Chapter 22 of 25

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Tamil Nadu (TNBSE)Class 9 Science★ 4-5 marks in assessments

World of Microbes

Samacheer Kalvi (TN State Board) Class 9 Science — Biology Chapter 22, World of Microbes. Microbes are microscopic living organisms. Understanding bacteria, viruses, fungi, and how infectious diseases spread enables us to design vaccines, manage hygiene, and use microbes in food (curd, yeast) and medicine. Notes, worked examples, common mistakes, and quick revision.

⏱ 20 min readMedium difficulty✓ Free · NCERT-aligned

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The chapter at a glance

Every concept and exam-favourite branch — revisable in 5 minutes.

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Study Guide

Night-before notes

Key points, glossary and the questions most likely to show up.

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

1Differentiate viruses, bacteria, fungi, and protozoa
2Explain useful applications of microbes in industries and soil
3Describe the mode of transmission of common infectious diseases
4Understand immunization, vaccines, and antibiotics

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Why this chapter matters

Microbes are microscopic living organisms. Understanding bacteria, viruses, fungi, and how infectious diseases spread enables us to design vaccines, manage hygiene, and use microbes in food (curd, yeast) and medicine.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Class 8 Science — Microorganisms

Basic types of microbes.

World of Microbes — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Biology — Chapter 22. Microbes are microscopic living organisms. Understanding bacteria, viruses, fungi, and how infectious diseases spread enables us to design vaccines, manage hygiene, and use microbes in food (curd, yeast) and medicine.

1. About this chapter

This chapter covers types of microbes, useful microbes, infectious diseases (air, water, vector-borne), vaccines, and antibiotics.

2. Types of Microbes

Bacteria: Unicellular prokaryotes. Useful: Rhizobium (nitrogen fixation), Lactobacillus (curd). Pathogen: Vibrio cholerae (cholera).
Viruses: Non-living outside host, have DNA/RNA. Pathogen: Poliovirus, HIV, Influenza.
Fungi: Saprophytic eukaryotes. Useful: Yeast (fermentation). Pathogen: Ringworm fungi.
Protozoa: Eukaryotic parasites. Pathogen: Plasmodium (malaria).

3. Disease Transmission

Airborne: TB (cough droplets), Common Cold, Influenza.
Waterborne: Cholera, Typhoid (contaminated food/water).
Vector-borne: Malaria (Female Anopheles), Dengue (Aedes mosquito).

4. Antibiotics and Vaccines

Antibiotics: Chemical substances produced by microbes that destroy or inhibit bacteria (e.g. Penicillin).
Vaccines: Introduced antigen to build antibodies (e.g. BCG for TB, Polio drops). Discovered by Edward Jenner (Smallpox).

Key formulas & results

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

Immunisation

Antigen/Vaccine -> Antibody production -> Immunity

Stimulates the body's immune system.

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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

✗ Taking antibiotics to cure viral infections.

✓ Antibiotics kill bacteria by blocking their cell wall synthesis or metabolism. They have no effect on viruses, which use host cell machinery. Viral diseases are treated with antivirals or prevented with vaccines.

WATCH OUT

✗ Assuming all microbes are harmful pathogens.

✓ Only a small percentage cause disease (pathogens). Most are useful in food production (lactobacillus in curd), sewage treatment, and nitrogen fixation (rhizobium).

NCERT exercises (with solutions)

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

Multiple Choice Questions

Questions on disease-causing agents, vaccines, and vector vectors.

Questions

Short Answer Questions

Bacterial vs viral diseases, TB prevention, malaria cycle, and vaccine types.

Questions

Practice problems

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

Q1EASY· Concept

Differentiate active and passive immunity.

▶ Show solution

1. Active immunity: produced when the body's own cells generate antibodies in response to infection or vaccine (slow but long-lasting). 2. Passive immunity: produced by transferring pre-made antibodies from outside (fast but temporary, e.g. Colostrum for infants).

Q2MEDIUM· Concept

Name two bacterial and two viral diseases in humans with their causative pathogens.

▶ Show solution

Bacterial: 1. Tuberculosis (Mycobacterium tuberculosis). 2. Cholera (Vibrio cholerae). Viral: 1. Common Cold (Rhinovirus). 2. Polio (Poliovirus).

5-minute revision

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

•Pathogens are disease-causing microbes.
•Antibiotics cure bacterial diseases; vaccines prevent diseases.
•Airborne: TB, cold. Waterborne: Cholera, typhoid. Vector: Malaria, dengue.
•First vaccine was developed by Edward Jenner for Smallpox.

Tamil Nadu (TNBSE) marks blueprint

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

Typical chapter weightage: 4-5 marks in assessments

Question type	Marks each	Typical count	What it tests
MCQ	1	1-2	Base concepts and definitions
Short Answer	2-3	1-2	Descriptive and application points

Prep strategy

Understand core definitions and solve standard textbook problems.
Review common mistakes to avoid losing easy marks.

Where this shows up in the real world

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

Antibiotics (Penicillin)

Discovered by Alexander Fleming, antibiotics save millions of lives annually from bacterial infections.

Fermentation

Yeast and lactobacillus are used globally in baking, brewing, and dairy industries.

Exam strategy

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

Write definitions precisely as defined in the textbook.
Draw neat, labeled diagrams for biology and physics chapters.

Going beyond the textbook

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

Read advanced reference materials to explore concepts beyond the school syllabus.

Where else this chapter is tested

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

Class 9 Annual ExamsHigh

NTSE Stage 1Medium

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Questions students ask

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

A preparation of dead or weakened pathogens introduced into the body to stimulate antibody production, providing immunity against that specific disease.

By the bite of an infected female Anopheles mosquito, which carries the Plasmodium parasite.

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