Study Material and Notes of Ch 2 Microorganisms: Friend and Foe Class 8th Science

Topics in the Chapter

 • Introduction
• Microorganisms
• Classifications of microorganisms
• Food Preservation
• Methods of food preservation
→ Chemical methods
→ Heat and cold Treatments
→ Storage and Packing
• Importance of microorganisms
→ In food industry
→ In beverage industry
→ In medicine production
→ In vaccine production
→ In increasing soil fertility
→ In cleaning the environment
• Nitrogen cycle
• Harmful microrganisms
• Examples of carriers
• Diseases in humans, animals and plants by microorganisms


→ Those living organisms around us which we cannot be seen with naked eyes are called
microorganisms or microbes.

→ Some of these can be seen with a magnifying glass while some cannot be seen without the help
of a microscope. That is why these are called microorganisms or microbes.


→ Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae.

Classification of microorganisms

(i) Bacteria

→ Single-celled organisms
→ Found in wide range of habitats ranging from glaciers to deserts and hot springs
→ For example: curd bacteria (Lactobacillus)

(ii) Fungi

→ Multicellular, heterotrophic organisms
→ Lack chlorophyll and are generally found in colonies
→ For example: Penicillium, Aspergillus

(iii) Protozoa

→ Unicellular or multicellular microorganisms
→ Usually found in water
→ For example: Amoeba and Paramecium

(iv) Algae

→ Unicellular or multicellular autotrophic organisms
→ Contain chlorophyll pigment and carry out photosynthesis
→ For example: Chlamydomonas and Spirogyra

(v) Viruses

→ Viruses are also microscopic which reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.
→ Ultramicroscopic organisms
→ Require host cells to reproduce and complete their life cycle.
→ For example: Influenza virus, polio virus.

Food Preservation

→ Process of preventing the spoilage of food items by the action of microbes is called food preservation.

→ Microorganisms act on food items and spoil them.

Methods of food preservation

(i) Chemical methods

→ The chemicals that controls the growth of microorganisms on food are called preservatives.
For example: sodium benzoate, sodium metabisulphite, salts and edible oil.

→ Common salt is used as preservative in pickles. It is also used to preserve meat and fish.

→ Sugar is used as preservative in jams and jellies.

→ Oil and vinegar are used as preservatives in pickles and vegetables.

(ii) Heat and cold treatments

→ Boiling the milk helps in killing microorganisms present in it.

→ Pasteurization is a technique of preserving milk in which it is boiled to about 70°C for 15 to 30 seconds and then suddenly chilled and stored.

(iii) Storage and packing

→ Dry fruits and vegetables are stored in sealed air tight packets to prevent microbial attack.

Importance of microorganisms

(i) In food industry

→ Lactobacillus bacteria promote the conversion of milk into curd.

→ Yeast is used in preparation of breads, pastries and cakes.

(ii) In beverage industry

→ Yeast is used for commercial production of alcohol, wine and vinegar (acetic acid).

→ Yeast acts on sugar and converts it into alcohol by the process of fermentation. Louis Pasteur discovered fermentation.

(iii) In medicine production

→ Medicines produced by certain microorganisms to kill or stop the growth of other disease-causing microorganisms are called antibiotics.

→ Antibiotics are obtained from bacteria and fungi.

→ Commonly used antibiotics are streptomycin, tetracycline, and erythromycin.

→ First antibiotic penicillin was prepared by Alexander Fleming

(iv) In vaccine production

→ Protection of the body from the attack of various disease-causing microorganisms through vaccines is known as vaccination.

→ Vaccine includes dead or weakened microbes that trigger the production of antibodies in the body.

→ These antibodies help in preventing the attack from disease-causing microorganisms.

→ Vaccination helps in controlling diseases such as cholera, polio, small pox, hepatitis etc.

→ Vaccine for small pox was discovered by Edward Jenner.

(v) In increasing soil fertility

→ Blue green algae and Rhizobium bacteria are called biological nitrogen fixers.

→ They fix free atmospheric nitrogen to enhance soil fertility.

(vi) In cleaning the environment

→ Microorganisms (decomposers) help in converting dead waste of plants and animals into simpler substances by the process of decomposition.

Nitrogen cycle

→ It involves circulation of nitrogen through living and non-living components of nature.

→ Nitrogen gas comprises 78% of the atmosphere.

→ First process of nitrogen cycle is fixation of nitrogen gas into nitrogenous compounds caused by bacterium Rhizobium and lightning.

→ Nitrogen compounds in soil are taken up by the plants through roots and used up in synthesis of plant proteins. Animals obtain nitrogen by feeding on plants.

→ Waste of plants and animals are converted to nitrogenous compounds by the action of bacteria and fungi in the soil.

→ Some bacteria convert nitrogenous compounds back to nitrogen to maintain atmospheric levels of nitrogen.

Harmful microorganisms 

→ Disease-causing microorganisms are called pathogens.

→ Diseases in humans caused by microorganisms

→ Diseases caused by microorganisms that spread from an infected person to a healthy person through air, water, or food are called communicable diseases.

→ The example includes cholera, chicken pox, and tuberculosis.

→ The organisms that transmit diseases from one place to the other are called carriers.

Example of carriers

→ Housefly spreads diseases such as cholera, dysentery, and typhoid.
→ Female Anopheles mosquito spreads malarial parasites.
→ Female Aedes mosquito spreads dengue virus.

• Examples of human diseases caused by bacteria


• Examples of human diseases caused by virus

Chicken pox

• Examples of human diseases caused by protozoa

Sleeping Sickness

• Diseases in animals caused by microorganisms

Anthrax is caused by bacteria
Foot and mouth disease in cattle is caused by virus

• Diseases in plants caused by microorganisms

Citrus canker disease is caused by bacteria
Rust of wheat is caused by fungi
Yellow vein mosaic of Bhindi (Okra) is caused by virus
Previous Post Next Post