Introduction

• Beside several kinds of plants and animals, there are other living organisms around us which could not be seen by naked eyes.
• These are called microorganisms or microbes.
• These microorganisms or microbes are so small in size that they cannot be seen with the unaided eye.

Micro organisms as observed under microscope

• We need a microscope to observe them.

• They are generally found in air, water and soil.
• Microorganisms are used for various purposes. They are used in the preparation of curd, bread and cake.
• They are also used in cleaning up of the environment.
• In agriculture they are used to increase soil fertility by fixing nitrogen.
• Elemental form of nitrogen cannot be used by directly from atmosphere by plants.
• It has to be first converted into nitrates and nitrites. This conversion is known as nitrogen fixation and it is done by micro organisms like nitrosomonas, nitrobacter etc.

    Nitrobacter

Nitrosomonas

• They can also be harmful as many of them can cause diseases in human beings, animals or plants.
• Thus they are of different types.