Microorganisms: Friend and Foe Notes
Microorganisms are tiny living things that cannot be seen with the naked eye. This chapter covers the main groups (bacteria, fungi, protozoa, algae, and viruses), where they live, their useful roles (curd, bread, alcohol, medicines, nitrogen fixation, decomposition), and their harmful roles (diseases in humans, animals and plants, and food spoilage), along with food preservation.
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- 10 practice questions
- Aligned to CBSE 2025–26 syllabus
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- Updated Sept 2026
Use this map for a first read, then return to the checks and formulas during revision.
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Groups of Microorganisms
Microorganisms are classified into four major groups — bacteria, fungi, protozoa, and some algae. Viruses are also microscopic but reproduce only inside a host cell, so they are on the border of living and non-living.
They may be single-celled (bacteria, many protozoa like Amoeba and Paramecium) or many-celled (many fungi and algae). Some cause diseases: bacteria (typhoid, tuberculosis), protozoa (malaria, amoebic dysentery), viruses (common cold, influenza, COVID-19), and fungi (ringworm).
Microorganisms live almost everywhere — in soil, water, air, inside our bodies, and even in hot springs and polar ice.
Useful Microorganisms
In the kitchen: the bacterium Lactobacillus turns milk into curd; yeast, which respires anaerobically and releases carbon dioxide, makes bread and idli/dosa batter rise and is used to produce alcohol from sugars (fermentation).
In medicine: antibiotics such as penicillin are made from fungi and bacteria and kill or stop disease-causing microbes (they do not work on viral diseases). Vaccines contain dead or weakened microbes that train the body's immune system to fight the real disease later — as with polio, tetanus, measles, and hepatitis vaccines.
In the environment and agriculture: bacteria and fungi decompose dead plants and animals and animal waste, returning nutrients to the soil. Certain bacteria (Rhizobium in the root nodules of pulses) and blue-green algae fix atmospheric nitrogen into compounds plants can use, increasing soil fertility.
Harmful Microorganisms and Food Preservation
Disease-causing microbes are called pathogens. They spread through air (cold, TB), water (cholera, typhoid), food, contact, and through carriers such as the housefly and the female Anopheles mosquito (malaria) and Aedes mosquito (dengue).
Microorganisms also spoil food. Some, like certain bacteria and fungi, produce toxins in food that cause food poisoning. Food is therefore preserved to stop microbial growth: adding salt (pickles, salted fish, meat), adding sugar (jams, jellies), adding chemical preservatives (sodium benzoate, sodium metabisulphite), heating and cooling (boiling, pasteurisation of milk, refrigeration, deep freezing), and removing moisture (drying, dehydration). Sealed cans and airtight packing keep microbes out.
Pasteurisation, invented by Louis Pasteur, involves heating milk to about 70 °C for a short time and then quickly cooling it, which kills most microbes without spoiling the taste.
Microorganisms: Friend and Foe: revision sheet
Close the notes and explain each idea in your own words. Then check the formulas and attempt the examples before opening their solutions.
Recall checklist
- Groups of Microorganisms
- Useful Microorganisms
- Harmful Microorganisms and Food Preservation
For each topic: state the rule, explain when it applies, give an example and identify one mistake to avoid. Return to any topic you cannot explain without looking.
Explain your method
Write the given information, choose a method, show the steps, then check units and whether the answer makes sense.
Check worked solution 1
Check worked solution 2
Turn mistakes into revision
Record the question, the step that went wrong and the corrected reason. Try it again tomorrow without the solution, then revisit it after a week. A correct guess still needs an explanation.
Practice and Revision
Test your understanding with quick chapter-level practice.
Chapter Q&A
Are all microorganisms harmful?
No. Many are extremely useful — they make curd and bread, produce antibiotics and alcohol, decompose waste, and fix nitrogen in the soil. Only some are pathogens.
Why don't antibiotics work against the common cold?
The common cold is caused by a virus, and antibiotics act only on bacteria and fungi. Viral infections are prevented by vaccines and are usually managed by rest and symptomatic treatment.
Why does food kept in a refrigerator last longer?
Low temperature slows down the growth and reproduction of the microorganisms that spoil food, so spoilage is delayed (though not stopped completely).