Microorganisms: Friend and Foe: Class 8 Study Guide
Microorganisms are living things too small to see without a microscope. Many are useful, and some cause disease.
The main groups of microorganisms are bacteria, fungi (such as yeast and moulds), protozoa, some algae and viruses. Viruses are different because they multiply only inside a living host cell.
Microorganisms live everywhere: in air, water, soil and inside our bodies. Most are harmless or helpful.
Useful microorganisms
| Use | Microorganism and how |
|---|---|
| Curd | Lactobacillus bacteria turn milk into curd |
| Bread and cake | Yeast ferments sugar and makes carbon dioxide that makes dough rise |
| Medicines | The mould Penicillium gives the antibiotic penicillin, found by Alexander Fleming in 1928 |
| Soil | Rhizobium bacteria in pulse roots fix nitrogen |
| Cleaning | Bacteria break down dead plants and animals |
Harmful microorganisms
Some cause diseases: cholera and typhoid (bacteria), common cold and dengue (viruses), malaria (a protozoan carried by female Anopheles mosquitoes). They also spoil food. Vaccines protect us: Edward Jenner made the first vaccine, against smallpox, in 1796. Antibiotics work against bacteria but not viruses, and should be taken only on a doctor's advice.
Wash hands, boil or filter water.
Keep flies away.
No stagnant water.
Follow the schedule.
Preserving food
Salting (pickles, fish), adding sugar (jams), oil and vinegar (pickles), heating and cooling all slow the growth of microorganisms. Pasteurisation, named after Louis Pasteur, heats milk to about 70 °C for a short time and cools it quickly. Refrigerators keep food cold, which slows spoilage.
Worked examples and common mistakes
Example: Which microorganism makes dough rise? Yeast. Example: Do antibiotics work against the common cold? No, it is caused by a virus. Common mistakes: thinking that all microorganisms are harmful, and thinking that viruses can multiply outside a living cell.
Check your understanding: model answers
1. Lactobacillus. 2. Edward Jenner. 3. It kills germs in the water.
✨ Key Takeaways
- Microorganisms are tiny and found everywhere.
- Many are useful: curd, bread, antibiotics.
- Some cause disease and spoil food.
- Cleanliness, vaccines and preservation protect us.
💬 Questions for Discussion & Reflection
- Which bacteria make curd?
- Who made the first vaccine?
- Why is boiling water useful?
Print a science observation log →