Introduction — What are Microorganisms?

Microorganisms (or microbes) are tiny living organisms that cannot be seen with the naked eye. They include bacteria, fungi (moulds and yeasts), protozoa, viruses and some algae. Microorganisms are measured in micrometres (µm) and are found almost everywhere — in soil, water, air, and inside living organisms. Their size, shape and method of nutrition differ, but all carry out essential life processes such as growth, reproduction and metabolism.

Types of Microorganisms & Key Features

Bacteria: Single-celled organisms without a true nucleus (prokaryotes). Shapes include rods (bacilli), spheres (cocci) and spirals (spirilla). Some bacteria form spores to survive harsh conditions. They reproduce mainly by binary fission (splitting into two).

Fungi: Include moulds and yeasts. Moulds are multicellular and form threads called hyphae; yeasts are unicellular and are used in fermentation. Fungi obtain nutrients by absorbing organic matter.

Viruses: Much smaller than bacteria, viruses are not complete cells — they are particles consisting of genetic material (DNA or RNA) inside a protein coat. Viruses can reproduce only inside living host cells and cause many diseases.

Protozoa: Single-celled eukaryotic organisms that often live in water and feed on other microbes. Some protozoa cause diseases like malaria (plasmodium — a protozoan parasite).

Algae: Simple photosynthetic organisms, some of which are microscopic (blue-green algae are actually bacteria called cyanobacteria). Algae produce oxygen and are a key part of aquatic food chains.

Where Microbes Live — Habitats & Requirements

Microbes inhabit diverse environments: soil, fresh and salt water, hot springs, deep-sea vents, and inside plant and animal bodies. Their growth depends on factors such as temperature, moisture, pH, oxygen and availability of nutrients. For example, many bacteria prefer warm, moist conditions; fungi grow well on decaying organic matter; certain extremophiles tolerate very high or low temperatures.

Microorganisms as Friends — Beneficial Uses

Though microbes are often associated with disease, many are extremely useful. Important benefits include:

• Food production: Yeast is used in bread-making (fermentation produces CO₂ making dough rise) and in alcohol production. Bacteria and fungi are used in making curd, cheese, idli/dosa batter, soy sauce and vinegar.
• Medicine: Antibiotics like penicillin are produced by moulds (Penicillium). Many bacteria are used to produce vitamins and drugs in pharmaceutical industries.
• Environmental services: Microbes help decompose dead plants and animals, recycling nutrients. Certain bacteria fix atmospheric nitrogen into forms plants can use (e.g., Rhizobium in legume root nodules).
• Industry and biotechnology: Microbes are used to produce enzymes, biofuels, and in wastewater treatment, bioremediation and genetic engineering (insulin production via recombinant bacteria).

Microorganisms as Foes — Diseases and Spoilage

Many microbes are pathogenic (disease-causing). They can affect humans, animals and plants. Examples include:

• Human diseases: Tuberculosis (caused by bacteria Mycobacterium tuberculosis), cholera (Vibrio cholerae), common cold and influenza (viruses).
• Plant diseases: Certain fungi and bacteria cause wilts, blights and rots leading to crop loss.
• Food spoilage: Microbial growth causes food to decay — moulds on bread, souring of milk, and rotting of fruits.

Transmission may occur through contaminated food, water, air droplets, direct contact or vectors (e.g., mosquitoes). Understanding how microbes spread helps in designing prevention strategies.

How to Control Harmful Microbes

Control measures aim to kill or reduce microbes, prevent infections and preserve food. Key methods include:

• Sterilisation: Killing all microbes and spores on an object by heat (autoclaving), chemicals or radiation. Used for surgical instruments and laboratory glassware.
• Disinfection: Using chemicals (disinfectants) to reduce microbial load on surfaces (e.g., lysol, bleach).
• Antiseptics: Mild chemicals safe for use on skin to prevent infection (e.g., alcohol, iodine-based solutions).
• Pasteurisation: Mild heating (e.g., milk at 72°C for 15 seconds) to kill most pathogens while preserving taste and nutrients — used in dairy processing.
• Refrigeration: Slows microbial growth and preserves food quality by lowering temperature.
• Vaccination and antibiotics: Vaccines stimulate immunity to prevent infections; antibiotics treat bacterial infections (note: antibiotics do NOT work against viruses).
• Hygiene practices: Handwash, safe water, proper cooking and storage prevent spread of microbes.

Practical Activities & Demonstrations

NCERT suggests simple classroom activities to reinforce learning. Examples students can perform under supervision:

• Preparing curd: Use boiled and cooled milk, add a spoon of curd (starter) and keep it warm. Observe curd formation — shows beneficial bacterial fermentation (Lactobacillus).
• Yeast fermentation: Mix yeast, sugar and warm water; observe bubble formation due to CO₂ — explains bread leavening and alcohol production.
• Observing mould on bread: Keep slices of bread in different conditions (moist/dry) and observe mould growth to learn about factors affecting microbial growth (moisture, temperature).
• Simple sterilisation demo: Boil water to demonstrate killing many microbes; discuss limitations (not all spores killed).

Safety: Perform experiments with teacher supervision, maintain hygiene, avoid tasting unless instructed, and dispose of cultures safely.

Important Definitions & Quick Notes

• Microbe/Microorganism: Organism too small to be seen with the naked eye.
• Pathogen: Microbe that causes disease.
• Fermentation: Anaerobic breakdown of sugars by microbes producing energy and by-products (CO₂, alcohol, lactic acid).
• Antibiotic: Drug that kills or inhibits bacteria.
• Vaccine: A preparation that stimulates the immune system to protect against specific diseases.

Exam Tips & Revision Strategy

To score well in CBSE exams for this chapter:

• Learn clear definitions and give one example for each microbe type.
• Memorise the uses of microbes (bread, curd, antibiotics, nitrogen fixation) and write short diagrams (e.g., labelled yeast fermentation setup).
• Understand control methods — sterilisation, pasteurisation, antibiotics, vaccines — and differences between them.
• Practice practical-based questions: explain simple experiments like curd making and yeast fermentation step-by-step.
• Revise common diseases and their causal agents (e.g., cholera — Vibrio cholerae, tuberculosis — Mycobacterium).

Summary (Concise)

Microorganisms are tiny life forms that can be both helpful and harmful. They play crucial roles in food production, medicine and the environment, but can also cause diseases and spoilage. Knowing where they live, how they spread and how to control them is essential. Students should focus on examples, simple experiments, and differences between sterilisation, pasteurisation, antiseptics and antibiotics. These NCERT-aligned notes will help strengthen core concepts and prepare for CBSE Class 8 assessments.

Practice Questions (Short)

1. What is fermentation? Give one example from food production.
2. Differentiate between sterilisation and pasteurisation in one line each.
3. Name two diseases caused by microbes and their causative agents.
4. Why are antibiotics not effective against cold and flu?
5. List three beneficial roles of microorganisms.