for clean water and for fertile soil, and the sections below take these up one by one.

8.1 Microbes in Household Products

Your kitchen is a small microbiology lab. Every time someone sets curd (दही) overnight, leaves idli batter to rise or bakes a loaf of bread, a population of microbes is doing the work.

To set curd, you stir a spoonful of yesterday's curd into lukewarm milk. That spoonful is the starter, or inoculum: it carries billions of lactic acid bacteria (LAB), such as Lactobacillus. Kept warm, the LAB multiply quickly and ferment the milk sugar, lactose, into lactic acid (Fig. 8.2).

The acid does two jobs. As the pH falls from about 6.6 to about 4.6, the particles of the milk protein casein lose their charge, clump together and trap the liquid in a soft gel: the milk sets. The acid and the bacterial enzymes also break down part of the protein, so curd is easier to digest than milk. The fermentation improves its food value too, notably by raising the level of vitamin ${\mathrm{B}}_{12}$.

Eaten alive, LAB help your gut too. They make the intestine more acidic and crowd out harmful microbes, which is one reason curd and buttermilk are counted among probiotic foods.

Idli and dosa batter works on the same principle. A batter of soaked rice and black gram (urad dal) left overnight turns frothy and swells, because bacteria such as Leuconostoc ferment its sugars and release carbon dioxide. The trapped bubbles make steamed idlis soft and spongy, and the acids give dosa its slight tang.

Bread rises for a similar reason, but here the worker is a fungus: baker's yeast, Saccharomyces cerevisiae. Kneaded into dough, the yeast cells ferment sugars to ethanol and carbon dioxide. The gas inflates the dough, and the ethanol escapes during baking.

Across India, fermentation gives many traditional foods and drinks their character. In Kerala, Tamil Nadu and other southern states, palm sap collected in pots begins to ferment within hours, through wild yeasts and bacteria, into the mildly alcoholic toddy. Fish, soybeans and bamboo shoots are fermented too, especially in the Northeast.

Why is Swiss (Emmental) cheese full of holes while Roquefort is streaked with blue? In both cases the answer is a microbe (Fig. 8.3). As Swiss cheese ripens, the bacterium Propionibacterium shermanii releases plenty of carbon dioxide, and the gas collects into large round holes, or 'eyes'.11Your textbook spells it sharmanii; the species is named after the American bacteriologist James M. Sherman. Roquefort is ripened with the mould Penicillium roqueforti, whose blue-green veins give it a sharp flavour. People have made cheese with microbes for thousands of years, and today's hundreds of varieties owe much of their texture and taste to the species added.

Paneer, by contrast, is not a microbial food. Hot milk is curdled with lemon juice or vinegar and pressed straight away, with no ripening at all.

8.2 Microbes in Industrial Products

Scale the curd bowl up a million times and you have a microbial factory. Industry grows chosen microbes by the tonne to make beverages, antibiotics, organic acids, enzymes and medicines.

The vessels used are fermentors (किण्वक): closed tanks, often of stainless steel and taller than a house. Inside, the conditions the microbe needs are held steady: temperature, pH, oxygen supply, nutrients and stirring. The vessel is sterilised before the chosen microbe is added, so no other microbe competes with it. You will meet fermentors again, as bioreactors, in Chapter 9.

8.2.1 Fermented Beverages

Every alcoholic drink, from beer to rum, starts the same way: yeast feeding on sugar. People brewed such drinks for thousands of years before anyone knew that yeast was alive. The species is Saccharomyces cerevisiae, the very yeast that raises bread; brewers call it brewer's yeast. Its food may be the sugar of germinated (malted) grain such as barley, or the sugar of fruit juice such as grape juice, and its main product is ethanol: $\mathrm{C}{\phantom{X}}_6\mathrm{H}{\phantom{X}}_{12}\mathrm{O}{\phantom{X}}_6\stackrel{}{\to }2\mathrm{C}{\phantom{X}}_2\mathrm{H}{\phantom{X}}_5\mathrm{O}\mathrm{H}+2\mathrm{C}\mathrm{O}{\phantom{X}}_2$.

Two choices then decide which drink comes out (Fig. 8.4): the sugar source, and whether the fermented liquid is distilled. Yeast cannot push ethanol much above 15 %, because the alcohol begins to poison it, so drinks sold straight from fermentation stay fairly weak: wine (from grapes) and beer (from malted barley). Spirits are made by heating the fermented liquid and condensing the vapour, which concentrates the ethanol to around 40 %: whisky from grain, brandy from wine or other fruit, and rum from sugarcane molasses.

8.2.2 Antibiotics

Antibiotic comes from the Greek anti (against) and bios (life): these substances act against the life of microbes, above all the pathogens that infect us. Few discoveries of the twentieth century have saved more human lives than antibiotics (प्रतिजैविक).

Penicillin, the first antibiotic, was found by accident (Fig. 8.5). In 1928 the London bacteriologist Alexander Fleming returned from a holiday to a stack of culture plates of Staphylococcus bacteria. A mould had landed on one plate as a contaminant, and around it the bacterial colonies had died, leaving a clear ring. The mould turned out to be Penicillium notatum, and the bacteria-killing substance it leaked into the jelly became known as penicillin.

Fleming could not purify enough penicillin to treat patients. Between 1939 and 1941, Howard Florey, Ernst Chain and their team at Oxford purified it, showed that it cured infected mice and then people, and helped start mass production. By the end of World War II it was saving thousands of wounded soldiers. Fleming, Florey and Chain shared the 1945 Nobel Prize in Physiology or Medicine.

Once penicillin had shown the way, scientists screened thousands of soil microbes for similar substances. Most antibiotics in use today trace back to two groups of microbes: moulds and, above all, the thread-like soil bacteria called Streptomyces (Table 8.3).

Tuberculosis, leprosy (कुष्ठ रोग, kushth rog), plague, diphtheria (गलघोंटू, galghotu), whooping cough (काली खाँसी, kali khansi) and bacterial pneumonia were once major killers; all can now be treated with antibiotics. Antibiotics also make surgery, organ transplants and cancer treatment far safer, because they control the infections that would otherwise follow.

Most antibiotics attack something that bacteria have and our cells lack. Penicillin, for example, stops bacteria from building their cell walls; human cells have no cell wall, so they are spared. For the same reason antibiotics are useless against viruses, which have no cell wall or metabolism of their own: an antibiotic will not cure a cold or flu.

8.2.3 Chemicals, Enzymes and other Bioactive Molecules

Microbes are cheap, fast chemical factories. Fed sugar and minerals in a fermentor, each species turns out its own products, which industry harvests by the tonne (Table 8.4).

Organic acids are a large group. Lactobacillus yields lactic acid, used in foods and to make a biodegradable plastic. The bacterium Acetobacter aceti oxidises ethanol to acetic acid, the acid of vinegar. The mould Aspergillus niger makes citric acid, which flavours and preserves soft drinks, jams and sweets, and Clostridium butylicum yields butyric acid for flavourings.22Industry today mostly uses the closely related Clostridium butyricum; your textbook names C. butylicum. For industrial ethanol, the workhorse is once again the yeast Saccharomyces cerevisiae.

Enzymes from microbes turn up in unexpected places. Freshly pressed juice is cloudy because fine pulp is held up by pectin and proteins; juice factories add pectinases and proteases to break these down, which is why bottled juice looks clear. Washing powders contain lipases, fat-splitting enzymes that loosen greasy stains. In hospitals, streptokinase from Streptococcus bacteria, improved by genetic engineering, serves as a 'clot buster': injected soon after a heart attack (myocardial infarction), it activates the blood's own clot-dissolving enzyme and reopens the blocked coronary artery.

Two other microbial molecules are everyday medicines. Statins, such as those made by the mould Monascus purpureus, lower blood cholesterol.33Your textbook calls Monascus a yeast; it is a filamentous fungus, a mould. A statin molecule resembles the substrate of HMG-CoA reductase, the key enzyme in cholesterol synthesis, so it competes for the enzyme's active site: a case of competitive inhibition, which you met in Class 11. Cyclosporin A, from the fungus Trichoderma polysporum, damps down the immune system; after an organ transplant it stops the body from attacking the new organ.

8.3 Microbes in Sewage Treatment

Every flush, bath and kitchen sink in a town sends water down the drains. The combined waste water of homes and towns is called sewage (वाहित मल). It is mostly water, but it carries human excreta, food scraps, soap and an enormous load of microbes, a good share of them pathogens.

Pouring it straight into a river does two kinds of harm. Its pathogens spread water-borne diseases such as cholera, typhoid and hepatitis A to people downstream. And its organic matter feeds river bacteria, which multiply and use up the oxygen dissolved in the water, so that fish and other animals suffocate.

The job is done in a sewage treatment plant (STP), and the workers are not chemicals but the microbes that arrive with the sewage itself. Most of them are heterotrophs, which feed on organic matter; the plant's tanks, pumps and blowers simply give them the right conditions. The process has a physical stage and a biological stage (Fig. 8.7).

Primary treatment

Primary treatment is purely physical. A series of bar screens and finer filters catches floating rubbish such as plastic, rags and leaves. The water then flows slowly through a grit chamber, where sand, silt and gravel drop out. In the primary settling tank, still finer solids sink as primary sludge, and the cloudy water above, the primary effluent, flows on. Settling alone removes roughly a third of the organic load.

Secondary (biological) treatment

Secondary treatment is where microbes take over. Blowers force air through the primary effluent in long aeration tanks while paddles keep it churning. With oxygen to spare, aerobic bacteria multiply fast and grow tangled up with fungal threads into flocs (ऊर्णक), loose mesh-like clumps you could see in a jar of tank water (Fig. 8.8). The flocs feed on the dissolved and suspended organic matter, turning much of it into carbon dioxide, water and more microbial cells.

How do operators know when the water is clean enough? They measure its biochemical oxygen demand (जैव रासायनिक ऑक्सीजन माँग), or BOD.

In the test, a sealed bottle of the sample, diluted if it is very dirty, is kept in the dark, usually for 5 days at 20 °C, and the fall in its dissolved oxygen is measured.44Indian laboratories often run a 3-day test at 27 °C, which gives similar values. The logic is simple: microbes can use oxygen only if they have organic matter to oxidise, so a large oxygen demand betrays a large organic load. That is also why high-BOD water harms a river: its bacteria would strip the oxygen that fish need. Operators keep the effluent in the aeration tanks until its BOD has dropped to a small fraction of the starting value.

Aeration leaves a mixture of cleaner water and a growing mass of flocs, and a second settling tank separates the two. The flocs that collect at its bottom form activated sludge (सक्रियित आपंक), a dense mass of living microbes. Because it is alive, some of it is recycled: pumped back to the head of the aeration tank, it starts the next batch of effluent off with ready-made flocs, much as a spoonful of curd starts fresh milk. The surplus, together with the primary sludge, is sent to large sealed tanks called anaerobic sludge digesters.

Inside a digester there is no oxygen, and a new set of microbes takes charge: anaerobic bacteria that break down the dead bacteria and fungi of the sludge. Their waste gases, mostly methane with carbon dioxide and a little hydrogen sulphide, make up biogas (Section 8.4); since methane is a fuel, many plants burn it to warm their digesters or make electricity. Meanwhile the clear water from the top of the secondary settling tank, now low in BOD, returns to a river or stream, often after disinfection, or is treated further for reuse in parks and industry.

The activated sludge process dates from 1914, when it was developed in Manchester, England, and it is still the backbone of sewage treatment worldwide: nothing engineered by people cleans such volumes of water as cheaply as microbes do. India's problem is capacity. In 2021 its cities produced roughly 72 000 million litres of sewage a day, but their plants could treat well under half of it. The rest reached rivers untreated, adding to pollution and water-borne disease.

Two national programmes target this gap on India's best-known rivers. The Ganga Action Plan, begun in the mid-1980s, and the Yamuna Action Plan of 1993, both run by the Ministry of Environment and Forests, paid for chains of STPs along the rivers to intercept town sewage before it reached the water. The Namami Gange programme, launched in 2014, continues the effort on a larger scale.

8.4 Microbes in Production of Biogas

In the sludge digesters of an STP, microbes turn waste into fuel. The same thing happens, on a smaller scale, beside thousands of village homes across India.

Look back at the gases met so far in this chapter: idli batter, Swiss cheese and the brewer's fermentor all gave off carbon dioxide. Biogas is different, because most of it is methane, and only one group of microbes can make methane: the methanogens, such as Methanobacterium. They live only where there is no oxygen, and they turn small molecules left by other microbes into methane.

They make methane in two main ways: from hydrogen and carbon dioxide, $\mathrm{C}\mathrm{O}{\phantom{X}}_2+4\mathrm{H}{\phantom{X}}_2\stackrel{}{\to }\mathrm{C}\mathrm{H}{\phantom{X}}_4+2\mathrm{H}{\phantom{X}}_2\mathrm{O}$, and by splitting acetic acid, $\mathrm{C}\mathrm{H}{\phantom{X}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\stackrel{}{\to }\mathrm{C}\mathrm{H}{\phantom{X}}_4+\mathrm{C}\mathrm{O}{\phantom{X}}_2$.

On their own, methanogens do not break down cellulose. In dung and plant waste they are the last link in a chain (Fig. 8.10). Some bacteria split cellulose into sugars, fermenting bacteria turn the sugars into acids, hydrogen and carbon dioxide, and the methanogens finish the job. By using up hydrogen, they also keep the earlier steps running. Strictly, methanogens are archaea rather than bacteria, although your textbook groups them with bacteria.

Cattle depend on such a chain. Their rumen, the first and largest chamber of the stomach, holds a dense community of microbes, methanogens included, that ferments the cellulose of grass and straw. The animal absorbs the acids produced as its main source of energy and belches out the methane. Cattle dung, or gobar, therefore comes already rich in these microbes and in half-digested plant fibre, which makes it the ideal raw material for biogas (जैव गैस), known in India as gobar gas.

Trace the gas backwards from a village kitchen (Fig. 8.11). The stove is connected by a pipe to a large steel drum that floats, upside down, on a pit full of dung slurry. As the microbes below release gas, it collects under the drum and lifts it, so the height of the drum shows how much fuel is stored; the drum's weight also pushes the gas along the pipe. This is the floating-drum design.

The pit itself is the digester, a concrete-lined tank about 3–4.5 m (10–15 feet) deep. Each day the family mixes fresh dung with roughly an equal volume of water, often adding kitchen and farm waste, and pours the slurry in through an inlet pipe. Sealed from air, the tank holds the whole anaerobic chain. Digested slurry is pushed out through an outlet on the far side; having lost carbon but little of its nitrogen or phosphorus, it is a rich manure for the fields.

Villages are where such plants make most sense: cattle dung is plentiful there, and a clean blue flame replaces smoky firewood and dung cakes for cooking and lighting. Two Indian institutions shaped the technology: the Indian Agricultural Research Institute (IARI), whose scientists built early gobar gas plants, and the Khadi and Village Industries Commission (KVIC), whose floating-drum design spread across the country from the 1960s. Cheaper fixed-dome designs such as Deenbandhu followed, and the GOBARdhan scheme, launched in 2018, now supports village and community plants. If there is a plant near your school, ask its owners how much dung they feed each day and how many people it cooks for, and compare with the worked example below.

8.5 Microbes as Biocontrol Agents

Farmers lose a large share of their crops to insects, weeds and plant diseases, and the usual answer has been chemicals: insecticides, fungicides and weedicides. Many of these poison more than their targets: the farm workers who spray them, cattle and birds, and the bees that pollinate crops. Residues linger in soil, seep into wells and reach our plates, while pests that survive repeated spraying breed resistant offspring.

Biocontrol (जैव नियंत्रण) takes a different view of a field. An organic farmer sees it as an ecosystem in which pests are only some of many interacting species. The aim is not to wipe pests out but to hold them below the level at which they do real damage, through a web of predators, parasites and diseases. Wiping them out would be neither practical nor wise: the ladybirds, wasps and spiders that hunt pests would starve without them. The more varied the life in and around a field, the steadier these checks become.

Biocontrol therefore starts with a survey of the field: which species live there, which are pests and which hunt them, when each appears and where it shelters. A farmer who notices, say, that ladybird larvae arrive in the mustard crop a week after the aphids can hold off spraying and let them work, or release extra natural enemies at the right moment.

Some biocontrol agents are animals. The red-and-black ladybird beetle eats aphids, the small sap-sucking insects that crowd young shoots. Dragonflies catch mosquitoes, both as flying adults and as underwater nymphs that eat mosquito larvae.

Microbes join in too. The best known is the soil bacterium Bacillus thuringiensis, or Bt, which packs a protein into crystals as it forms spores. The crystals are harmless until they reach the alkaline gut of a caterpillar, where they dissolve into an active toxin that punches holes in the gut lining; the larva stops feeding and dies within days (Fig. 8.12). Bees, ladybirds, other insects and people lack this trigger and are unharmed. Farmers buy Bt as a powder of spores and crystals, stir it into water and spray it wherever caterpillars are eating: on cabbage, cauliflower and other brassicas, or on fruit trees.

Scientists have gone a step further and moved the Bt toxin gene into crop plants, which then make the toxin in their own leaves. Bt cotton, grown in India since 2002, is the best-known example; you will see how it is made in Chapter 10.

Around plant roots lives a fungus that preys on other fungi: Trichoderma. It coils around the threads of the fungi that cause root rot, wilt and damping-off and digests them, and it also competes with them for space and food. Seed and soil treatments with Trichoderma are now widely used in India.

Insects catch viral diseases too, and some of those viruses are now farm tools. Baculoviruses infect only insects and their arthropod relatives; the ones sold for pest control mostly belong to the genus Nucleopolyhedrovirus (NPV). Their great strength is precision. A given NPV kills one pest species or a few close relatives, and tests have found it harmless to crops, fish, birds, mammals and other insects. Farmers following integrated pest management (IPM) value this because the natural enemies of pests survive the spray, and so do managers of ecologically sensitive areas, where a broad poison would do great damage.

8.6 Microbes as Biofertilisers

Chemical fertilisers helped India grow enough food, but heavy use has a cost. Much of the nitrogen applied washes into rivers and ground water, feeding algal blooms and raising nitrate levels in wells, and years of overuse can leave soil poorer in organic matter and life. This has pushed many farmers towards organic farming, which relies on biofertilisers (जैव उर्वरक).

Biofertilisers come from three groups of microbes: bacteria, fungi and cyanobacteria (Fig. 8.13).

Nitrogen gas makes up 78 % of the air, but plants cannot use it in that form. Rhizobium bacteria enter the roots of leguminous plants, such as pea, gram, groundnut and soybean, and live inside swollen root nodules. There they convert atmospheric nitrogen into ammonia, which the plant builds into amino acids, and in return they receive sugars. Azotobacter and Azospirillum do the same job without a host: they live free in the soil or loosely on root surfaces, and the nitrogen they fix enriches the soil for whatever crop is grown.

Roots get help from fungi as well. In a mycorrhiza (कवकमूल), fine fungal threads, often of the genus Glomus, grow into or around the roots and fan out through the soil far beyond the root's own reach. Phosphorus is scarce and slow-moving in soil; the threads gather it and hand it over to the plant. The benefits do not stop there: a mycorrhizal plant fends off root pathogens better, survives drought and salty soil better, and simply grows bigger.

Flooded paddy fields have their own nitrogen suppliers: cyanobacteria (blue-green algae) such as Anabaena, Nostoc and Oscillatoria. These photosynthetic microbes thrive in shallow, sunlit water and on damp soil, and many of them fix nitrogen from the air. When they die and decay they leave organic matter that improves the soil, so the paddy farmer gains twice.