The artificial scheme of classification in Bergey's Manual of Systematic Bacteriology is widely used. Knowing the source of the bacterium is also important. Can also use DNA probes. Some live harmlessly in our mouths. Chlamydiae alternate between 2 cell types, elementary bodies and vegetative cells .
Elementary bodies are tiny, round structures released when an infected host cell lyses.
They then differentiate into elementary bodies again before the host cell lyses. All lack a rigid cell wall. All are parasites of humans, animals, or plants. Their colonies have a distinctive fried egg appearance. Their wall-less structure allows them to squeeze through even the tiny pores in filters used to sterilize liquids.