Microorganisms: Definition of Microorganisms, Types of Microorganisms (For CBSE, ICSE, IAS, NET, NRA 2022)

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Definition of Microorganisms

  • Those minute organisms that are invisible to the naked eye are called Microorganisms.
  • A microorganism may exist either in single celled form or in a colony of cells.
  • These organisms can be unicellular or multicellular.
  • They are widespread in nature and are beneficial to life.
  • Some of them are harmful as well.

Six Major Types of Microorganisms

Six Major Types of Microorganisms


  • These are unicellular organisms.
  • They exist in four major shapes:
    • Bacillus (rod shape)
    • Coccus (spherical shape)
    • Spirilla (spiral shape)
    • Vibrio (curved shape)
  • They can be classified as either Gram-positive or Gram-negative based on their cell wall structure.
  • Classification based on the response to gaseous oxygen:
    • Aerobic (living in the presence of oxygen)
    • Anaerobic (living without oxygen


  • They differ from true bacteria in their cell wall structure and lack peptidoglycans.
  • They are also known as Archaebacteria.
  • Classification based on the habitats:
    • Methanogens (methane-producing organisms)
    • Halophiles (archaeans that live in salty environments)
    • Thermophiles (archaeans that live at extremely hot temperatures)
    • Psychrophiles (cold-temperature Archaeans)


  • They are unicellular or multicellular eukaryotes.
  • They are also called cyanobacteria or blue-green algae.
  • They live in water, damp soil, and rocks.
  • They are also responsible for producing oxygen and carbohydrates used by other organisms.


  • They are unicellular eukaryotes.
  • They have a nucleus, complex organelles.
  • They are useful in mineralizing nutrients and making them available to the plants and other soil organisms.
  • They feed on bacteria and regulate the bacterial population.


  • They consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat.
  • They are no cellular entities.
  • They cannot reproduce outside a host cell.
  • They cannot metabolize their own.

Useful Microorganisms

  • These microorganisms help in the production of several food items, medicines, manufacturing, and research.
  • Some of the useful microorganisms are Bacteria, Fungi, and Protozoa.
  • They are a significant part of the ecosystem and participate in the production of minerals and gases like oxygen, carbon dioxide.
  • They feed on the dead and decaying matters.
  • One of the important examples is that of the biogeochemical cycle such as the nitrogen cycle.
  • These organisms can be further used in various industries to produce various metabolites such as ethanol, riboflavin, lactic acid, and butanol.

Harmful Microorganisms

  • These are the microorganisms responsible for food spoilage, diseases, and infections.
  • Bacteria are one of the most harmful microorganisms responsible for several infectious diseases such as tuberculosis, cholera, etc.
  • Fungi can cause skin infections and allergies.
  • Viruses may cause AIDS, Influenza, etc.
  • Microorganisms multiple on food and release toxic products causing food poisoning.


Q 1. What is the simple definition of Microorganism?

Answer: Those minute organisms that are invisible to the naked eye are called Microorganisms.

Q 2. What are the six major types of microorganisms?

Answer: Six major types of microorganisms are namely Bacteria, Archaea, Fungi, Protozoa, Algae, and Viruses.

Q 3. What are the four major shapes into bacteria exist?

Answer: The four major shapes into which a bacterium exist are namely Bacillus (rod shape) , Coccus (spherical shape) , Spirilla (spiral shape) , and Vibrio (curved shape) .

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