In the 1970s and 80s a new type of bacterium
was analyzed that eventually lead to the classification
of a new prokaryotic cell type, the archeabacteria (or
Archaea). Even when viewed with an electron microscope,
the structural differences between different bacteria are
minor compared to other groups of organisms. Because the
structural differences are so slight, bacteria are classified
based primarily upon their metabolic and genetic characteristics.
Bacteria can be characterized properly only when
they are grown on a defined medium because the characteristics
of these organisms often change, depending on
their growth conditions.