Class 8 Microorganisms : Friend and Foe Notes


Microorganisms : Friend and Foe Notes


The living organisms which cannot be seen by the naked eyes but are only seen under the microscope are called microorganisms or microbes.

  • The study of microorganisms is known as microbiology.

Types of microorganisms:-

  • Bacteria:- Staphylococcus, Lactobacillus.
  • Fungi:- Bread mould, Yeast.
  • Protozoans:- Paramoecium, Amoeba, Euglena.
  • Algae:- Chlamydomonas, Spirogyra.

Viruses are also microscopic. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal. Without the cells of the host, they are not able to reproduce and multiply. So, they are considered as the boundary of living and non-living.

➣ Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as algae and fungi.

Uses of microorganisms:-

  • Microorganisms are vastly used for baking purpose.Ex:- yeast is used for making cake, pastries or bread.
  • Bacteria are involved in the making of cheese, pickles and many other food items. Ex:- Lactobacillus bacterium helps in the formation of curd.
  • Yeast helps in the production of alcohol, wine and vinegar (acetic acid) by the process of fermentation.
  • Microorganisms like fungi and bacteria are used to produce medicines called antibiotics. Some examples of antibiotics are penicillin, streptomycin, tetracycline and erythromycin.
  • Microorganisms are used to make vaccines which help our body to produce antibodies to protect from-disease causing microbes.
    Several diseases, including cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.
  • Microorganisms help to keep the environment clean by decomposing and converting the biodegradable wastes into manure.
  • Microorganisms are used in the treatment of sewage.
  • Some bacteria like rhizobium and blue green algae increase the fertility of soil by fixing nitrogen. So, they are called biological nitrogen fixers.

Fermentation:- The process of conversion of sugar into alcohol with the help of yeast is known as fermentation.

Antibiotics:- These are the medicines which kill or stop the growth of the disease-causing microorganisms.
Ex:- Penicillin, streptomycin, tetracycline and erythromycin.

Antibodies:- When a diseases-carrying microbe enters our body, the body produce antibodies.

Immunity:- If any microorganism enters the body again, it gets recognized and destroyed much faster. This is called immunity.

Harmful effects of microorganisms:-

  • Some diseases in living being are caused by microorganisms. These harmful microbes are known as pathogens.
  • Some microorganisms make the food poisonous causing serious illness and even death.
  • Some microorganisms spoil clothing and leather.
fungus on strawberry
fungus on strawberry

Some common human diseases caused by microorganisms:-

Causative MicroorganismHuman DiseasesMode of TransmissionPreventing measures
Chicken pox
i. Keep the patient in complete isolation.
ii. Vaccination to be given at suitable age.
i. Maintain personal hygiene and good sanitary habits.
ii. Consume properly cooked food
VirusHepatitis AWateri. Drink boiled drinking water.
ii. Take proper Vaccination.
ProtozoaMalaria Mosquito
i. Use mosquito net and repellents.
ii. Spray insecticides.
iii. Clean stagnant Water.

Some common plant diseases caused by microorganisms:-

MicroorganismPlant DiseasesMode Of Transmission
BacteriaCitrus cankerAir
FungiRust of wheatAir/Seeds
VirusYellow vein mosaic of okraInsects

Pathogens:- The disease-causing-microorganisms are called pathogens.

Communicable diseases:- The diseases that can spread from an infected person to healthy person through air, water, food, insects, animals or physical contact are called communicable diseases.
Ex:- Cholera, common cold, chicken pox and tuberculosis.

  • Anopheles mosquito carries the parasite of malaria.
  • Female Aedes mosquito carries dengue virus.

Pasteurization:- The process in which milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled and stored to prevent the growth of microbes is called pasteurization.

Food Preservation:-

Preservation by chemical method:-

Preservatives:- The common chemicals like salts and edible oils which are generally used to check the growth of microorganisms are called preservatives.

  • Salt like sodium benzoate and sodium metabisulphite or acid preservative is added to pickles, jams and squashes to prevent the attack of microbes.

Preservation by common salt:-

Preservation by common salt
Fish preservation
  • Meat and Fish are covered with dry salt to check the growth of bacteria.
  • Salting is generally used at house to preserve pickles because salt removes water, dry condition is not good for the growth of microbes.
  • It is also used to preserve amla, tamarind and raw mangoes.

Preservation by sugar:-

Preservation by sugar
Jams preservation
  • Sugar is used as preservative in jams, jellies and squashes.
  • Sugar reduce the moisture content which inhibits the growth of bacteria which spoil food.

Preservation by oil and vinegar:-

Preservation by oil and vinegar
Pickles preservation
  • Some food are like pickles are preserved by adding either oil or vinegar to them.

Uses of hot and cold treatment:-

preservation by cooling treatment
cooling treatment
  • Heating of food kills many microbes.
  • Refrigerating food at low temperature restricts the growth of microbes.
  • Pasteurization is a process in which milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled and stored. By doing so, it prevents the growth of microbes.

Use of sealed air tight packets:-

Preservation by sealed air tight packets
Sealed air tight packets
  • Dry fruits and even vegetables and many other edible items are sold in sealed air- tight packets because it prevent the attack of microbes.

Some important discoveries in this field:-

Louis Pasteur
Louis Pasteur
  • Louis Pasteur discovered fermentation in 1857. He also discovered the process of pasteurization.
  • Alexander Fleming prepared penicillin by the help of green mould in 1929.
  • Edward Jenner discovered the vaccine for small pox in 1798.
  • Robert Koch discovered bacterium (Bacillus Anthracis ) which cause anthrax disease.


The maximum upload file size: 100 MB. You can upload: image. Drop file here