What are Microbes?
Microbes or microorganisms are tiny organisms which are so small that we cannot see them with an unaided eye. Some microorganisms can be seen with the help of a magnifying glass (such as fungus that grows on bread) while some can only be seen when you use a microscope (such as bacteria and protozoa). Microorganism were first observed by Anton Van Leeuwenhoek.Viral, Bacterial, and Protozoan Diseases

A microorganism or microbe is a microscopic organism that may exist in its single-celled or colony of cells.

Nature and occurrence: -

▪ Life on earth depends on microorganisms. They play a very critical role in the ecosystem of our planet. Microbes perform the important role of breaking down dead organisms into the basic nutrients that are essential for life on our planet.
▪ Bacteria, fungi, algae, and protozoa are the 4 major groups of microbes.
▪ Viruses are another class of microbes which totally depend on the host for food, growth and reproduction. They are incapable of independent existence.
▪ Nature: Microbes may be unicellular e.g. bacteria, protozoa, or multicellular (2 or more cells) e.g. some Algae and Fungi.

Features of Micro-organisms

  • Microorganisms are too small and are not visible with the naked eye.
  • They can live in all kinds of environment, ranging from ice cold climate to hot springs, deserts and marshy lands.
  • Microorganisms are found in air, water and in the bodies of plants and animals.
  • They may be unicellular or multicellular.
  • Microorganisms include bacteria, fungi, protozoa and some algae. Viruses though different from the above-mentioned living organisms are referred to as microbes.
  • Viruses are quite different from other microorganisms. They reproduce only inside the cells of the host organism: bacterium, plant or animal.


There are four major types of microorganisms:

Fig 1: Types of Microorganisms

Bacteria: These are single-celled organisms with a rigid cell wall. They can only be seen under a microscope which enlarges images from 100 to 1000 times.

Ultrastructure of a Bacterial Cell

Types of Bacteria (based on their Shapes) and their Examples


  • Fungi: These are non-green plants and hence, cannot make their own food. They either live as parasites (deriving nutrition from host organisms, for example, Puccinia which causes wheat leaf rust) or grow on the organic matter (such as bread mould). 

Puccinia triticina

Fig 8: Bread Mould

  • Fungi, like mushrooms, moulds, mildews, and years, are eukaryotic. It means that they have a true nucleus.
  • The main components of fungi are:
  • i. Hyphae: They are thread-like filaments which penetrate into substrates, secrete enzymes to break down nutrients into smaller molecules, and absorb them.
  • ii. Spores are a unit of sexual or asexual reproduction. They can adapt for dispersal and survival for extended periods of time in unfavourable conditions.
  • Algae: These are simple plant- like organisms which are usually aquatic in nature. They contain a cell wall and chlorophyll and can make their own food by photosynthesis. Algae can be unicellular or multicellular. Some of the common examples are diatoms, Chlamydomonas, and seaweed.
  • Protozoa: Protozoa are unicellular organisms. Some of them live independently while others live as parasites. Many of the parasitic protozoans cause diseases in plants, domestic animals, and human beings. Example of some protozoans are Amoeba, Plasmodium and Paramecium

Fig 9: Paramecium

How are Viruses different from other microbes?

  • Viruses are microscopic organisms but they are different from other microbes because they reproduce only inside the cells of the host organism (which can be a plant, animal, or a bacterium).

 Types of Viruses

  • Viruses are much smaller than bacteria and come in a wide variety of shapes and sizes. A complete virus particle is known as Virion.
    Virion consists of a nucleic acid surrounded by 'capsid'. Capsid is a protective coat made of protein. The subunits of this protein called 'Capsomeres'. Viruses can be seen only by an electron microscope as they are ultramicroscopic in size.
  • Outside the body of a living organism, they do not show any reaction and hence, can be crystallized and stored like non-living things.