Big picture
Microbes are often thought of as germs, but most are harmless and many are extremely useful. They make our curd and bread, give us antibiotics and industrial chemicals, clean our sewage, produce fuel, control pests and fertilise crops.
This chapter is a list of uses and the organisms responsible. The exam asks "Name the microbe that…" questions, so build a table of organism–product pairs and learn it thoroughly.
Learning goals
Explain the role of microbes in household products.
Name the microbes used to produce beverages, antibiotics, chemicals, enzymes and bioactive molecules.
Explain primary and secondary sewage treatment and the meaning of BOD.
Explain biogas production and the biogas plant.
Explain the use of microbes as biocontrol agents and in integrated pest management.
Explain the use of microbes as biofertilisers.
Core ideas
1. Microbes in household products
Product | Microbe | Role |
|---|---|---|
Curd | Lactic acid bacteria (LAB), such as Lactobacillus | Convert milk to curd by producing lactic acid, which coagulates and partly digests milk proteins. LAB also increase vitamin B₁₂ and check disease-causing microbes in the stomach. A small amount of curd added to fresh milk acts as the inoculum (starter) |
Dough for bread | Baker's yeast, Saccharomyces cerevisiae | Fermentation releases CO₂, which puffs up the dough |
Dosa and idli batter | Bacteria | Fermentation produces CO₂, which makes the batter rise and gives a puffed appearance |
Toddy (traditional drink of south India) | Microbes ferment sap from palms | |
Swiss cheese | Propionibacterium sharmanii | Produces a large amount of CO₂, giving the large holes |
Roquefort cheese | Fungi | Ripened by a specific fungus, giving its particular flavour |
2. Microbes in industrial products
Industrial production uses microbes grown in very large vessels called fermentors.
Fermented beverages. Saccharomyces cerevisiae (brewer's yeast) ferments malted cereals and fruit juices to produce ethanol.
Without distillation: wine and beer.
With distillation: whisky, brandy and rum (higher alcohol content).
Antibiotics. Chemical substances produced by some microbes that kill or retard the growth of other (disease-causing) microbes.
Penicillin, the first antibiotic, was discovered by Alexander Fleming. He noticed that a mould, *Penicillium notatum, growing on unwashed culture plates of Staphylococcus* bacteria stopped their growth around it.
Its full potential as an effective antibiotic was established later by Ernest Chain and Howard Florey. It was used extensively to treat soldiers in World War II. Fleming, Chain and Florey received the Nobel Prize in 1945.
Antibiotics have greatly reduced deaths from diseases such as plague, whooping cough, diphtheria and leprosy.
Chemicals, enzymes and bioactive molecules
Product | Microbe | Use |
|---|---|---|
Citric acid | Aspergillus niger (fungus) | Food and drinks |
Acetic acid | Acetobacter aceti (bacterium) | Vinegar |
Butyric acid | Clostridium butylicum (bacterium) | |
Lactic acid | Lactobacillus (bacterium) | |
Ethanol | Saccharomyces cerevisiae (yeast) | Industrial alcohol and fuel |
Lipases | Microbes | In detergent formulations to remove oily stains from laundry |
Pectinases and proteases | Microbes | Clarify bottled fruit juices (juices are clearer than home-made ones) |
Streptokinase | Streptococcus (modified by genetic engineering) | A "clot buster": removes blood clots in blood vessels of patients who have had a heart attack (myocardial infarction) |
Cyclosporin A | Trichoderma polysporum (fungus) | An immunosuppressive agent used in organ transplant patients |
Statins | Monascus purpureus (yeast) | Lower blood cholesterol by competitively inhibiting the enzyme that synthesises cholesterol |
3. Microbes in sewage treatment
Sewage is the large volume of waste water from cities and towns, containing organic matter and microbes, many of them pathogenic. It is treated in sewage treatment plants (STPs) before being discharged into rivers.
Primary treatment (physical):
Removes particles, large and small, by filtration and sedimentation.
Floating debris is removed by sequential filtration; grit (soil and small pebbles) by sedimentation.
The settled solids form primary sludge; the supernatant is the primary effluent, which goes to secondary treatment.
Secondary treatment (biological):
The primary effluent is passed into large aeration tanks, where it is constantly agitated and air is pumped in.
This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments in a mesh-like structure).
These microbes consume most of the organic matter, greatly reducing the BOD of the effluent.
When BOD has been reduced significantly, the effluent goes to a settling tank, where the bacterial flocs settle. This sediment is called activated sludge.
A small part of the activated sludge is pumped back into the aeration tank as inoculum. The rest is pumped into large tanks called anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi, producing a mixture of gases such as methane, hydrogen sulphide and CO₂. These gases form biogas.
The treated effluent is released into rivers and streams.
BOD (biochemical oxygen demand) is the amount of oxygen consumed if all the organic matter in one litre of water were oxidised by bacteria. It is a measure of the organic matter in water: the greater the BOD, the more polluted the water.
The Ministry of Environment and Forests started the Ganga Action Plan and Yamuna Action Plan to build STPs so that only treated sewage enters these rivers.
4. Microbes in biogas production
Biogas is a mixture of gases, predominantly methane, produced by microbial activity. It is used as fuel.
Bacteria that produce methane from cellulosic material are called methanogens, for example *Methanobacterium*.
They are found in anaerobic sludge in STPs and in the rumen (part of the stomach) of cattle, where they help break down cellulose. So cattle dung (gobar) is rich in methanogens and is used to make gobar gas.
Biogas plant: a concrete tank (10–15 feet deep) in which bio-wastes and dung slurry are fed. A floating cover rises as gas is produced. Gas is piped for cooking and lighting, and the spent slurry is used as fertiliser.
Biogas technology in India was developed mainly by the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).
5. Microbes as biocontrol agents
Biocontrol means using biological methods to control plant diseases and pests, instead of chemical pesticides that harm humans, animals and the environment.
Biocontrol agent | Controls |
|---|---|
Ladybird beetle | Aphids |
Dragonflies | Mosquitoes |
*Bacillus thuringiensis* (Bt) | Butterfly caterpillars. Spores are sprayed on plants; when eaten by larvae, a toxin is released in the gut that kills them. Bt toxin genes have also been introduced into plants (Bt cotton) |
*Trichoderma* (free-living fungi in the root ecosystem) | Plant pathogens |
Baculoviruses, especially genus *Nucleopolyhedrovirus* | Insects and arthropods. They are species-specific, narrow-spectrum, with no negative impact on plants, mammals, birds or non-target insects; ideal for integrated pest management (IPM), especially in ecologically sensitive areas |
Integrated pest management relies on natural predation rather than eliminating every pest. Organic farmers keep a balance of pests and their predators, because killing all insects also kills beneficial ones.
6. Microbes as biofertilisers
Biofertilisers are organisms that enrich the nutrient quality of the soil, reducing the need for chemical fertilisers.
Biofertiliser | Role |
|---|---|
*Rhizobium* (symbiotic bacteria) | Forms root nodules on leguminous plants and fixes atmospheric nitrogen into organic forms used by the plant |
*Azospirillum and Azotobacter* (free-living bacteria in soil) | Fix atmospheric nitrogen, enriching soil nitrogen |
Mycorrhiza (symbiosis between fungi, e.g. genus *Glomus*, and plant roots) | The fungus absorbs phosphorus from the soil and passes it to the plant; it also gives resistance to root pathogens, tolerance to salinity and drought, and better growth |
Cyanobacteria (autotrophic, aquatic and terrestrial), such as *Anabaena, Nostoc, Oscillatoria* | Fix atmospheric nitrogen; important biofertilisers in paddy fields; also add organic matter and increase soil fertility |
A reliable approach to questions
For "name the organism" questions, recall the table pairs: citric acid (Aspergillus niger), acetic acid (Acetobacter aceti), Swiss cheese (Propionibacterium sharmanii), statins (Monascus purpureus), cyclosporin A (Trichoderma polysporum), streptokinase (Streptococcus), methane (Methanobacterium).
For sewage treatment, separate physical steps (primary) from biological steps (secondary), and use BOD correctly.
For biocontrol, match the agent to the pest.
For biofertilisers, decide whether the organism is symbiotic (Rhizobium, mycorrhiza) or free-living (Azotobacter, Azospirillum, cyanobacteria).
Connections that matter
Biotechnology and its applications explains Bt crops and genetically engineered streptokinase.
Ecosystem covers decomposition and nutrient cycling by microbes.
Human health and disease covers pathogens and immunity, which antibiotics and cyclosporin A affect.
Common mistakes
Saying higher BOD means cleaner water. Higher BOD means more organic pollution.
Calling primary treatment biological. It is physical; secondary treatment is biological.
Confusing *Trichoderma* uses. Trichoderma species act as biocontrol agents, and Trichoderma polysporum gives cyclosporin A.
Saying wine is distilled. Wine and beer are made without distillation.
Calling *Azotobacter* symbiotic. It is free-living.
Revision checklist
I can name the microbes behind curd, bread, idli and cheeses.
I can name the microbes behind beverages, antibiotics, acids, enzymes and bioactive molecules, and their uses.
I can explain primary and secondary sewage treatment and BOD.
I can explain biogas production.
I can match biocontrol agents to pests and explain IPM.
I can explain each biofertiliser.