From extracellular to intracellular: the establishment of a symbiosis.

The colonization of host cells by modern symbionts is surveyed. The morphological distinction between extracellular and intracellular symbionts is not sharp, and the various kinds of association can be arranged in a graded series of increasing morphological integration of the symbiont into the host cell. Apart from some aggressive parasitic infections, the great majority of symbionts are enclosed by a host membrane in a vacuole. Those not enclosed in a host vacuole usually cannot be cultivated outside the cell. It is therefore surmised that encirclement by a vacuolar membrane would only disappear, if at all, in the later stages of the evolution of intracellular symbiosis. Recognition mechanisms between host and symbiont occur, but have been little studied. In some associations, recognition at surface contact occurs, and there is evidence for the involvement of lectins in certain cases. In other associations, recognition may occur wholly or in part after the entry of symbiont into host cells. After entry, special mechanisms for the biotrophic transfer of nutrients from symbiont to host develop. Both the symbiont population size and its rate of increase are strictly regulated by the host cell; symbiont metabolism may be controlled likewise. Rates of evolution of intracellular symbionts are probably very rapid, owing in part to responses of the host cell to its symbiont.