Microbes in Human Welfare

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 8.

The curd in your fridge is a microbe farm

Most of the microbes you will ever meet are not out to harm you. A spoonful of curd holds millions of living bacteria, working for you. The bread on your table rose because of yeast. The antibiotic your doctor prescribes, the clean water in your river, the gas in a village kitchen and the nitrogen in a farmer's field all depend on microbes. This chapter is a catalogue of what each one does, and CUET asks it as "match the microbe with the product".

Where microbes live and what they are

Microbes are found everywhere: soil, water, air, inside your body and in other animals and plants. They even live where no other life can, such as deep thermal vents where the temperature may reach about 100 °C, deep soil, under metres of snow and in highly acidic places. The group includes protozoa, bacteria, fungi, microscopic animal and plant viruses, viroids and prions (proteinaceous infectious agents). Bacteria and many fungi can be grown on nutrient medium, where they form colonies visible to the naked eye.

Microbes in household food

Curd. Microbes called lactic acid bacteria (LAB), such as Lactobacillus, grow in milk and convert it to curd. They make acids that coagulate and partly digest the milk proteins, which is what turns liquid milk into a solid. A small amount of curd added to fresh milk, called the inoculum or starter, contains millions of LAB. At a suitable temperature they multiply and convert the milk to curd. LAB also improve nutrition by adding vitamin B12, and in the stomach they help check disease-causing microbes.

Dosa, idli and bread. The dough for dosa and idli is fermented by bacteria. The puffed-up look comes from the carbon dioxide (CO2) they release. Bread dough is fermented using baker's yeast, Saccharomyces cerevisiae. Toddy, a traditional drink of southern India, is made by fermenting palm sap. Fish, soyabean and bamboo shoots are also fermented to make foods.

Beyond the textbook. For the exam: idli and dosa batter is fermented by bacteria, and the CO2 they release makes it puff up. In reality: the main bacteria are lactic acid bacteria, especially Leuconostoc, and yeasts also take part. They make both the CO2 that raises the batter and the acid that gives the sour taste. Why it matters: the answer "bacteria produce CO2" is right for the exam, and the real picture is a small ecosystem of microbes working together.

Cheese. Cheese is one of the oldest foods made with microbes, and different microbes give different flavours and textures. The large holes in Swiss cheese come from the large amount of CO2 made by the bacterium Propionibacterium sharmanii. Roquefort cheese is ripened by growing a specific fungus on it, which gives its particular flavour.

Pause and predict: your grandmother sets curd at night in summer and it is ready by morning, but in a cold winter kitchen it stays milk. What does this tell you about LAB?

Check your answer: LAB multiply only in a suitable warm temperature. In cold conditions they grow slowly, so too little acid is made to coagulate the milk. This is why curd sets only "at suitable temperatures".

Microbes in industry

Fermentors. On an industrial scale microbes are grown in very large vessels called fermentors (bioreactors), where conditions can be controlled.

Fermented drinks. Yeast has been used since ancient times to make wine, beer, whisky, brandy and rum. The same yeast, Saccharomyces cerevisiae, is called brewer's yeast when used for this purpose. It ferments malted cereals and fruit juices to make ethanol.

Antibiotics. The word means "against life" (Greek anti = against, bio = life), against the life of pathogens but for the life of humans. An antibiotic is a chemical made by some microbes that can kill or slow the growth of other, disease-causing microbes.

The story begins with Alexander Fleming. While working on Staphylococci bacteria he noticed a mould growing on one plate, and bacteria could not grow around it. It was a chance discovery. He named the substance penicillin, after the mould Penicillium notatum. Its full potential as an effective antibiotic was established much later by Ernest Chain and Howard Florey. Penicillin was used widely to treat American soldiers wounded in World War II. Fleming, Chain and Florey shared the Nobel Prize in 1945. Antibiotics let doctors treat plague, whooping cough, diphtheria and leprosy.

Beyond the textbook. For the exam: penicillin comes from Penicillium notatum. In reality: modern studies have placed Fleming's mould in the species Penicillium rubens, and factories now use high-yielding strains of P. chrysogenum. Also, antibiotics work against bacteria and not against viruses, and careless overuse is making bacteria resistant. Why it matters: names in science can be revised, but the name Penicillium notatum is the one the exam wants.

Chemicals, enzymes and bioactive molecules. Microbes are used to make organic acids, alcohols and enzymes. This is one of the most tested tables in the chapter.

Enzymes are also useful. Lipases are added to detergents to remove oily stains. Pectinases and proteases clarify bottled fruit juices. Streptokinase dissolves blood clots and is given to patients who have had a heart attack (myocardial infarction). Cyclosporin A is given to organ-transplant patients to stop the immune system from rejecting the new organ. Statins lower blood cholesterol by competitively inhibiting the enzyme that makes cholesterol.

Beyond the textbook. For the exam: cyclosporin A comes from Trichoderma polysporum and statins from the yeast Monascus purpureus. In reality: the fungus that makes cyclosporin is now named Tolypocladium inflatum, and Monascus is a filamentous mould, the same one that gives "red yeast rice", despite the yeast label. Why it matters: Trichoderma appears again in this chapter as a biocontrol agent, so keep the two roles apart. Also, streptokinase (a clot buster) must not be confused with streptomycin (an antibiotic).

Sewage treatment: cleaning the water we spoil

Every city produces sewage, the waste water from homes and industry. It is full of organic matter and pathogens and must not be thrown into rivers raw. It is cleaned in sewage treatment plants (STPs) by the microbes that already live in sewage, mostly heterotrophic ones. It happens in stages.

Flocs are masses of bacteria held together with fungal filaments, forming mesh-like structures. They are the working units of the aeration tank, and their large surface lets them trap and digest organic matter quickly.

BOD, explained. Biochemical oxygen demand is the amount of oxygen that bacteria would use up if they oxidised all the organic matter in one litre of water. Microbes eat organic matter and use oxygen to do it. So the more organic matter, the more oxygen is demanded, and the higher the BOD. That is why higher BOD means more polluting potential, and why we measure BOD to see how well secondary treatment has worked. BOD is an indirect measure of organic matter.

Pause and predict: a sample of treated water has a lower BOD than the raw sewage it came from. Is the treated water more or less polluted?

Check your answer: less. Lower BOD means less organic matter left for microbes to consume, so the water will not strip oxygen from a river.

Ganga and Yamuna Action Plans. These were started by the Ministry of Environment and Forests to build STPs so that only treated sewage flows into the rivers.

Biogas: fuel from dung

Biogas is a mixture of gases, mostly methane, produced by microbes and used as fuel. It is made by methanogens, bacteria that grow anaerobically on cellulosic material and produce methane, CO2 and hydrogen. A common one is Methanobacterium. Methanogens are found in anaerobic sludge and in the rumen (a part of the stomach) of cattle, where they help to digest cellulose and add to the animal's nutrition. Cattle dung (gobar) is therefore rich in them, and the gas made from it is called gobar gas.

A biogas plant has a concrete tank 10 to 15 feet deep into which a slurry of dung is fed. A floating cover rises as gas collects. The gas goes by pipe to nearby houses, and the spent slurry leaves through another outlet and is used as fertiliser. The technology was developed in India mainly by IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission).

Biocontrol: using nature to protect crops

Biocontrol means using living organisms to control pests and diseases instead of toxic chemicals. The organic farmer's aim is not to kill every pest but to keep pests at manageable levels. A modern pest-control plan that uses biocontrol and spares other insects is called Integrated Pest Management (IPM).

Bt toxin genes have also been engineered into crop plants, which is how Bt cotton was produced.

Beyond the textbook. For the exam: Bt spores are sprayed, the larva ingests them and the toxin is released in its gut. In reality: the Bt toxin is made as a crystal of an inactive protein. In the alkaline gut of the insect larva it is activated and punches holes in the gut wall. Mammals have an acidic stomach, so the same protein does not act in the same way. Why it matters: this explains why Bt kills caterpillars but leaves other animals unharmed, and it is the link to the Bt cotton story in Biotechnology and its Applications.

Biofertilisers: microbes that feed the soil

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources are bacteria, fungi and cyanobacteria. They are used to cut down the use of chemical fertilisers.

Pause and predict: why are cyanobacteria such good biofertilisers for flooded paddy fields in particular?

Check your answer: they are photosynthetic and need only light, water and air to grow, and they fix their own nitrogen. A flooded paddy field gives them all of that, and when they die they add organic matter to the soil.

Exam traps

60-second revision

Where this chapter connects

• Earlier: Biological Classification introduced methanogens, cyanobacteria with heterocysts, Trichoderma and the fungal classes that appear here as microbes with a use.
• Same class: Biotechnology and its Applications picks up the Bt toxin and shows how its gene was put into cotton.
• Immunity link: Human Health and Disease explains why cyclosporin A, the immunosuppressant, prevents organ rejection.
• Ecology link: Organisms and Populations covers mutualism, of which mycorrhiza and root nodules are classic examples.