ABSTRACT
Introduction Rickettsia may be considered the epitome of infectious, pathogenic bacteria. High morbidity and mortality, environmental stability, low infectious dose, and persistence in infected hosts have contributed to Rickettsia prowazekii being weaponized as a biological warfare agent (Azad, 2007). Rickettsiae have also been implicated as the origin of the quintessential end product in the evolution of symbiosis and inheritance of bacteria, the mitochondria. Rickettsia seemed to be characterized by two extreme phenotypic manifestations, with an absence of any representatives in between; rickettsiae as infectious pathogens and rickettsiae as inherited and obligate organelles of eukaryotic cells. Only recently has the genomic sequencing of pathogenic Rickettsia allowed insight into the natural history of the pathogenic arm. The discovery of Rickettsia that are obligatory symbionts of their insect hosts has demonstrated that Rickettsia do indeed span the whole spectrum of host-pathogen associations from symbionts without which a host cell cannot survive to pathogens that will kill their host cells readily. The term symbiosis denotes here a long-term association that renders the symbiotic bacterium noninfectious for all practical purposes, but which in itself does not connote any benecial or detrimental qualities.
