Rickettsia

The genus Rickettsia encompasses a diverse group of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that are obligate intracellular parasites of mammals, plants and insects. Among the 24 validated species within the genus, Rickettsia rickettsii, the etiologic agent of Rocky Mountain spotted fever (RMSF), is the prototypic pathogenic species. Being the first rickettsiosis described, RMSF was characterized at the end of the nineteenth and beginning of the twentieth century. However, despite the long history, our current knowledge about the mechanisms of infection, ecology, and epidemiology of the etiologic agent is still very fragmentary. Part of the problem is that a limited number of laboratories found primarily in the western hemisphere work with R. rickettsii, and the number of investigations has ³uctuated greatly over time depending upon the priorities of funding agencies. Modern safety requirements and Select Agent regulations have contributed additional limitations for work with this organism. Moreover, different aspects of rickettsial biology were studied with a limited number of species; consequently, attempts to extrapolate observations from one species to the other should be taken very cautiously because the genus Rickettsia displays significant variability. This statement is well supported by comparisons of the complete genome sequences of different species; however, for the purpose of this review, we will refer to some findings obtained with different spotted fever group rickettsiae (SFGR) and comment about the values of such extrapolations to R. rickettsii.