How do microbiologists determine the correct kingdom for a microbe? Several characteristics are used to describe a living organism; and some of the primary (most important) characteristics include size, complexity, motility, nutritional habits, and location of the DNA.

Observing microbes can be used to estimate the size of the organism. If you can easily see the organism, then the organism is called macroscopic. Organisms that require a microscope to study and observe them are called microscopic. Remember that most macroscopic organisms are at least 200-300 microns in size.

Living organisms vary in complexity. Many (most) larger organisms are composed of many different types of specialized cells called tissues. Organisms like the majority of plants, animals, and fungi are composed of a variety of different types of cells and are called multicellular. A recently found organism in the gut of the sturgeon fish is known to be about 500 microns in size, yet is considered to be a one-celled organism. Most other organisms like the bacteria and protists are unicellular (composed of one cell); however, some of these microscopic organisms form a filament (strand) or mass (clump) of similar cells and are called colonial.