The Invisible Living World | Science Grade 8 CBSE

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The microscopic cast

Microbes come in a few main groups, plus the tool that reveals them. Tap each term to see what it is and where you meet it.

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The three big ideas

Microorganisms (microbes) are living things too small to see with the naked eye. They live almost everywhere — in air, water, soil, on our skin and even inside us.
A microscope magnifies them hundreds of times. Before microscopes, no one knew this living world existed; the instrument literally opened our eyes to it.
Though invisible, their effects are easy to spot — milk turning to curd, bread rising, fruit rotting, a wound healing or going septic.
Most microbes fall into a few groups: bacteria, fungi, protozoa and algae — all truly living — plus viruses, which behave like living things only inside a host.

Bacteria — single-celled, found in huge numbers everywhere. Some set curd or fix nitrogen; others cause illness.
Fungi — include moulds and yeasts. Yeast raises bread; moulds rot food but also give us medicines.
Protozoa — single-celled animal-like microbes. Some, like the malaria parasite and the one causing amoebic dysentery, cause disease.
Algae — plant-like microbes that make their own food by photosynthesis; a green pond bloom is millions of them.
Viruses are different: they are not active on their own and can multiply only inside a living cell. They cause colds, influenza, dengue and many other diseases.

Common mistake: calling every germ a "bacteria". A cold and dengue are caused by viruses, not bacteria — and antibiotics, which fight bacteria, do not work on viruses.

Useful microbes: setting curd, fermenting idli and dosa batter, raising bread, making cheese; fixing nitrogen to enrich soil; decomposing dead matter to recycle nutrients; and producing medicines and vaccines.
Harmful microbes: some cause disease (typhoid, tuberculosis, malaria, the common cold), some spoil food, and some attack crops.
Diseases spread by air, water, food, contact and carriers such as mosquitoes and houseflies. Clean water, hygiene and covered food cut the chain of spread.
Food preservation — salting, drying, sugaring, refrigeration and boiling — works by denying microbes the warmth and moisture they need to grow.

Worked example. Why does milk last longer in a refrigerator than on the kitchen shelf?

Spoilage bacteria multiply fast in warmth. The cold of a fridge slows their growth to a crawl, so the milk stays fresh for far longer — though it is not killed, only slowed, which is why even chilled milk eventually spoils.

Where you'll meet it

Microbes in everyday life

The kitchen ferment

Curd, idli, dosa, dhokla and bhatura all depend on microbes. Bacteria sour the milk for curd; a mix of bacteria and wild yeast bubbles through soaked rice-and-dal batter overnight, giving idlis their soft, airy texture.

Vaccines and antibiotics

Vaccines train the body to fight a germ before it strikes — the reason polio and smallpox no longer terrify families. Antibiotics, first discovered from a mould, treat bacterial infections, though they do nothing against viruses.

Soil and the nitrogen cycle

Farmers grow pulses like moong and arhar partly because nitrogen-fixing bacteria in their roots refill the soil with nutrients. Decomposer microbes then break down crop waste, keeping the soil alive for the next season.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and case studies, testing whether you can use the ideas, not just recall them.

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The Invisible Living World: Beyond Our Naked Eye