Intracellular pathogens: living in an extreme environment.

The phylogenetic tree segregates into three major groups, which probably descended from a common ancestor: the bacteria, the archaea, and the eucarya (1–3). While bacteria and archaea lack a true nucleus and intracellular organelles, eucarya possess these. The origin of the ancestor remains elusive, but it emerged about 4 billion years ago. The bacteria probably started life 3.5 billion years ago and eukaryotes emerged 1.75 billion years thereafter. Sometime in between archaea appeared. Eukaryotes began as unicellular organisms but soon also developed multicellular forms. Bacteria and archaea had sufficient time to exploit all niches of the abiotic environment, ranging from ice cold glacial lakes to fiery geysers – from the highest mountain peaks to the deepest sea beds. They also quickly developed forms of mutualism by forming colonies composed of mixed populations. With the evolution of eucarya, new environments could be exploited. Today prokaryotes are the most prevalent form of life on earth, making up the majority of our biomass. It has been estimated that more than 10 prokaryotic organisms live on our globe. Thus, archaea and bacteria are far more prevalent than eucarya. Even human beings comprise more prokaryotic cells than mammalian cells, which range on the order of 10 human cells and 10 prokaryotes mostly concentrated in our intestinal system (4–7).