Page 15 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

Microbiology

• The study of microorganisms, which are tiny organisms that live around us and
inside our body.
• A microorganism is simply a very, very small organism that you cannot see

with your naked eye, but you sure feel its effect

• A microorganism can be one cell or a cluster of cells that can be seen only by
using a microscope.

The three-domain system
The microbial world, in fact all living organisms, can be separated into three

distinct groups called domains.
Organisms in each domain consist of cells with the same properties that distinguish

them from members of the other domains.

The three domains are:

• Bacteria
• Archaea

• Eukarya

• The three domains of life were further classified into 6 kingdoms

General Microbiology & Immunology (PM 401) theoretical handbook Page 4