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Microorganisms: Friend and Foe

CBSE Class 8 Science • Chapter 2 • Detailed Master Notes

Chapter Overview

This chapter explores the invisible world of microscopic organisms. We will learn how they are classified, their immense beneficial effects in food, medicine, and environment (our friends), and their harmful impacts as pathogens causing diseases and food spoilage (our foes).

2.1 What are Microorganisms?

Microorganisms (Microbes): Living organisms around us which are so small that they cannot be seen with the naked eye are called microorganisms or microbes.

Some of these, such as the greyish-white fungus that grows on moist bread during the rainy season, can be seen with a magnifying glass. Others strictly require a microscope to be visible.

Microorganisms can survive under all types of hostile environments, ranging from ice-cold climate to boiling hot springs, and from deserts to marshy lands. They are also found inside the bodies of animals, including humans.

2.2 Classification of Microorganisms

Microorganisms are broadly classified into four major groups, with viruses existing as a unique exception.

Bacteria: Single-celled, simple organisms. They can be spiral (Spirilla), spherical (Cocci), or rod-shaped (Bacilli). (Examples: Lactobacillus, Rhizobium).
Fungi: Mostly multicellular, plant-like organisms that lack chlorophyll. Because they cannot photosynthesise, they feed on dead or decaying organic matter (Saprophytes) or live as parasites. (Examples: Bread mould, Penicillium, Aspergillus, Yeast - which is unicellular).
Protozoa: Single-celled microscopic animals usually found in water. (Examples: Amoeba, Paramecium, Plasmodium).
Algae: Simple, plant-like organisms that contain chlorophyll and can make their own food via photosynthesis. Mostly aquatic. (Examples: Chlamydomonas, Spirogyra).

The Unique Case of Viruses:

Viruses are also microscopic but are fundamentally different from other microbes. They lie on the borderline of living and non-living. They do not possess a cellular structure (no distinct cell membrane or cytoplasm). They reproduce only when they enter inside the cells of a living host organism (a bacterium, plant, or animal). Outside a host, they behave like non-living crystals.

Examples of viral diseases: Common cold, Influenza (flu), Polio, Chickenpox, and COVID-19.

AI Image Prompt: A stunning, vibrant 3D microscopic collage showing the 5 major types of microorganisms isolated in glowing circular zones on a dark, sleek scientific background. From left to right: green rod-shaped Bacteria dividing; a fuzzy bread mould structure (Fungus); an irregularly shaped, translucent Amoeba (Protozoa); beautiful green spiral algae (Spirogyra); and a mechanical, geometric bacteriophage Virus injecting DNA. Provide clean, glowing white textbook labels for each.

Topic Practice: Classification

Q1. Why are viruses considered to be on the borderline of living and non-living?

Ans: Viruses are considered borderline because outside a host body, they behave like non-living, inactive crystals. They only show signs of life (like reproduction) when they enter the cells of a living host organism.

Q2. Name two examples of Protozoa.

Ans: Amoeba and Paramecium.

2.3 Microorganisms as Friends (Friendly Microbes)

Microorganisms play an indispensable role in our daily lives. Many are used for commercial production, medicinal purposes, and maintaining the ecological balance.

A. Commercial and Domestic Uses

Making Curd: Curd contains several microorganisms. A specific bacterium called Lactobacillus multiplies in warm milk, producing lactic acid which converts the milk into curd.
Making Bread and Pastries: Yeast (a unicellular fungus) reproduces rapidly during dough making. It respires anaerobically and produces carbon dioxide gas. The bubbles of this gas fill the dough and increase its volume (making it rise/fluffy), which is the basis of baking.
Production of Alcohol/Wine: Yeast is grown on natural sugars present in grains (like barley, wheat, rice, or crushed fruit juices). It converts the natural sugar into alcohol.
Fermentation: The process of conversion of sugar into alcohol by the action of yeast is called fermentation. (Discovered by Louis Pasteur in 1857).

B. Medicinal Uses

Antibiotics: These are medicines produced by certain microorganisms (bacteria and fungi) that kill or stop the growth of other disease-causing microorganisms.
Examples: Streptomycin, Tetracycline, Erythromycin.
Note: The very first antibiotic, Penicillin, was discovered accidentally by Alexander Fleming in 1929 from a green mould (fungus) called Penicillium notatum which inhibited bacterial growth.
Warning: Antibiotics are not effective against viral infections like cold and flu!
Vaccines: When a disease-carrying microbe enters our body, the body produces antibodies to fight the invader and "remembers" it. A vaccine introduces dead or weakened (attenuated) microbes into a healthy body. The body fights and kills them by producing specific antibodies. These antibodies remain in the blood permanently, protecting the body from future attacks by that actual disease.
(Edward Jenner discovered the vaccine for smallpox in 1798).

C. Cleaning the Environment & Soil Fertility

Cleaning up Waste: Microbes (bacteria and fungi) decompose dead organic waste of plants and animals, converting them into simple, harmless substances (manure) that are returned to the soil.
Biological Nitrogen Fixers: Certain bacteria (like Rhizobium found in root nodules of leguminous plants) and blue-green algae fix free atmospheric nitrogen gas into usable nitrogenous compounds (nitrates). This naturally enriches soil fertility.

AI Image Prompt: A vibrant split-screen educational illustration showing friendly microbes in action. On the left pane: a microscopic view of spherical yeast cells embedded in bubbling, rising bread dough, with tiny CO2 bubbles expanding. On the right pane: glowing, nodular Rhizobium bacteria intensely clinging to the roots of a green bean plant (legume) in cross-sectioned soil, pulling glowing blue nitrogen gas particles from the air.

Topic Practice: Friendly Microbes

Q1. What is fermentation, and who discovered it?

Ans: Fermentation is the process of conversion of sugar into alcohol by the microscopic action of yeast. It was discovered by Louis Pasteur in 1857.

Q2. Why does the dough rise when yeast is added to it?

Ans: Yeast respires rapidly and produces carbon dioxide gas. The bubbles of this gas get trapped in the dough, filling it up and causing the dough's volume to increase (rise).

Q3. Who discovered the antibiotic Penicillin?

Ans: Alexander Fleming discovered Penicillin from a fungus in 1929.

2.4 Microorganisms as Foes (Harmful Microbes)

Pathogens: Some microorganisms cause diseases in human beings, plants, and animals. Such disease-causing microbes are specifically called pathogens.

Pathogens can enter our body through the air we breathe, the water we drink, or the food we eat. They can also be transmitted by direct contact with an infected person or carried by an animal or insect.

Carrier Insects

Insects and animals that act as vehicles to carry disease-causing microbes from a sick person (or dirty place) to a healthy person are called carriers or vectors.

Housefly: Flies sit on garbage and animal excreta. Pathogens stick to their hairy bodies. When these flies sit on uncovered food, they transfer the pathogens. Whoever eats the contaminated food is likely to fall sick (e.g., Cholera).
Female Anopheles Mosquito: Acts as the carrier of the deadly parasite of malaria (which is a protozoan called Plasmodium). By biting a human, she injects the parasite into the bloodstream.
Female Aedes Mosquito: Acts as a carrier of the dengue virus.

Prevention Hack: All mosquitoes breed in stagnant water. By keeping the surroundings dry and free of stagnant water in coolers, tyres, and flower pots, we can prevent mosquitoes from breeding and halt the spread of Malaria and Dengue.

Common Human Diseases Caused by Microbes (NCERT Focus)

Human Disease	Causative Micro-organism	Mode of Transmission
Tuberculosis (TB)	Bacteria	Air
Measles / Chicken Pox / Polio	Virus	Air / Direct Contact
Cholera / Typhoid	Bacteria	Water / Food
Hepatitis B	Virus	Water
Malaria	Protozoa (Plasmodium)	Mosquito (Female Anopheles)

Topic Practice: Pathogens & Vectors

Q1. What are pathogens?

Ans: Disease-causing microorganisms are called pathogens.

Q2. Name the carrier of the malaria-causing protozoan.

Ans: The Female Anopheles mosquito acts as the carrier for the malaria parasite (Plasmodium).

2.5 Food Poisoning and Preservation

Microorganisms that grow on our food sometimes produce toxic (poisonous) substances. This makes the food poisonous, causing serious illness and sometimes even death. This is known as food poisoning.

To prevent this spoilage, we use several techniques collectively called Food Preservation. These techniques work by either killing microbes or stopping their growth by removing moisture or altering the chemical environment.

Chemical Methods: Special chemical preservatives like Sodium benzoate and Sodium metabisulphite are added to jams and squashes to check microbial growth.
Preservation by Common Salt: Salting draws out water from the cells of microbes, killing them. Used for preserving meat, fish, amla, and raw mangoes.
Preservation by Sugar: Sugar reduces the moisture content which inhibits the growth of bacteria. Used for making jams, jellies, and squashes.
Preservation by Oil and Vinegar: Bacteria cannot survive in such an acidic and oxygen-deprived environment. Extensively used for pickles.
Pasteurisation: Discovered by Louis Pasteur. Milk is heated to a high temperature (about 70°C) for 15-30 seconds and then suddenly chilled to cold storage. The blast of heat essentially kills microbes, and sudden chill prevents remaining ones from growing.

2.6 The Nitrogen Cycle

Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids, and vitamins. Our atmosphere has 78% nitrogen gas, but plants and animals cannot use this huge reservoir directly in its gaseous form.

The Cycle Steps:

Fixation: Certain bacteria (like Rhizobium) and blue-green algae "fix" atmospheric nitrogen, converting it into usable nitrogenous compounds (nitrates) in the soil. Sometimes lightning strikes also fix nitrogen.
Assimilation: Plants absorb these soluble compounds from the soil through their roots and use them to synthesize plant proteins. Animals get these proteins by eating the plants.
Decomposition: When plants and animals die, bacteria and fungi present in the soil decompose their complex organic waste back into simple nitrogenous compounds to be used by plants again.
Denitrification: Certain other specialised bacteria in the soil take a portion of these nitrogenous compounds and convert them back into gaseous nitrogen, which escapes into the atmosphere.

As a result of this continuous biological cycle, the percentage of nitrogen in the atmosphere remains more or less constant.

AI Image Prompt: A vibrant, clean, highly professional educational flowchart diagram of the Nitrogen Cycle on a white background. Include a blue sky area representing atmospheric nitrogen (N2), a dramatic lightning bolt, green leguminous plants with roots embedded in soil, a cow eating grass, and soil indicating decomposition by bacteria back into the air. Use bright, well-organized arrows connecting the continuous cycle.

Topic Practice: Preservation & Nitrogen Cycle

Q1. Explain the process of pasteurisation.

Ans: In pasteurisation, milk is heated to about 70°C for 15 to 30 seconds to kill microbes, and then immediately chilled and stored to prevent the growth of any surviving bacteria. This keeps the milk safe for consumption without boiling.

Q2. Name the microbe that fixes atmospheric nitrogen in the roots of leguminous plants.

Ans: Rhizobium bacteria, which live in the root nodules of leguminous plants, fix atmospheric nitrogen into the soil.