CBSE – 8th Standard Science


Microorganisms – Friends & Foe

  • Microorganisms are too small and are not visible to the unaided eye.
  • Microorganisms are found in air, water and in the bodies of plants and animals.
  • Diseases like dysentery and malaria are caused by protozoans whereas typhoid and tuberculosis (TB) are bacterial diseases.
  • Microorganisms may be unicellular or multicellular.

  • Microorganisms have been used for the production of alcohol since ages they are also used in cleaning up of the environment.
  • Microorganisms can live in all kinds of environment, ranging from ice cold climate to hot springs and deserts to marshy lands.
  • Microorganisms like amoeba can live alone, while fungi and bacteria may live in colonies.
  • Viruses are also microscopic they reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.


  • Viruses are quite different from other microorganisms. They reproduce only inside the host organism; bacterium, plant or animal cell.
  • Curd contains several microorganisms. Of these, the bacterium Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd.

  • Microorganisms play an important role in our lives. Some of them are beneficial in many ways whereas some others are harmful and cause diseases.
  • These days a number of antibiotics are being produced from bacteria and fungi Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics.

  • Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar)
  • Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals. They are also used to control many plant diseases.
  • These days’ vaccines are made on a large scale from microorganisms to protect humans and other animals from several diseases.
  • Some bacteria are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility. These microbes are commonly called biological nitrogen fixers.

  • The microorganisms decompose dead organic waste of plants and animals converting them into simple substances.
  • Microorganisms can be used to degrade the harmful and smelly substances and thereby clean up the environment.
  • The plant waste is always converted into manure by the action of microbes.
  • The disease causing micro organisms are called as Pathogens
  • Pathogens enter our body through the air we breathe, the water we drink or the food we eat.

  • Pathogens can also get transmitted by direct contact with an infected person or carried through an animal.
  • Robert Köch (1876) discovered the bacterium (Bacillus anthracis) which causes anthrax disease.
  • Anthrax is a dangerous human and cattle disease caused by a bacterium.
  • Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and others. the diseases reduce the yield of crops.
  • Food poisoning could be due to the consumption of food spoilt by some microorganisms.
  • Microorganisms that grow on our food sometimes produce toxic These make the food poisonous causing serious illness and even death.
  • Sodium benzoate and sodium metabisulphite are common preservatives.
  • Preservatives are also used in the jams and squashes to check their spoilage.
  • Common salt has been used to preserve meat and fish for ages.
  • Meat and fish are covered with dry salt to check the growth of
  • Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food.
  • Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment.
  • These days’ dry fruits and even vegetables are sold in sealed air tight packets to prevent the attack of
  • Our atmosphere has 78% nitrogen gas.
  • Some microorganisms reside in the root nodules of leguminous They can fix nitrogen from air into soil and increase the soil fertility.
  • Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins.
  • Sometimes nitrogen gets fixed through the action of lightning.
  • Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into compounds of nitrogen.
  • When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again.

  • Certain bacteria convert compounds of nitrogen present in the soil into nitrogen gas which is released to the atmosphere.
  • When nitrogen is converted into these usable compounds, it can be utilized by plants from the soil through their root system.
  • Nitrogen is then used for the synthesis of plant proteins and other compounds.

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