Microbes in household products and industry
The curd in your kitchen, the soft idli batter and the bread in a bakery all depend on microbes that work for us quietly.
Lactic acid bacteria (LAB), such as Lactobacillus, turn milk into curd. Baker's yeast (Saccharomyces cerevisiae) makes dough rise by producing CO₂. In industry, fermentation by microbes gives alcoholic drinks, organic acids, antibiotics and enzymes.
In the figure : The starter has millions of LAB; warmth helps them grow.
In the figure : Many foods and chemicals depend on fermentation.
Rule 1: Household products — LAB also check disease-causing microbes in our stomach. The puffed-up dough is due to CO₂ released by yeast. Roquefort cheese is ripened by a particular fungus; the flavour of different cheeses comes from different microbes.
- Easy : LAB convert lactose to lactic acid.
- Medium : The large holes of Swiss cheese are due to CO₂.
- Exam-type : Q: Which gas makes dough rise? A: Carbon dioxide.
Rule 2: Industrial products — Fermented beverages use the same yeast Saccharomyces cerevisiae (brewer's yeast) on malted cereals and fruit juices. Antibiotics: penicillin was noticed by Alexander Fleming (on Staphylococcus plates) and developed by Chain and Florey; the three shared the Nobel Prize in 1945. Enzymes and bioactive molecules: lipases (detergents), pectinases and proteases (clearing fruit juices), streptokinase (Streptococcus; clot buster), cyclosporin A (fungus Trichoderma polysporum; immunosuppressant) and statins (yeast Monascus purpureus; lower blood cholesterol).
- Easy : Streptokinase removes blood clots.
- Medium : Cyclosporin A is used in organ transplants.
- Exam-type : Q: Name the source of statins. A: The yeast Monascus purpureus.
Fleming, Chain and Florey got the Nobel Prize in 1945 for penicillin. Wine and beer are not distilled; whisky, brandy and rum are distilled.
Writing that all alcoholic drinks are distilled. Wine and beer are only fermented.
Memory clue: WB no-D, WBR D — Wine and Beer: no Distillation; Whisky, Brandy, Rum: Distilled.
Spot the difference
| Point | Wine, beer | Whisky, brandy, rum |
|---|---|---|
| Process | Fermentation only | Fermentation then distillation |
| Yeast | Saccharomyces cerevisiae | Saccharomyces cerevisiae |
| Alcohol content | Lower | Higher |
Practice questions
- 1Which bacteria change milk to curd?
- 2Name the yeast used in bread making.
- 3Who discovered penicillin?
- 4What is streptokinase used for?
- 5Which microbe gives cyclosporin A?
- 6Which product lowers blood cholesterol?
- 7Why are holes formed in Swiss cheese?
- 8Why is a little curd added to warm milk to set curd?
Show answers (try them yourself first)
- 1Lactic acid bacteria, such as Lactobacillus.
- 2Saccharomyces cerevisiae.
- 3Alexander Fleming.
- 4To remove blood clots (clot buster).
- 5The fungus Trichoderma polysporum.
- 6Statins, from Monascus purpureus.
- 7The bacterium Propionibacterium sharmanii produces large amounts of CO₂.
- 8It acts as a starter with LAB that multiply and convert milk to curd.
Sewage treatment and biogas
Every city produces huge amounts of dirty water. Microbes clean it in a series of tanks before it returns to the river.
Sewage is cleaned in two stages. Primary treatment is physical: filtration and sedimentation remove solids. Secondary (biological) treatment uses aerobic microbes that form flocs in aeration tanks, reducing the BOD (biochemical oxygen demand). The settled flocs are called activated sludge. Biogas (mainly methane) is made by methanogens in anaerobic digesters.
In the figure : Part of the activated sludge is pumped back as inoculum to the aeration tank.
In the figure : The slurry left behind is used as manure.
Rule 1: Sewage treatment — High BOD means more organic matter and more pollution. In the aeration tank the sewage is stirred with air so that aerobic microbes grow into flocs, consume organic matter and BOD falls. Settled sludge goes to anaerobic sludge digesters where anaerobic bacteria digest the bacteria and fungi and release gases such as methane, hydrogen sulphide and CO₂ (biogas). The effluent is released to rivers.
- Easy : BOD measures the organic matter in water.
- Medium : Low BOD means clean water.
- Exam-type : Q: What is activated sludge? A: Sediment of bacterial flocs in the settling tank.
Rule 2: Biogas — Methanogens such as Methanobacterium grow anaerobically on cellulosic material and are also present in the rumen of cattle. Biogas (gobar gas) is a fuel for cooking and lighting; the technology was developed in India through IARI and KVIC. Dung is fed in a digester tank with a floating gas holder.
- Easy : Biogas is mostly methane.
- Medium : Methanogens live in cattle rumen too.
- Exam-type : Q: Name a methanogen. A: Methanobacterium.
Biogas: methane + CO₂ (+ H₂, H₂S). Sewage treatment: primary (physical) → secondary (biological).
Thinking primary treatment uses microbes. It only removes solids physically.
Memory clue: P-physical, S-microbes — Primary = Physical; Secondary = microbes (biological).
Spot the difference
| Point | Primary treatment | Secondary treatment |
|---|---|---|
| Type | Physical | Biological |
| Removes | Floating and settling solids | Organic matter (BOD falls) |
| Microbes | No | Aerobic microbes, flocs |
Practice questions
- 1What does BOD mean?
- 2What are flocs?
- 3Which gas is the main component of biogas?
- 4Name two institutions that developed biogas technology in India.
- 5What is the use of an anaerobic sludge digester?
- 6Name the bacteria that make methane.
- 7Why does high BOD mean polluted water?
- 8Why is part of the activated sludge pumped back to the aeration tank?
Show answers (try them yourself first)
- 1Biochemical oxygen demand.
- 2Masses of bacteria associated with fungal filaments.
- 3Methane.
- 4IARI and KVIC.
- 5To digest the sludge anaerobically and make biogas.
- 6Methanogens (e.g. Methanobacterium).
- 7More organic matter needs more oxygen for microbes, reducing oxygen for aquatic life.
- 8It works as inoculum for the next batch.
Microbes as biocontrol agents and biofertilisers
Instead of spraying poison on crops, a farmer can use a ladybird, a bacterial powder or a fungus-friend, and enrich the soil without chemical fertilisers.
Biocontrol is the use of living organisms to control pests. Biofertilisers are organisms that enrich the soil with nutrients, such as Rhizobium, Azospirillum, Azotobacter, cyanobacteria and mycorrhiza.
In the figure : These replace chemical pesticides and fertilisers.
Rule 1: Biocontrol — Bt is sold as dried spores in sachets and sprayed on plants like brinjal and apple; the toxin kills the larvae but not other organisms. Bt cotton carries the Bt toxin gene. Trichoderma controls root diseases. Baculoviruses are species-specific and narrow-spectrum; they are useful in integrated pest management.
- Easy : Bt controls butterfly caterpillars.
- Medium : Baculoviruses attack insects, not plants.
- Exam-type : Q: What is Bt? A: Bacillus thuringiensis, a bacterium that produces insect-killing proteins.
Rule 2: Biofertilisers — Rhizobium forms nodules in legume roots and fixes atmospheric nitrogen; Azospirillum and Azotobacter fix nitrogen in the soil; cyanobacteria are important in paddy fields; mycorrhiza (Glomus) absorbs phosphorus and gives resistance to salinity and drought. Biofertilisers reduce chemical fertiliser use.
- Easy : Rhizobium lives in root nodules.
- Medium : Mycorrhiza improves phosphorus uptake.
- Exam-type : Q: Which cyanobacteria are used as biofertilisers? A: Anabaena, Nostoc and Oscillatoria.
Biocontrol reduces toxic pesticide use. Biofertilisers: bacteria, cyanobacteria, fungi that improve nutrient supply.
Saying Rhizobium is free-living. Rhizobium is symbiotic; Azotobacter and Azospirillum are free-living.
Memory clue: R-symbiont, A-free — Rhizobium = in roots (symbiotic); Azotobacter/Azospirillum = free in soil.
Spot the difference
| Point | Rhizobium | Azotobacter |
|---|---|---|
| Habitat | Root nodules of legumes | Free in the soil |
| Association | Symbiotic | Free-living |
| Function | Fixes N₂ | Fixes N₂ |
Practice questions
- 1Define biocontrol.
- 2Name a bacterium used to kill caterpillars.
- 3Which fungus controls plant diseases?
- 4Which biofertilisers live in root nodules?
- 5What does mycorrhiza help the plant to absorb?
- 6Name a nitrogen-fixing cyanobacterium.
- 7Why are biofertilisers better than chemical fertilisers?
- 8Why is Bt cotton resistant to bollworms?
Show answers (try them yourself first)
- 1Control of pests by using living organisms.
- 2Bacillus thuringiensis (Bt).
- 3Trichoderma.
- 4Rhizobium.
- 5Phosphorus (and water).
- 6Anabaena (or Nostoc).
- 7They are eco-friendly, renewable and do not pollute soil and water.
- 8It contains the gene for the Bt toxin protein that kills the pest larvae.