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Microorganisms – Friend and Foe Notes for Class 8, CBSE

Micro Organism - Friend and Foe notes class 8

Important topics covered From Microorganisms – Friend and Foe Chapter:

  • Introduction to microorganisms
  • Commercial and Medicinal use of Microorganisms
  • Harmful microorganisms
  • Food poisoning
  • Food preservation
  • Nitrogen fixation
  • Nitrogen cycle

Introduction to microorganisms:

  • Those organisms which are too small to be seen with our naked eye are said to be microorganisms.
  • They are also called microbe.
  • Generally microbes are unicellular while some other is multicellular.
  • They can survive under different types of climatic environment, ranging from hot to ice cold climate.

microorganisms notes for class 8

Types of microorganisms:

Microorganisms are mainly classified into four groups. They are:

  • Bacteria are the microorganisms with defined cell wall but cell has no nucleus.
  • Some of the bacteria are autotrophs while some other are heterotrophs.
  • These are categorized in the plant kingdom.
  • The part of it can be the category which can be in the same manner.
  • Lactobacillus, Escherichia coli etc. are some examples of bacteria.
  • These are multicellular organisms.
  • Chlorophyll is absent in fungi so they can’t prepare their own food.
  • They are dependent on heterotrophs and dead organic matter for their food.
  • These are mainly found in colonies.
  • Aspergillus and Penicillium are some of the examples of fungi.
  • These are green substances floating on the surface of stagnant water, pond, lake, river, moist soil etc.
  • These are either unicellular or multicellular organisms.
  • Also, they are mainly autotrophs and contains chlorophyll, thus carry photosynthesis to prepare their own food.
  • Chlamydomonas and spirogyra are some examples of algae.
  • Protozoa are unicellular organisms.
  • Some protozoans are free-living while the others are parasites.
  • Protozoa do not have cell wall and chloroplast but they contain a nucleus.
  • Some of the parasitic protozoans cause disease in humans.
  • Amoeba and Plasmodium are some examples of protozoa.
  • Reproduction of viruses takes place in host organism.
  • The host organism may be bacteria, plants or animals.
  • Viruses are considered as the borderline between living and non-living organisms. As they possess the character of living as well as non-living organisms.
  • Outside a host cell, the virus behaves as non-living as when it is outside a host it does not carry on nutrition, respiration or reproduction.
  • Inside a host it carries all the essential life processes, thus behaves as living organisms.
  • HIV, H1N1, TMV are some examples of virus.

Where do microorganisms live?

where do microorganisms live?

  • Microorganisms are present almost everywhere on earth.
  • They are observed in all the three habitats i.e. land, water and air.
  • They are even observed in harsh conditions such as high-temperature zones (volcano), extremely cold climates, desert and marshy lands.
  • Microbes are also being observed inside the body of living organisms including human beings.
  • Amoeba lives alone or freely while fungi and bacteria live in colonies.

Friendly microorganisms:

importance of microorganisms 8th class CBSE

  • Some of the microorganisms are much helpful to us.
  • Lactobacillus bacteria helps in converting milk into curd.
  • Also, many products like cake, bread, antibiotics and beverages are prepared with the help of microorganisms.
  • They also help in cleaning our environment by acting as bio-fertilizers and decomposers.
  • Sewage treatment also uses microorganisms.
  • The organic wastes (vegetable peels, remains of animals, faces, etc.) are broken down into usable and harmless substances by bacteria.
  • They are used in agriculture to increase soil fertility by nitrogen fixation.

Commercial use of microorganisms:

Commercially microorganisms are of great use. Several food and beverage industries, as well as many other industries, uses it as:

Making of Curd and Bread:

making of curd by bacteria

  • Lactobacillus bacteria helps in the formation of curd.
  • When a little amount of bacteria is added to the milk it starts multiplying and gets converted to curd.
  • Bacteria are also involved in the making of cheese, pickles and many other food items.

Preparation of alcohol:

preparation of alcohol

  • From ages alcohol preparation through yeast has been the most viable method.
  • Saccharomyces cerevisiae is popularly known as Brewer’s yeast is used for alcohol preparation.

Preparation of different food items:

prepartion of food items

  • Microorganisms are used for the preparation of vinegar (acetic acid).
  • Yeast microbe is also used in baking.
  • It also plays an important role in fermenting idli/dosa batter (containing urad pulses).

Medicinal use of Microorganisms:

medicinal use of microorganisms

Microorganisms are used on a large scale in the medicinal sector. Several medicines make use of microbes in one way or the other. Some of them are listed below:

  • Doctors prescribe us antibiotics, injections or capsules such as penicillin, all these are made of microorganisms.
  • Bacteria and fungi are used nowadays to prepare medicines which fight against various disease-causing germs. Such medicines are called antibiotics.
  • Streptomycin, erythromycin, tetracycline, amoxicillin, cefotaxime, cefoperazone etc. are made from bacteria and fungi and are some antibiotics.
  • Microorganisms are also used to cure diseases in plants.



  • One of the main quality of human body is that it has tendency to fight against diseases.
  • A small amount of disease-causing weak microbes (mainly they are dead) is inserted in our body, this makes the antibodies present in our body to fight against the particular microbe.
  • Once the antibodies are activated it remembers how to fight with that particular microbe throughout life.
  • This is the way how vaccine
  • Cholera, TB, hepatitis, smallpox, hepatitis etc. are some diseases which can be prevented by vaccination.

Soil fertility:

  • Blue-green algae are the most favourable microorganism which helps in increasing soil fertility.
  • They help in nitrogen fixation.
  • Leguminous plants contain rhizobium bacteria in its root nodule for nitrogen fixation.
  • Plants utilize nitrogen from soil through their roots.

In the cleaning environment:

  • Decomposers are the microorganisms which help in converting dead and decaying matters into simpler substances.
  • Microbes have the main role in clearing organic waste from our surroundings.

Harmful microorganisms

harmful microorganisms

  • Many microbes cause several diseases in plants, animals & human beings.
  • That is why we read this chapter as microorganism friend and foe.
  • We call such disease-causing organisms as Pathogens.
  • Bacteria, viruses, protozoa & fungi are pathogens.
  • Typhoid is an example of diseases which is caused pathogen bacteria named as Salmonella typhi. This microorganism enters our body through contaminated food and water.
  • Food, clothing, leather and many of our useful items get spoiled by microorganisms.
  • Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called Communicable diseases.

Food poisoning

  • Microbes produce toxic substances in the food and thus it gets poisoned.
  • Eating such food makes us ill.
  • Microbes do not attack a particular food, only it searches a suitable environment to survive.
  • Moisture and warm temperature make it easy to survive thus releases toxic substance in food. Thus food poisoning occurs.

Food preservation

food preservation

  • Prevention of food from getting it spoiled by the action of microorganisms by taking suitable measures are said to be food preservation.
  • It is very important to preserve food items so that microbes cannot grow & spoil them.

Methods of food preservation:

There are many methods of food preservation. We will discuss each one by one.

Chemical Methods of food preservation:

  • Preservatives are the chemicals which check the growth of microorganisms on the food.
  • In pickles, we use salt as preservatives.
  • Also, salt is used to preserve meat and fish.
  • Sugar is used in jams as a preservative material.
  • Vinegar is used as preservative in vegetables.
  • Sodium benzoate, oils (edible) and salts are common preservatives.

Heat and cold treatments

  • We boil milk to kill microorganisms present in it.
  • Milk for commercial use is pasteurized.
  • The technique to boil milk at 70⁰C for 15 to 30 sec then suddenly cooled and stored is said to be pasteurization.
  • We use refrigerators to preserve fruits, vegetables and edible items.

Proper storage and packing:

  • Properly sealed packets or airtight containers are necessary to store some kind of food such as dry fruits.
  • Such containers would inhibit the growth of microbes as they need moisture, air etc. for growth.

Nitrogen fixation

  • The process of converting Nitrogen (N2) in the atmosphere into ammonia (NH3) inside the soil is said to be nitrogen fixation.
  • Those bacteria which is involved in such process is said to be nitrogen-fixing bacteria.
  • Rhizobium bacteria which lives in the root nodules of leguminous plants (peas and beans) helps in nitrogen fixation.
  • Nitrogen also gets fixed through the action of lightning.
  • However the atmospheric nitrogen is fixed, it neither decreases nor increases.

Nitrogen cycle

  • Our atmosphere contains 78% nitrogen gas and is most essential for living organisms.
  • Nitrogen is one of the main constituents of protein, chlorophyll and vitamins.
  • Atmospheric nitrogen can’t be utilized directly by living beings.
  • Blue-green algae and certain bacteria present in soil fix the nitrogen of atmosphere.
  • Nitrogen gets utilised by plants from the soil through their root system then used for the synthesis of plant proteins and other important compounds.
  • Heterotrophic animal feeds on plants and get protein and essential nutrients based on nitrogen.
  • When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds.
  • Plants use these nitrogenous compounds again.

nitrogen cycle

We will have a detailed study of each topic in higher sections. However, the necessary material is provided in class 8 chapter 2 microorganism friend and foe notes.

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