Microbes in Human Welfare

NCERT opens this chapter by reminding you that microbes are the most diverse and the most widely spread living things on Earth. The list of microbes includes bacteria, fungi, protozoa and viruses, and also the viroids (naked RNA without a protein coat) and prions (infectious proteins) that you met in Class 11. Bacteria and many fungi can be grown on nutritive media in the laboratory; a single cell multiplies into a visible mass called a colony, and it is these colonies on a plate that let us count and identify microbes we cannot otherwise see.

Microbes occupy places where no other life survives. NCERT gives four examples that turn up as 1-mark questions: they live inside thermal vents (springs) where the temperature may exceed 100 °C, deep inside the soil, under layers of snow several metres thick, and in highly acidic environments. Many of them are on or inside our own bodies as well, mostly without harming us.

The chapter is not about disease. It takes the opposite view: we use microbes every day, and the uses are grouped into six areas that form the sections of the chapter, in this order: household products (8.1), industrial products (8.2, covering fermented beverages, antibiotics, and chemicals, enzymes and bioactive molecules), sewage treatment (8.3), biogas (8.4), biocontrol (8.5) and biofertilisers (8.6). The examiner tests this chapter almost entirely on names and mechanisms, so as you read each section keep a running table with three columns: the microbe (with its correct genus and species spelling), what it makes or does, and where that is used. That table is most of your revision for this chapter.

The most familiar example is curd. Micro-organisms called lactic acid bacteria (LAB), such as Lactobacillus, grow in milk and convert it into curd. A small amount of curd, the starter or inoculum, is added to fresh milk; it contains millions of LAB. At a suitable temperature these multiply, produce acids that coagulate and partially digest the milk proteins, and the milk sets. Two extra points earn marks whenever curd is asked. First, curd improves the nutritional quality of milk by increasing vitamin B₁₂. Second, in our stomach the LAB play a very useful role in checking disease-causing microbes.

The batter of dosa and idli is also fermented by bacteria; the puffed-up appearance of the dough comes from the CO₂ they release. Bread dough is fermented by baker's yeast, Saccharomyces cerevisiae, and again CO₂ from fermentation makes the dough rise. A number of other traditional foods depend on microbes too: toddy, a drink of some parts of southern India, is made by fermenting the sap of palms; and microbes are used to ferment fish, soybean and bamboo shoots into foods.

Cheese is one of the oldest microbial foods. Different cheeses are known by their texture, flavour and taste, and each is produced by a specific microbe. The large holes in Swiss cheese are caused by a large amount of CO₂ produced by the bacterium Propionibacterium sharmanii (NCERT spelling). Roquefort cheese is ripened by growing a specific fungus on it (commonly given as Penicillium roqueforti), which gives the cheese its particular flavour.

NCERT exercise 2 asks you to prove that microbes release gases during metabolism, and this section is the answer: the CO₂ that puffs dosa and idli batter, the CO₂ that raises bread dough, and the CO₂ that makes holes in Swiss cheese are all visible evidence of gas released by microbes. Exercise 1, which sample you would carry to demonstrate microbes under a microscope, is answered by curd: a drop of curd shows rod-shaped Lactobacillus cells and is safe, cheap and easy to obtain.

The same processes that work in a kitchen are scaled up in industry. On an industrial scale microbes are grown in very large vessels called fermentors, and the products recovered include beverages, antibiotics, organic acids, enzymes and other bioactive molecules. Producing them in bulk needs the right microbe, a suitable medium and controlled conditions inside the fermentor.

Fermented beverages. The microbe is once again the yeast Saccharomyces cerevisiae, here called brewer's yeast. It is used to ferment malted cereals and fruit juices and produce ethanol. The type of raw material used and the type of processing decide which beverage results. Wine and beer are produced without distillation; whisky, brandy and rum are produced by distillation of the fermented broth. This one distinction is the entire examinable content of section 8.2.1 and is a favourite 1-mark question.

Notice that the same organism has two names depending on use: baker's yeast when it raises bread and brewer's yeast when it makes alcohol, but the scientific name Saccharomyces cerevisiae is the same, and it also appears later as the producer of ethanol among the industrial chemicals. Write the scientific name, not 'yeast', whenever a question asks for the microbe.

Remember that the fermentor itself is a large tank and not a magic device: the microbe still does the chemistry, and the engineering only keeps temperature, oxygen, pH and nutrients where the microbe grows and secretes best. Questions here are direct: name the microbe used for fermenting malted cereals and fruit juices; name two beverages made without distillation; name two made with distillation.

The word antibiotic comes from the Greek anti, against, and bios, life. NCERT defines antibiotics as chemical substances produced by some microbes that can kill or retard the growth of other, disease-causing, microbes. Both parts of the definition matter: they are made by microbes, and they act against other microbes.

Penicillin was the first antibiotic to be discovered, and it was a chance discovery. Alexander Fleming was working on Staphylococci bacteria when he noticed that a mould growing on one of his unwashed culture plates had stopped the bacteria from growing around it. He found that the mould was Penicillium notatum and that it produced a chemical he named penicillin. The full potential of penicillin as an effective antibiotic was established later by Ernest Chain and Howard Florey. It was used extensively to treat American soldiers wounded in World War II. Fleming, Chain and Florey were awarded the Nobel Prize in 1945 for the discovery.

Antibiotics have greatly improved our capacity to treat deadly diseases. NCERT names four: plague, whooping cough (pertussis), diphtheria (gal ghotu) and leprosy (kusht rog). In the CBSE syllabus the phrase attached to this topic is 'antibiotics: production and judicious use', so be ready to write a line on why antibiotics must not be misused: unnecessary or incomplete courses select for resistant bacteria, and the drug then fails against the very infection it was meant for.

NCERT exercise 6 asks for two species of fungus used to produce antibiotics. Give Penicillium notatum (penicillin) and Penicillium chrysogenum (the high-yielding industrial strain), or name Cephalosporium acremonium for cephalosporins; either pair is accepted. Note that many antibiotics, such as streptomycin, come from the bacterium Streptomyces, not from a fungus, so read the question for the word 'fungus'.

This section is a list, and the examiner tests the list as a table. Learn each pair and whether the microbe is a bacterium or a fungus.

Organic acids and alcohol. Aspergillus niger, a fungus, produces citric acid. Acetobacter aceti, a bacterium, produces acetic acid. Clostridium butylicum, a bacterium, produces butyric acid. Lactobacillus, a bacterium, produces lactic acid. The yeast Saccharomyces cerevisiae is used for the commercial production of ethanol.

Enzymes. Lipases are used in detergent formulations, where they help remove oily stains from laundry. Pectinases and proteases are used to clarify bottled fruit juices, which is why bottled juices are clearer than home-made ones: the enzymes break down the pectin and protein that make fresh juice cloudy. Streptokinase, produced by the bacterium Streptococcus and modified by genetic engineering, is used as a 'clot buster' to remove clots from the blood vessels of patients who have undergone myocardial infarction (a heart attack).

Bioactive molecules. A bioactive molecule is a microbial product that has a specific biological effect on the body when used as a drug. NCERT gives two. Cyclosporin A, produced by the fungus Trichoderma polysporum, is an immunosuppressive agent used in organ-transplant patients, where it suppresses the immune rejection of the graft. Statins, produced by the yeast Monascus purpureus, are commercialised as blood-cholesterol lowering agents; they act by competitively inhibiting the enzyme responsible for the synthesis of cholesterol.

When a table question gives you six organisms and asks for the product and its use, write both columns: the product alone is usually half the mark. For example, Trichoderma polysporum: cyclosporin A, immunosuppressant for transplant patients; Monascus purpureus: statins, lower blood cholesterol; Streptococcus: streptokinase, dissolves blood clots after a heart attack; Aspergillus niger: citric acid; Acetobacter aceti: acetic acid; Clostridium butylicum: butyric acid.

Cities and towns generate huge amounts of municipal waste-water, called sewage. It contains large amounts of organic matter and microbes, many of them pathogenic, so it cannot be discharged into rivers and streams directly. It is treated in sewage treatment plants (STPs), and the treatment is carried out by the heterotrophic microbes naturally present in the sewage. The treatment has two stages.

Primary treatment is physical. Particles, large and small, are removed from the sewage by filtration and sedimentation, in stages. First, floating debris is removed by sequential filtration (screens). Then the grit (soil and small pebbles) is removed by sedimentation. All the solids that settle form the primary sludge, and the supernatant is the primary effluent, which goes for secondary treatment.

Secondary or biological treatment uses microbes. The primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped in. This allows vigorous growth of useful aerobic microbes into flocs: masses of bacteria associated with fungal filaments to form mesh-like structures. While growing, these microbes consume the major part of the organic matter in the effluent, which brings down its BOD.

Biochemical oxygen demand (BOD) 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, so BOD is an indirect measure of the organic matter present: the greater the BOD of waste water, the greater its polluting potential. Sewage is treated until its BOD is reduced.

Once the BOD has fallen significantly, the effluent is passed into a settling tank, where the bacterial flocs are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank as inoculum, so that the next batch starts with a ready population of flocs. The remaining major part is pumped into large tanks called anaerobic sludge digesters. Here anaerobic bacteria digest the bacteria and fungi of the sludge and, during this digestion, produce a mixture of gases, methane, hydrogen sulphide and carbon dioxide. This mixture is biogas and can be used as a source of energy. The effluent from secondary treatment is then released into natural water bodies such as rivers and streams.

NCERT closes the section with the river clean-up: because untreated sewage is the main polluter of Indian rivers, the Ministry of Environment and Forests initiated the Ganga Action Plan and the Yamuna Action Plan to save these rivers by building a large number of sewage treatment plants.

Biogas is a mixture of gases, predominantly methane, produced by microbial activity and used as a fuel. Which gases a microbe produces during growth depends on the microbe and on the organic substrate it uses. 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 the common example NCERT names is Methanobacterium.

Methanogens occur in two places you must be able to name. They are found in the anaerobic sludge during sewage treatment, which is why the sludge digester produces biogas. They are also present in the rumen (a part of the stomach) of cattle, where they help break down the cellulose of the fodder and so play an important part in the nutrition of cattle. Because of this, the excreta (dung) of cattle, called gobar, is rich in methanogens, and dung is the ideal feed for a biogas plant; biogas from dung is therefore called gobar gas.

The biogas plant. A typical plant is a concrete tank, 10 to 15 feet deep, in which bio-wastes are collected and a slurry of dung and water is fed through an inlet. A floating cover (gas holder) is placed over the slurry; it rises as gas is produced in the tank by microbial activity. The plant has an outlet connected to a pipe that carries the biogas to nearby houses for cooking and lighting. The spent slurry is removed through another outlet and used as fertiliser, so nothing is wasted.

Biogas plants are common in rural areas because cattle dung is available in large quantities there and the plant is simple and cheap to run. The technology of biogas production was developed in India mainly through the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

The sequence of events inside the digester is the answer to the 3-mark question: organic waste and dung slurry are fed in; anaerobic conditions develop; the methanogens (and other anaerobic bacteria) act on the cellulosic material; methane, CO₂ and H₂ are released and collect under the floating cover; the gas is piped out; the spent slurry leaves as fertiliser. Note that an inoculum of methanogens is not added separately because dung already carries them.

Biocontrol is the use of biological methods for controlling plant diseases and pests. Chemical insecticides and pesticides are toxic, they harm non-target organisms, and they pollute soil and water and reach our food. The biological approach is the one that organic farming relies on. It starts from an ecological view: a field is a web of predators and prey, and the farmer's job is to keep pests below damaging levels, not to eradicate them. Complete eradication would also remove the food of the useful predators and parasites that depend on the pest.

NCERT gives two non-microbial examples first: the ladybird (a beetle with red and black markings) is used to get rid of aphids, and dragonflies are used against mosquitoes. The rest of the section is about microbes.

Bacteria: Bacillus thuringiensis (Bt). This is the microbial biocontrol agent used against butterfly caterpillars. It is sold in sachets as dried spores, which are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees. The insect larvae eat the sprayed leaves, and in the gut of the larva the toxin is released and the larva is killed. Because the toxin acts only when eaten by the larva, other insects are unaffected. By the methods of genetic engineering, the Bt toxin genes have been introduced into plants, giving crops such as Bt cotton that are resistant to insect pests; this is taken up in the biotechnology chapters.

Fungi: Trichoderma. Trichoderma species are free-living fungi that are very common in the root ecosystems. They are effective biocontrol agents of several plant pathogens, protecting the roots from soil-borne fungal diseases.

Viruses: baculoviruses. Baculoviruses are pathogens that attack insects and other arthropods. The majority of baculoviruses used as biological control agents belong to the genus Nucleopolyhedrovirus. They are excellent candidates for species-specific, narrow-spectrum insecticidal applications: they have no negative impact on plants, mammals, birds, fish or even non-target insects. This makes them especially desirable when beneficial insects must be conserved as part of an integrated pest management (IPM) programme, and when an ecologically sensitive area is being treated.

Chemical fertilisers raise yields but they are a major source of pollution of soil and water, and the Green Revolution made farmers depend on them. Biofertilisers are the alternative. NCERT's definition is short and should be quoted as it is: biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources are bacteria, fungi and cyanobacteria.

Bacteria. Rhizobium lives in a symbiotic association in the root nodules of leguminous plants, where it fixes atmospheric nitrogen into organic forms that the plant uses as nutrient. It is called symbiotic because both partners gain: the plant gets fixed nitrogen and the bacterium gets food and shelter in the nodule. Other bacteria fix nitrogen while living free in the soil: Azospirillum and Azotobacter. These are the two names to give when a question asks for free-living nitrogen-fixing biofertilisers, and they are the ones you recommend to a farmer growing a non-leguminous crop who cannot use Rhizobium.

Fungi. Many fungi form symbiotic associations with plants called mycorrhiza. Members of the genus Glomus form mycorrhiza. The fungal symbiont absorbs phosphorus from the soil and passes it to the plant. Plants with mycorrhiza show three further benefits that NCERT lists and the examiner expects: resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development.

Cyanobacteria. These are autotrophic microbes widely distributed in aquatic and terrestrial environments, and many of them fix atmospheric nitrogen: Anabaena, Nostoc, Oscillatoria. In paddy fields cyanobacteria serve as an important biofertiliser. Blue-green algae also add organic matter to the soil and increase its fertility, which is the second reason for adding them to agricultural soil.

A number of biofertilisers are now sold commercially in India, and farmers use them to replenish soil nutrients and reduce dependence on chemical fertilisers. When asked why farmers prefer biofertilisers, give three reasons: they are cheap and renewable, they do not pollute soil or water, and they improve soil structure and fertility over time instead of degrading it.

NCERT exercise 13 also asks about the role of microbes in single cell protein (SCP): microbes such as Spirulina, yeast and Methylophilus methylotrophus are grown in bulk and their biomass is used as a protein-rich food or feed; and in soil, where microbes decompose organic matter, cycle nutrients and fix nitrogen.