Microorganisms: Friend and Foe



  • Living organisms which are not visible through naked eyes are known as micro-organisms.
  • These micro-organisms are present everywhere.
  • Some micro-organisms like fungus on bread can be seen with the help of  a magnifying glass.
  • Micro-organisms are classified into 4 major groups: Bacteria, Fungi, Protozoa and some algae.
  • Micro-organisms can live in all types of environments.
  • They can live in icy cold areas to hot places like desserts.
  • They also live under human body and animals.
  • Micro-organisms are both helpful and harmful to humans.
  • Friendly Micro-organisms are used in the preparation of curd, bread , cake etc..
  • They also help in the production of alcohols.
  • They are used to make various medicines.
  • They are also used in agriculture to increase soil fertility.
  • The microbes which are capable of converting the atmospheric nitrogen into fixed nitrogen are called as nitrogen-fixers.
  • Some of the biological nitrogen-fixers are Cyanobacteria, Azolla, Frankia etc..
  • Harmful micro-organisms causes various types of diseases in human beings , animals and in plants.
  • Disease causing micro-organisms are called Pathogens.
  • Microbial diseases which can be spread through air, water, food or through physical contact are known as Communicable Diseases.
  • Examples of communicable diseases are Cholera, Common Cold, Chicken Pox etc..
  • Some insects and animals spread diseases by acting as Carriers of disease-causing microbes.
  • Example of carriers are: Housefly, female Anopheles Mosquito( causes Malaria),  Female Aedes Mosquito (causes dengue).
  • Some common Plant diseases caused by Micro-organisms are: Rust of wheat caused by fungi, Citrus Canker caused by Bacteria etc..
  • Some common Animal diseases caused by Micro-organisms are: Anthrax in cattle caused by Bacteria, Foot and mouth disease of cattle caused by a virus.


  • Bacteria are prokaryotic unicellular organisms.
  • They develop through Binary Fission.
  • Prokaryotic organisms are those organisms which do not have a nucleus.
  • Diseases caused by bacteria are Typhoid, Tuberculosis, cholera, Plague etc..


  • Fungi are eukaryotic organisms.
  • Eukaryotic organisms are those organisms which contain a nucleus.
  • Fungi develop through Spore Formation.
  • Diseases caused by Fungi are skin problems.


  • Protozoans are eukaryotic organisms,
  • They are single- celled organisms.
  • They can be free- living or parasitic.
  • Protozoans develop asexually through binary fission or multiple fission.
  • Disease caused by protozoans are Dysentery, Malaria, Leishmaniasis etc..


  • Algae are eukaryotic organisms.
  • Algae is developed both  through sexual and asexual reproduction.
  • Diseases caused by algae are Diarrhoea, skin problems etc..


  • Virus replicates only inside the living cells of organisms.
  • They contain both RNA and DNA.
  • They do not get affected by antibiotics.
  • Diseases caused by Viruses are Common Cold, Flu, Polio etc..

Making of Curd and Bread:

  • Lactobacillus helps in the formation of Curd.
  • Lactobacillus multiplies in milk and thus curd is formed.
  • Yeast is used in the making of breads and cakes.
  • Yeast produces Carbon dioxide during respiration.
  • We can tell this by seeing the formation of bubbles where yeast is used.


  • The process in which sugar is converted into alcohol with the help of enzymes is called Fermentation.
  • Fermentation process was discovered by Louis Pasteur.


  • Vaccine stimulates the production of antibodies in our body.
  • It provides immunity against various diseases.
  • Antibodies helps us to fight the disease carrying microbe in our body.
  • Antibodies also remember the disease if it comes again.
  • This memory of antibodies helps us to get cured from a particular disease forever.
  • For example: when we were kids, we have been vaccinated against polio and now we are cured from that disease forever.
  • Some diseases which can be prevented from vaccination are Cholera, tuberculosis, small pox etc..

Food Preservation:

  • We need to preserve our food from micro-organisms as it spoils our food making it poisonous and unhealthy to eat.
  •  Salt and edible oils are the common preservatives used to check micro-organisms.
  • Sodium benzoate and sodium meta bisulphite  are common preservatives.
  • Common salt is used to preserve meat and fish.
  • Jam and Jellies are prevented by sugar.
  • Oil and vinegar prevents the spoilage of pickles.
  • Keeping the food at low temperature prevents the growth of micro-organisms.
  • Storing in air-tight packets prevent the entry of micro-organisms.

Nitrogen Cycle:

  •  78% of nitrogen is present in our atmosphere.
  • This atmospheric nitrogen cannot be taken directly by plants and animals.
  • The atmospheric nitrogen is converted into fixed nitrogen with the help of certain bacteria and blue-green algae.
  • This converted nitrogen is used for the synthesis of plant proteins and other compounds.
  • After the death of plants and animals, their nitrogenous waste is converted into nitrogenous compounds by bacteria and fungi.
  • These nitrogenous compounds can be used by the plants again.
  • Some part is also converted into nitrogen gas which goes into the atmosphere.
  • And thus the concentration of nitrogen in the atmosphere remains constant.

Related Articles


For Worksheets & PrintablesJoin Now