NCERT Class VIII Science Chapter 2 Microorganisms - Friend and Foe

Table of Contents


Organisms that are so small that they can only be seen through a microscope are called microorganisms or microbes.
The study of microorganisms is known as microbiology.
Most organisms are hardy and can be found in almost any kind of environment – hot desert, polar ice caps, salt water, marsh lands and inside other organisms.

Types of microorganisms:

There are five major groups of microorganism. These are as follows:
  1. Bacteria
  2. Algae
  3. Protozoa
  4. Fungi
  5. Viruses


  • They are simple living organisms which are found all around us. They are mostly single- celled tiny organisms, but they live together in colonies of millions.
  • Bacteria are found in three different shapes:
  1. Rod shaped, called bacilli.
  2. Spherical shaped, called cocci.
  3. Spiral called spirilla.


  • Algae are simple plant- like organisms which are usually aquatic in nature.
  • They contain a cell wall and chlorophyll and can make their own food by photosynthesis.
  • Algae can be unicellular or multicellular.
  • Some of the common examples are diatoms, chlamydomonas, and seaweed.


  • These are unicellular microscopic organisms similar to animals that can move about to capture food and are heterotrophic in nature.
  • They are mostly aquatic in nature.
  • Amoeba, paramecium are some examples of protozoa.


  • Plant -like organisms that do not contain chlorophyll are called Fungi.
  • Fungi may be unicellular (Yeast) or multicellular (Bread mould) and are found in warm and moist places.
  • Fungi can be heterotrophic, saprophytic or parasitic in nutrition.
  • Examples: Yeast, Rhizopus (Bread mould), mushrooms, puffballs.


  • Viruses are smaller than any known cell.
  • Viruses can only be seen with electron microscope.
  • Viruses can reproduce only inside the bodies of other organisms, which mean they need a host.
  • A virus is like a non- living thing outside the body of other organisms. Therefore, it is a borderline between a living and a non- living thing.


Microorganisms are useful in the following ways:
  • In food and beverage industry
  • In making medicines and vaccines
  • In agriculture
  • In cleaning the environment

Food and beverage industry:

  • Bacteria help us in making certain foods. Lactobacillus is a bacterium that sours milk and is used to makes curd from milk.
  • Many fungi like Mushrooms and morels are edible which are rich
  • Yeast is commonly employed in preparing food items like bread, cakes, idli, dosa, dhokla etc.
  • Yeast is also used in breweries for making wines by the process of fermentation.

Making medicines and vaccines:

  • Bacteria and fungi are used to make medicines called antibiotics.
  • The first antibiotic, penicillin was discovered by Alexander Flemming using the fungus penicillium.
  • Some common antibiotics are streptomycin and terramycin.
  • Microbes are used to make vaccines. Vaccines are weakened or dead germs introduced into the body. They offer protection against diseases like tuberculosis, polio etc. by producing anti- bodies.
  • Bacteria and Yeast are used in making vitamin B complex tablets.
  • The human hormonal called insulin can also be obtained from bacteria.

In Agriculture:

  • Some bacteria like Rhizobium, blue green algae like Nostoc, live in the root nodules of plants such as gram, pea etc. These bacteria absorb the nitrogen of the atmosphere and convert it into nitrate which serves as natural fertilizers for plants, hence, enhancing soil fertility.

In cleaning the environment:

  • Microbes help to keep our environment clean by decomposing dead matter. They decomposed substances are recycled as these get reused from the soil by plants.
  • Some bacteria decompose sewage and other waste in water. This is nature’s method of keeping the environment free from pollution.


  • Microorganisms that cause diseases in human, animals and plants are called pathogens or germs.
  • Germs may enter the body of living organisms through air, contaminated food and water, from an infected person by direct or indirect contact or by a carrier.
  • Diseases that can spread from an infected person to a healthy individual through air, water or direct contact are called communicable diseases. Examples: Common cold, chicken pox, AIDS etc.
  • Micro organisms also cause diseases in animals and plants. For examples-
  • Anthrax is a disease caused by bacterium and affects human and cattle.
  • A virus causes the dangerous foot and mouth disease in cattle.
  • Citrus canker is a bacterial disease that affects trees of citrus fruits and is spread by air.
  • Rust of wheat is a viral disease that affects vegetable like bhindi & spread by insects.
  • Malaria is actually caused by pathogen called plasmodium (protozoa) which is transmitted by female Anopheles mosquito.
  • Dengue is caused by dengue virus and spread by female Aedes mosquito.

Preventing the spread of Communicable diseases:

Some simple methods of limiting the spread of communicable diseases are:
  • To keep the infected person separated from others & to advice his/ her to keep a handkerchief on the nose and mouth while sneezing.
  • To keep our environment or surrounding clean.
  • Never let garbage collect in the neighborhood.
  • Timely vaccination against diseases should also be taken.
  • To prevent mosquitoes from breeding we should not allow water to collect anywhere in our neighborhoods.

Food spoilage:

Many bacteria and fungi grow on food items and produce certain toxic substances. This makes the food unfit for consumption. Consuming such food can cause a serious illness called food poisoning.

Food preservation:

  • Processing of food to prevent their spoilage and to retain their nutritive value for period is called food preservation.
  • Food can be preserved using many methods:-
    1. Heating: heating food to a high temperature kills microbes. For example: Milk and water are boiled to kill microbes.
    2. Cooling: food can be kept in refrigerator at about 5˚C which delays its spoilage.
    3. Canning: canning is done to package or preserve food or drink by putting it in sealed, airtight containers.
    4. Salting: Fruit and vegetables can be preserved by using salt and then drying. Salts prevent the growth of microbes.
    5. Sweetening: Excess sugar can also works on the same principle as salting. Jams, jellies & squashes are preserved by this method.
    6. Dry or dehydration: Dehydration of food remaining water from it. This stops microorganisms from growing as they cannot grow without water.
Chemical preservatives: Chemicals like sodium benzoate and sodium metabisulphite prevent microbial attack and are often use to preserve jams, sauces & ketchup.

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Class 8 Maths Class 8 Science
Reference Books for class 8 science

Given below are the links of some of the reference books for class 8 science.

  1. Oswaal NCERT & CBSE Pullout Worksheets Class 8 Science These worksheets can help student evaluate whatever he/she has studied from the text books.
  2. Science for Class 8 Paperback by Lakhmir Singh Detailed science book to clear basics and concepts. I would say it is a must have book for class 8 student.
  3. Pearson Foundation Series (IIT -JEE / NEET) Physics, Chemistry, Maths & Biology for Class 8 (Main Books) | PCMB Combo : These set of books could help your child if he aims to get extra knowledge of science and maths. These would be helpful if child wants to prepare for competitive exams like JEE/NEET. Only buy if you can provide help to the child while studying.
  4. Reasoning Olympiad Workbook - Class 8 :- Reasoning helps sharpen the mind of child. I would recommend students practicing reasoning even though they are not appearing for Olympiad.

You can use above books for extra knowledge and practicing different questions.

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