ABSTRACT
Prokaryotes are organisms whose cells lack extensive internal compartments. There are two very different groups of prokaryotes, distinguished from one another by characteristic genetic and biochemical features:
The bacteria, which include most of the commonly encountered prokaryotes including the Gram-negatives (such as Escherichia coli), the Gram-positives (such as Bacillus subtilis), the cyanobacteria (such as Anabaena), and many more
The archaea, which are less well studied and were once thought to be extremophiles, living only in inhospitable environments such as hot springs and acidic streams. Now we know that they are much more common and are present in many nonextreme environments, including the human gut.
