Rickettsia: The Typhus Group

The phylogenetic position of Rickettsia, as inferred from rRNA sequence data, is within the α-proteobacteria group (1). The genus Rickettsia comprises two main groups: the typhus group Rickettsia and the spotted fever group Rickettsia. However, several species, such as Rickettsia bellii and Rickettsia canada, although phylogenetically close, have been shown to be distinct from the two main groups (2,3). Rickettsia prowazekii and Rickettsia typhi are the only two members of the typhus group. Both species are pathogenic to humans (4). The Latin name of the typhus pathogen, Rickettsia prowazekii, was given to honor the scientists who in 1909 first described this organism, H. T. Ricketts and S. J. M. Prowazek. Unfortunately, both scientists contracted typhus and died during their studies of the pathogen.