Chapter 16 of 23

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Tamil Nadu (TNBSE)Class 8 Science★ 5-8 marks in the TN Class 8 annual exam through book-back MCQs, fill-ups, true/false and short answers

Microorganisms

Samacheer Kalvi (TN State Board) Class 8 Science — Biology Chapter 16, Microorganisms. The five groups of microbes (bacteria, viruses, fungi, protozoa, algae), the shapes of bacteria, useful microbes (Lactobacillus in curd, yeast in bread, Rhizobium fixing nitrogen), harmful microbes and diseases, and how we fight them — pasteurization, antibiotics (penicillin, Fleming) and vaccines (Edward Jenner). Notes with the actual book-back questions, common mistakes and quick revision.

⏱ 16 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1Name the five groups of microorganisms
2Classify bacteria by shape
3Give examples of useful microbes and their roles
4List microbial diseases
5Explain pasteurization, antibiotics and vaccines

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Why this chapter matters

Microorganisms explains the microbes that shape daily life — curd and bread, nitrogen fixing in soil, the diseases they cause and how pasteurization, antibiotics and vaccines protect us. Lactobacillus, Rhizobium, Fleming and Jenner are directly tested book-back content in the TN Class 8 exam.

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Class 7 — Living World

Cells and living things.

Microorganisms — Class 8 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Science, Biology — Chapter 16. The tiny living things that help and harm us.

1. About this chapter

This chapter covers the groups of microorganisms, the shapes of bacteria, useful and harmful microbes, and how we control them through pasteurization, antibiotics and vaccines.

2. Groups of microorganisms

Microorganisms (microbes) are too small to see with the naked eye and are studied under a microscope. The main groups are bacteria, viruses, fungi, protozoa and algae.
Bacteria are classified by shape into four types: bacilli (rod), cocci (spherical), spirilla (spiral) and vibrio (comma).
Viruses can multiply only inside a living host cell.

3. Useful microorganisms

Lactobacillus bacteria convert milk into curd.
Yeast carries out fermentation — it makes bread rise (releasing CO₂) and is used to make alcohol.
Rhizobium bacteria live in the root nodules of leguminous plants and fix atmospheric nitrogen into the soil.
Microbes also help in making antibiotics and in decomposing dead matter (cleaning the environment).

4. Harmful microbes and how we fight them

Many microbes cause diseases: bacteria (tuberculosis, cholera, typhoid), viruses (common cold, polio, measles), protozoa (malaria), fungi (ringworm).
Pasteurization (Louis Pasteur): milk is heated to about 70 °C and cooled suddenly to kill harmful microbes.
Antibiotics are substances produced by microbes that kill or stop other microbes — penicillin, the first antibiotic, was discovered by Alexander Fleming.
Vaccines make the body immune to a disease — Edward Jenner developed the smallpox vaccine. Diseases prevented by vaccination include polio, measles, mumps and tuberculosis.

5. Worked examples

Example 1. Which bacterium converts milk into curd? Lactobacillus.

Example 2. Who discovered penicillin? Alexander Fleming.

Example 3. Which bacterium fixes atmospheric nitrogen? Rhizobium (in root nodules of legumes).

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

Curd is formed from milk by — (a) yeast / (b) Lactobacillus. Ans: (b) Lactobacillus.
The nitrogen-fixing bacterium present in the root nodules of legumes is — (a) Rhizobium / (b) Vibrio. Ans: (a) Rhizobium.
Penicillin was discovered by — (a) Edward Jenner / (b) Alexander Fleming. Ans: (b) Alexander Fleming.
A comma-shaped bacterium is — (a) bacillus / (b) vibrio. Ans: (b) vibrio.
The vaccine for smallpox was developed by — (a) Edward Jenner / (b) Louis Pasteur. Ans: (a) Edward Jenner.

II. Fill in the blanks 6. The process of heating milk to kill microbes is called pasteurization. 7. Yeast is used in the baking of bread. 8. Spherical bacteria are called cocci.

III. True or False 9. Viruses can multiply only inside a living host. — True. 10. All microorganisms are harmful. — False (many are useful).

7. Common mistakes

Mistake: Saying yeast makes curd. Fix: Lactobacillus makes curd; yeast ferments bread and alcohol.
Mistake: Crediting Edward Jenner with penicillin. Fix: Jenner → smallpox vaccine; Fleming → penicillin.
Mistake: Thinking all microbes cause disease. Fix: Many are useful (Lactobacillus, Rhizobium, yeast, decomposers).

8. Quick revision

Biology Ch 16 · microbes, useful and harmful.
Groups: bacteria, viruses, fungi, protozoa, algae; viruses need a host.
Bacteria shapes: bacilli, cocci, spirilla, vibrio.
Useful: Lactobacillus (curd), yeast (bread), Rhizobium (nitrogen fixing).
Defence: pasteurization (Pasteur), antibiotics (penicillin — Fleming), vaccines (smallpox — Jenner).

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Groups

bacteria, viruses, fungi, protozoa, algae

Viruses need a living host.

Bacteria shapes

bacilli (rod), cocci (sphere), spirilla (spiral), vibrio (comma)

Four shapes.

Useful microbes

Lactobacillus (curd), yeast (bread), Rhizobium (nitrogen)

Everyday helpers.

Defence

pasteurization · antibiotics · vaccines

Pasteur · Fleming · Jenner.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying yeast makes curd

✓ Lactobacillus makes curd; yeast ferments bread and alcohol.

WATCH OUT

✗ Crediting Edward Jenner with penicillin

✓ Jenner → smallpox vaccine; Fleming → penicillin.

WATCH OUT

✗ Thinking all microbes cause disease

✓ Many are useful (Lactobacillus, Rhizobium, yeast, decomposers).

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Choose the correct answer

Book-back MCQs on Lactobacillus, Rhizobium, Fleming, vibrio and Jenner.

Questions

Fill in the blanks

Pasteurization, yeast and cocci.

Questions

True or False / Answer briefly

Viruses, useful microbes, defence methods.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· MCQ

Curd is formed from milk by ____.

▶ Show solution

Lactobacillus.

Q2EASY· MCQ

The nitrogen-fixing bacterium in the root nodules of legumes is ____.

▶ Show solution

Rhizobium.

Q3EASY· MCQ

Penicillin was discovered by ____.

▶ Show solution

Alexander Fleming.

Q4EASY· Fill in the blanks

The process of heating milk to kill microbes is called ____.

▶ Show solution

pasteurization.

Q5EASY· Answer briefly

Name two useful microorganisms and state their uses.

▶ Show solution

Lactobacillus — converts milk into curd; yeast — ferments dough to make bread rise (also Rhizobium fixes nitrogen).

Q6MEDIUM· Answer briefly

How does a vaccine protect the body?

▶ Show solution

A vaccine introduces a weakened or dead microbe so the body builds immunity (antibodies) against that disease without falling ill — e.g. Edward Jenner's smallpox vaccine.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Biology Chapter 16 of Samacheer Kalvi Class 8 Science.
•Groups: bacteria, viruses, fungi, protozoa, algae; viruses need a host.
•Bacteria shapes: bacilli, cocci, spirilla, vibrio.
•Useful: Lactobacillus (curd), yeast (bread), Rhizobium (nitrogen fixing).
•Pasteurization (Pasteur) heats milk to kill microbes.
•Antibiotics: penicillin discovered by Fleming.
•Vaccines: smallpox vaccine by Edward Jenner.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 5-8 marks across book-back MCQ, fill-ups, true/false and short answers

Question type	Marks each	Typical count	What it tests
MCQ / Fill	1	3-5	Useful microbes, scientists, bacteria shapes
True / False	1	1-2	Viruses, useful vs harmful
Short Answer	2-3	1-2	Pasteurization, vaccines, antibiotics

Prep strategy

Match each useful microbe to its role
Learn the four bacteria shapes
Pair scientists with their discoveries (Pasteur, Fleming, Jenner)
Note viruses need a living host

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Food

Curd, bread, cheese and idli all depend on microbes.

Agriculture

Rhizobium enriches soil with nitrogen for free.

Medicine

Antibiotics and vaccines save millions of lives.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Quote Lactobacillus for curd and Rhizobium for nitrogen
Pair Fleming (penicillin) and Jenner (smallpox)
Define pasteurization with the heating step
Name the four bacteria shapes

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Explain how nitrogen fixing by Rhizobium fits into the nitrogen cycle.
Describe why antibiotics do not work against viral colds.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN Class 8 Annual ExamHigh

Foundation / NMMS ScienceMedium

School unit testsHigh

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

An antibiotic (like penicillin) is a medicine that kills or stops microbes already in the body; a vaccine is given beforehand to make the body immune so it can fight a future infection.

Lactobacillus bacteria added to warm milk ferment its sugar (lactose) into lactic acid, which thickens the milk into curd.

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