Symbiosis and pathogenesis: Evolution of the microbe-host interaction

Symbiotic and pathogenic bacteria have in common that they live in or on ho st organisms or host cells. To make a successful living in eukaryotic hosts , bacteria must possess the traits to recognize a given host and establish adherence. When the bacterial location is internal or intracellular, they m ust further have the ability to invade, to establish a niche, and finally t o multiply within a host. The underlying mechanisms which allow this form o f existence show similarities between symbiotic and pathogenic bacteria. Th e final outcome, however. may result in a wide spectrum of consequences for the host ranging from the acquisition of novel metabolic pathways to damag e or death. Despite the vastly different forms of interactions, symbiotic a nd pathogenic bacteria have in common that they are adapted to a particular environmental niche represented by the host organism or compartment thereo f. This contribution reviews the evolutionary forces which have shaped the microbial-host interactions. Particular emphasis is placed on the genetic a nd molecular mechanisms that drive bacterial evolution in response to the s elective pressures of the host environment.