Microorganisms are those which are not visible to the naked eye.
We need a microscope to see them.
Though they are very minute in size, they carry out their life autonomously like large animals.
These organisms are mostly single cellular and sometimes multi-cellular.
The differences among microbes can be described based on their structure, physiology, habitats and also food habits.
Types of Microorganisms
These are single cellular beings which have cell wall and also flagella. They are considered as prokaryotic due to lack of nucleus. Bacteria are omni present i.e. they can be found on the earth surface, inside, in the air, in the living bodies etc.
They obtain their food by parasitic nature or saprophytic nature. They eat up almost anything.
Bacteria cause many diseases like tuberculosis, leprosy, cholera etc.
Archaea: These are similar to bacteria but live in more harsh conditions and extreme temperatures. They differ from bacteria in few biochemical molecules.
Algae: These multi-cellular yet microscopic in nature.
They can be seen when they grow in large colonies as patches. They live in marine and also fresh water environments.
Fungi: These are multi-cellular and some of them are microscopic while others are macroscopic. They grow on bread and other food stuffs at home. They can be invisible when present as single strands. But as they grow into bunch of strands, they can be visible as fluffy material.
Protozoa: These are large class of animals which are visible only under a microscope. They have ability to move and find their food. They can cause diseases in man and other large animals. They are mostly found in fresh water and in slimy moist soil.
You can read more about them at examples of protozoa.
Diseases caused by protozoa include malaria, gingivitis etc. They are the only examples of single celled parasite.
Yeast: These are class of fungi but are single celled. They are some of the widely used organisms for human purposes like production of alcohol, baking as nutritional supplements etc.
Diatoms: These are a type of micro-algae enclosed in the cell wall made of silica. They are mostly present in fresh water and marine water.
Virus : These are not living organisms. But these virus is biological agent which when enter into living being take up its nuclear machinery and multiply.
They are absolute parasites and cannot live without a host. They have a protein coat and nucleic acid like DNA or RNA.