Microbes in Human Welfare: NEET notes

1. Microbes all around

NCERT §8 (chapter introduction)

• Microbes are major parts of every biological system, alongside the large plants and animals. They live in soil, water and air, and inside the bodies of humans, other animals and plants.
• They survive where nothing else can: inside geysers (thermal vents) at temperatures up to 100°C, deep in the soil, under snow many metres thick, and in strongly acidic places.
• The microbial world is varied: protozoa, bacteria, fungi, microscopic plant and animal viruses, viroids, and prions, which are infectious agents made of protein.
• Bacteria and many fungi grown on nutritive media form colonies large enough to see without a microscope; such cultures are used to study micro-organisms.
• Microbes cause many diseases of humans, animals and plants, but most are not harmful, and many serve human welfare in several ways.

2. Curd, dough and cheese

NCERT §8.1

• Curd is made from milk by Lactobacillus and other lactic acid bacteria (LAB). The acids they produce while growing coagulate the milk proteins and partly digest them.
• A spoonful of curd added to fresh milk acts as the inoculum or starter: it carries millions of LAB, which multiply at a suitable temperature and turn the milk into curd.
• Curd is more nutritious than milk because LAB raise its vitamin B12 content. In our stomach, LAB also help keep disease-causing microbes in check.
• The dough for dosa and idli is fermented by bacteria; it puffs up because of the CO₂ gas produced.
• Bread dough is fermented with baker's yeast, Saccharomyces cerevisiae.
• Toddy, a traditional drink in parts of southern India, comes from palm sap left to ferment. Fish, soyabean and bamboo shoots are likewise fermented by microbes to make foods.
• Cheese is among the oldest foods made with microbes; each variety owes its texture, flavour and taste to particular microbes.
• The large holes in Swiss cheese come from the large amount of CO₂ produced by the bacterium Propionibacterium sharmanii. Roquefort cheese gets its flavour from a particular fungus grown on it during ripening.

3. Fermented beverages

NCERT §8.2, §8.2.1

• Industrial production needs microbes grown in very large vessels called fermentors.
• Yeasts have been used since ancient times to make wine, beer, whisky, brandy and rum.
• The yeast used is Saccharomyces cerevisiae, the same species used in bread-making; in brewing it is called brewer's yeast.
• It ferments malted cereals and fruit juices, producing ethanol.
• The kind of drink depends on the raw material fermented and on whether the fermented broth is distilled.
• Wine and beer are made without distillation. Whisky, brandy and rum are made by distilling the fermented broth.

4. Antibiotics

NCERT §8.2.2

• Antibiotics are chemicals made by some microbes that kill or slow the growth of other, disease-causing microbes. They count among the major discoveries of the twentieth century.
• The word comes from Greek: 'anti' (against) and 'bio' (life). They act against the life of pathogens, but for human beings they are 'pro life'.
• Penicillin was the first antibiotic discovered, and its discovery was a chance one.
• Alexander Fleming, working on Staphylococci bacteria, saw a mould growing on an unwashed culture plate; around the mould, Staphylococci failed to grow.
• He traced this to a chemical made by the mould and named it Penicillin, after the mould Penicillium notatum.
• Ernest Chain and Howard Florey later established its full value as an antibiotic. It was used widely to treat American soldiers wounded in World War II.
• Fleming, Chain and Florey shared the Nobel Prize in 1945 for this discovery.
• Many more antibiotics were later purified from other microbes. They transformed our ability to treat killer diseases: leprosy (kusht rog), plague, diphtheria (gal ghotu) and whooping cough (kali khansi).

5. Chemicals, enzymes and bioactive molecules

NCERT §8.2.3

• Microbes are used to manufacture organic acids, alcohols and enzymes on a commercial scale.
• Acid producers: Aspergillus niger (a fungus) gives citric acid; Acetobacter aceti (a bacterium) gives acetic acid; Clostridium butylicum (a bacterium) gives butyric acid; Lactobacillus (a bacterium) gives lactic acid.
• Ethanol is produced commercially using the yeast Saccharomyces cerevisiae.
• Lipases go into detergents and help lift oily stains from laundry.
• Bottled fruit juices look clearer than home-made ones because they are clarified with pectinases and proteases.
• Streptokinase comes from the bacterium Streptococcus and is altered by genetic engineering. It serves as a 'clot buster', clearing clots from the blood vessels of patients whose myocardial infarction led to a heart attack.
• Cyclosporin A, an immunosuppressive agent for organ-transplant patients, comes from the fungus Trichoderma polysporum.
• Statins, made by the yeast Monascus purpureus, are sold as blood-cholesterol lowering agents. They work by competitively inhibiting the enzyme that synthesises cholesterol.

6. Primary sewage treatment

NCERT §8.3

• Towns and cities produce huge volumes of waste water every day, much of it human excreta. This municipal waste water is sewage.
• Sewage carries a lot of organic matter and microbes, many of them pathogenic, so it cannot be let straight into rivers and streams.
• So it first passes through sewage treatment plants (STPs), which cut its polluting load. The work is done by heterotrophic microbes already present in the sewage.
• Treatment has two stages: primary and secondary.
• Primary treatment is physical: particles large and small are removed by filtration and sedimentation, in stages.
• Floating debris is removed first by sequential filtration; grit (soil and small pebbles) is then removed by sedimentation.
• All the solids that settle make up the primary sludge; the liquid above (supernatant) is the effluent, which goes on to secondary treatment.

7. Secondary treatment and BOD

NCERT §8.3

• Secondary treatment is biological. The primary effluent is led into large aeration tanks, stirred mechanically all the time, with air pumped in.
• This lets useful aerobic microbes grow vigorously into flocs: masses of bacteria joined with fungal filaments into mesh-like structures.
• As they grow, the microbes use up most of the organic matter in the effluent, which greatly reduces its BOD.
• BOD (biochemical oxygen demand) is the amount of oxygen that would be used up if bacteria oxidised all the organic matter in one litre of water.
• The BOD test measures how fast micro-organisms in a water sample take up oxygen, so it is an indirect measure of organic matter. The higher the BOD of waste water, the more it can pollute.
• Once BOD has fallen enough, the effluent goes to a settling tank where the flocs sediment; this sediment is activated sludge. A small part is pumped back into the aeration tank as inoculum.
• Most of the sludge goes to large anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi of the sludge, giving off methane, hydrogen sulphide and CO₂. These gases form biogas, which burns and can be used as fuel.
• The effluent of secondary treatment is usually released into rivers and streams. This microbial method has been used for more than a century, and no man-made technology has yet matched it.
• Treatment plants have not kept pace with the growth of sewage, so untreated sewage often reaches rivers, polluting them and spreading water-borne diseases. The Ministry of Environment and Forests began the Ganga Action Plan and Yamuna Action Plan, which propose many new STPs so that only treated sewage enters these rivers.

8. Biogas

NCERT §8.4

• Biogas is a mixture of gases, mostly methane, made by microbial activity and usable as fuel.
• The gas a microbe produces depends on the microbe and its substrate. In dough, cheese and beverages the main gas is CO₂.
• Some bacteria that grow anaerobically on cellulosic material make large amounts of methane along with CO₂ and H₂. They are called methanogens; Methanobacterium is a common one.
• Methanogens live in the anaerobic sludge of sewage treatment and in the rumen (a part of the stomach) of cattle, where they help break down cellulose and so support the animal's nutrition.
• Cattle dung (gobar) is therefore rich in methanogens and can be used to generate biogas, called gobar gas.
• A biogas plant has a concrete tank 10-15 feet deep into which bio-wastes and a slurry of dung are fed. A floating cover over the slurry rises as microbes produce gas.
• One outlet pipes the gas to nearby houses for cooking and lighting; another removes the spent slurry, which can be used as fertiliser.
• Biogas plants are built mostly in rural areas, where cattle dung is plentiful. The technology was developed in India mainly through the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

9. Biocontrol agents

NCERT §8.5

• Biocontrol is the use of biological methods to control plant diseases and pests. The chemical insecticides, pesticides and weedicides used instead are toxic to humans and animals and pollute soil, ground water and crops.
• Organic farming relies on natural predation and holds that biodiversity furthers health: a varied landscape is more sustainable, and pests are kept at manageable levels by checks and balances instead of being wiped out.
• Eradicating pests is seen as undesirable, since the useful predatory and parasitic insects that feed on them or use them as hosts would then die out. Knowing the field's predators and pests, their life cycles, feeding and habitats, helps design biocontrol.
• Ladybird beetles (red and black) control aphids, and dragonflies control mosquitoes.
• The bacterium Bacillus thuringiensis (Bt) controls butterfly caterpillars. Its dried spores, sold in sachets, are mixed with water and sprayed on plants such as brassicas and fruit trees.
• When larvae eat the spores, the toxin is released in their gut and kills them, while other insects are unharmed. Genetic engineering has put Bt toxin genes into plants such as Bt-cotton, making them resistant to insect pests.
• The free-living fungus Trichoderma, common in root ecosystems, is being developed to treat plant disease; its species control several plant pathogens.
• Baculoviruses attack insects and other arthropods; most used in biocontrol belong to the genus Nucleopolyhedrovirus. They are species-specific and narrow-spectrum, with no harm shown to plants, mammals, birds, fish or non-target insects, which suits integrated pest management (IPM) and ecologically sensitive areas.

10. Biofertilisers

NCERT §8.6

• Overuse of chemical fertilisers has added to pollution, pushing farmers towards organic farming with biofertilisers: organisms that improve the nutrient quality of soil.
• Biofertilisers come mainly from three groups: bacteria, fungi and cyanobacteria.
• Rhizobium lives symbiotically in the root nodules of leguminous plants and fixes atmospheric nitrogen into organic forms the plant uses.
• Azospirillum and Azotobacter fix atmospheric nitrogen while living freely in the soil, raising its nitrogen content.
• Fungi form symbiotic associations with plant roots called mycorrhiza; many members of the genus Glomus do so. The fungus absorbs phosphorus from the soil and passes it to the plant.
• Plants with mycorrhiza also resist root-borne pathogens, tolerate salinity and drought, and grow and develop better overall.
• Cyanobacteria are autotrophic microbes found in water and on land; many fix atmospheric nitrogen, for example Anabaena, Nostoc and Oscillatoria. They are an important biofertiliser in paddy fields, where blue-green algae also enrich the soil with organic matter.
• Several biofertilisers are now sold commercially in India, and farmers use them to replenish soil nutrients and depend less on chemical fertilisers.

Must-know facts

1. Microbes survive in geysers at up to 100°C, under thick snow, deep in soil and in acidic places; prions are protein infectious agents.
2. Curd: Lactobacillus (LAB); acids coagulate milk proteins; vitamin B12 rises.
3. Idli and dosa dough: fermented by bacteria, puffed by CO₂. Bread: baker's yeast, Saccharomyces cerevisiae.
4. Toddy: fermented palm sap.
5. Swiss cheese holes: CO₂ from Propionibacterium sharmanii. Roquefort cheese: ripened by a fungus.
6. Wine and beer: no distillation. Whisky, brandy, rum: distilled.
7. Penicillin from Penicillium notatum; Fleming noticed it on a Staphylococci plate; Chain and Florey proved its worth; Nobel Prize 1945.
8. Citric acid: Aspergillus niger. Acetic acid: Acetobacter aceti. Butyric acid: Clostridium butylicum. Lactic acid: Lactobacillus.
9. Lipases in detergents; pectinases and proteases clarify fruit juice.
10. Streptokinase (Streptococcus) is a clot buster for heart-attack patients.
11. Cyclosporin A: Trichoderma polysporum, immunosuppressant. Statins: Monascus purpureus, lower cholesterol by competitive inhibition.
12. Primary treatment: physical, filtration and sedimentation; gives primary sludge and effluent.
13. Secondary treatment: aeration tanks, flocs of bacteria and fungal filaments, BOD falls; activated sludge settles, part returned as inoculum.
14. BOD: oxygen used if bacteria oxidised all organic matter in one litre of water; higher BOD means more polluting.
15. Anaerobic sludge digesters give biogas: methane, H₂S, CO₂.
16. Methanogens (Methanobacterium) live in sludge and the cattle rumen; biogas plant tank 10-15 feet deep; IARI and KVIC.
17. Ladybird eats aphids; dragonfly eats mosquitoes; Bt kills butterfly caterpillars; Trichoderma controls plant pathogens.
18. Baculoviruses (Nucleopolyhedrovirus) are species-specific, narrow-spectrum insecticides suited to IPM.
19. Rhizobium: symbiotic in legume nodules. Azospirillum, Azotobacter: free-living nitrogen fixers.
20. Glomus forms mycorrhiza and supplies phosphorus. Anabaena, Nostoc, Oscillatoria fix nitrogen in paddy fields.

Common traps

Key terms

Lactic acid bacteria (LAB)

Bacteria such as Lactobacillus that turn milk into curd by making acids.

Inoculum (starter)

A small amount of a culture added to start fermentation, like curd added to milk.

Fermentor

A very large vessel for growing microbes on an industrial scale.

Distillation

The step that separates whisky, brandy and rum from undistilled wine and beer.

Antibiotic

A chemical made by a microbe that kills or slows the growth of other, disease-causing microbes.

Streptokinase

A clot-dissolving enzyme from Streptococcus, used for heart-attack patients.

Cyclosporin A

An immunosuppressant for transplant patients, made by Trichoderma polysporum.

Statins

Cholesterol-lowering agents from the yeast Monascus purpureus.

Sewage

Municipal waste water, rich in organic matter and microbes.

Primary sludge

The solids that settle out during primary treatment.

Flocs

Mesh-like masses of bacteria and fungal filaments that grow in aeration tanks.

BOD

Biochemical oxygen demand: the oxygen bacteria would use to oxidise all the organic matter in one litre of water.

Activated sludge

The settled flocs after aeration; part returns to the aeration tank as inoculum.

Methanogens

Anaerobic bacteria such as Methanobacterium that make methane from cellulosic material.

Biocontrol

Controlling plant pests and diseases by biological methods instead of chemicals.

Bt

Bacillus thuringiensis, a bacterium whose toxin kills caterpillars.

Baculoviruses

Insect-attacking viruses, mainly Nucleopolyhedrovirus, used as narrow-spectrum insecticides.

Biofertiliser

An organism that enriches the nutrient quality of soil.

Mycorrhiza

A symbiotic association of a fungus, such as Glomus, with plant roots.