The cell as an extreme environment.

Living cells and their intracellular parasites show many of the characteristics ascribed to extreme environments and their dominant species. The diversity of species colonizing intracellular habitats is low, and successful inhabitants exhibit special fitness traits that often render them obligately dependent on residence within a host cell. However, the diversity-limiting factor in the extreme environment of the host cell interior is not abiotic, as it is in conventional extreme environments. It is biotic: the living cell itself and its many activities. Host cells bar the entrance to most would-be parasites, they destroy most of those that do manage to get inside, and they deny parasites free access to many components of their soluble metabolite pools. Successful intracellular parasites have evolved fitness traits that give them the capacity to survive in the face of diversity-limiting factors or to modify the intracellular habitat so that those factors no longer operate. Looking on the cell as an extreme habitat emphasizes its simultaneous roles as environment, antagonist, and competitor.