Microorganisms Friend and Foe
Focused last-minute revision notes with clear concepts, exam traps, key comparisons and compact practice.
Chapter Snapshot
- Microorganisms are organisms too small to be seen clearly without magnification.
- Major groups include bacteria, fungi, protozoa and microscopic algae; viruses are acellular infectious agents that reproduce only inside host cells.
- Useful microbes help make curd, bread, alcohol, medicines and organic manure.
- Some microorganisms fix atmospheric nitrogen and improve soil fertility.
- Harmful microbes cause diseases in humans, animals and plants and can spoil food.
- Food can be preserved by salt, sugar, oil, vinegar, drying, refrigeration, heating and suitable packaging.
- Antibiotics act against certain microorganisms, especially bacteria; they must not be misused.
- Vaccines train the immune system to respond to particular diseases.
What You Will Learn
- Classify major groups of microorganisms
- Explain useful roles of microbes
- Explain harmful effects and common diseases
- Describe fermentation and food preservation
- Differentiate antibiotics and vaccines
- Explain nitrogen fixation and the nitrogen cycle at a basic level
Last-Minute Chapter Map
Microorganisms can be useful, harmful or both depending on species and context.
1. What Are Microorganisms?
Microorganisms or microbes are extremely small living forms usually observed with a microscope. Major groups studied here are bacteria, fungi, protozoa and microscopic algae. Viruses are different from cellular organisms: they reproduce only inside living host cells.
2. Useful Microorganisms
Lactobacillus helps convert milk into curd. Yeast ferments sugars and is used in baking and production of alcohol. Microorganisms decompose dead matter, recycle nutrients and help produce compost. Certain microbes are used to manufacture medicines such as antibiotics.
3. Nitrogen Fixation
Plants need nitrogen compounds to make proteins and other important substances, but they cannot directly use most atmospheric nitrogen. Bacteria such as Rhizobium, living in root nodules of legumes, convert atmospheric nitrogen into usable compounds. Other microorganisms also contribute to the nitrogen cycle.
4. Harmful Microorganisms
Disease-causing microorganisms are called pathogens. They may spread through air, water, food, direct contact or vectors such as mosquitoes. Examples include tuberculosis (bacterial), malaria (protozoan) and several viral diseases. Plant diseases can also be caused by microbes and reduce crop yield.
5. Antibiotics and Vaccines
Antibiotics are medicines used against certain microbial infections, particularly bacterial infections. They should be taken only on medical advice and for the prescribed duration. Antibiotics do not treat viral illnesses such as the common cold.
Vaccines prepare the immune system to recognise particular disease-causing agents and help prevent disease or reduce severity.
6. Food Spoilage and Preservation
Microorganisms grow rapidly when moisture, suitable temperature and nutrients are available. Food preservation works by making conditions less favourable. Common methods include drying, refrigeration, salting, adding sugar, pickling with oil or vinegar, heating and pasteurisation.
7. Pasteurisation
In pasteurisation, a liquid such as milk is heated to a controlled temperature for a specified time and then cooled quickly. This reduces harmful microorganisms while preserving the food better than prolonged boiling.
Useful and Harmful Microorganisms
| Useful Role | Example | Harmful Effect |
|---|---|---|
| Food production | Lactobacillus / yeast | Food spoilage by microbes |
| Medicine | Antibiotic-producing microbes | Disease-causing pathogens |
| Agriculture | Rhizobium fixes nitrogen | Plant diseases |
| Environment | Decomposers recycle matter | Contamination of food/water |
Exam Focus
Must Revise Before the Exam
- Groups of microorganisms
- Lactobacillus and yeast
- Fermentation
- Antibiotics vs vaccines
- Communicable diseases and vectors
- Food preservation methods
- Pasteurisation
- Rhizobium and nitrogen fixation
Common Mistakes
Key Terms
Key Terms
Quick Revision
Quick Revision
- Lactobacillus → curd; yeast → fermentation and baking.
- Viruses reproduce only inside host cells.
- Pathogens cause disease; vectors can transmit pathogens.
- Use antibiotics only when medically indicated.
- Salt, sugar, oil, vinegar, drying and cooling help preserve food.
- Pasteurisation reduces harmful microbes in liquids such as milk.
- Rhizobium helps fix nitrogen in legume root nodules.
Practice Questions
Which microorganism helps convert milk into curd?
A. Amoeba
B. Lactobacillus
C. Rhizobium
D. Plasmodium
View Solution
Correct Answer: B. Lactobacillus
Lactobacillus produces acids that help convert milk into curd.
Which microorganism is used in baking?
A. Yeast
B. Virus
C. Amoeba
D. Algae
View Solution
Correct Answer: A. Yeast
Yeast releases carbon dioxide during fermentation, helping dough rise.
Which organism helps nitrogen fixation in legume roots?
A. Rhizobium
B. Plasmodium
C. Lactobacillus
D. Virus
View Solution
Correct Answer: A. Rhizobium
Rhizobium lives in root nodules of many legumes.
Antibiotics should generally be used against:
A. all infections
B. bacterial infections when prescribed
C. only injuries
D. all viral colds
View Solution
Correct Answer: B. bacterial infections when prescribed
Antibiotics are useful against susceptible bacteria and should be used under medical guidance.
Why does adding salt or sugar help preserve food?
View Solution
High concentrations of salt or sugar reduce water availability for many microorganisms, slowing their growth.
Differentiate an antibiotic and a vaccine.
View Solution
An antibiotic treats susceptible bacterial infections; a vaccine prepares the immune system to recognise a particular pathogen and helps prevent disease.
Why should cooked food not be left warm and uncovered for long?
View Solution
Warm, nutrient-rich food can support rapid microbial growth. Cooling, covering and hygienic storage reduce contamination and spoilage.
Chapter Summary
Chapter Summary
- Lactobacillus → curd; yeast → fermentation and baking.
- Viruses reproduce only inside host cells.
- Pathogens cause disease; vectors can transmit pathogens.
- Use antibiotics only when medically indicated.
- Salt, sugar, oil, vinegar, drying and cooling help preserve food.
- Pasteurisation reduces harmful microbes in liquids such as milk.
- Rhizobium helps fix nitrogen in legume root nodules.