What are the major groups of microorganisms?

Microorganisms are classified into four major groups. These groups are:

1. Bacteria — They are single celled. They do not have true nucleus or membrane bound cell organelles. They may be spherical, rod shaped or comma shaped.
2. Fungi — They are multicellular.
3. Protozoa — They are single celled.
4. Algae — They may be single celled or multi cellular.