Prokaryotic microbes

The prokaryotes are a group of organisms whose cells lack a membrane-bound nucleus (karyon ). Those organisms whose cells have a well defined membrane bound nucleus and organelles are called eukaryotes. Most prokaryotes are unicellular organisms, although a few such as myxobacteria have multicellular stages in their life cycles or create large colonies like cyanobacteria. The word prokaryote comes from the Greek πρό- (pro-) "before" and καρυόν (karyon) "nut or kernel". Prokaryotes do not have a membrane bound nucleus, mitochondria, or any other membrane-bound organelles. In other words, all their intracellular water-soluble components (proteins, DNA and metabolites) are located together in the same volume enclosed by the cell membrane, rather than in separate cellular compartments.

The division between prokaryotes and eukaryotes reflects two distinct levels of cellular organization rather than biological classification of species. Prokaryotes include two major classification domains of life: the Bacteria and the Archaea. Archaea were recognized as a domain in 1990. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.