Recent studies of Rickettsia tsutsugamushi have demonstrated clearly the phenotypic and genotypic differences between this microorganism and other species belonging to the genus Rickettsia. Therefore, classification of R. tsutsugamushi in a new genus, Orientiu gen. nov., is proposed. The genus Rickettsia includes etiological agents of human diseases, including typhus, spotted fever, and scrub typhus. All of the microbes belonging to this genus are similar in the following respects: they exhibit obligate intracellular parasitism, they are morphologically similar to gram-negative bacteria, they survive in both vertebrate and arthropod hosts, and human infection is mediated by arthropods. However, recent studies of Rickettsia tsutsugamushi, which is the only species in the scrub typhus group, have revealed some differences between this species and other species belonging to the genus Rickettsia. The most striking difference is in the structure of the outer envelope; as revealed by electron microscopy, the outer leaflet of the cell wall of R. tsutsugamushi is considerably thicker than the inner leaflet, while the opposite is true of the other Rickettsia species (36). In addition, chemically, R. tsutsugamushi lacks constitutional components of peptidoglycan and lipopolysaccharide, such as muramic acid, glucosamine, hydroxy fatty acids, and 2-keto-3-deoxyoctonic acid, suggesting that neither peptidoglycan nor lipopolysaccharide is present in R tsutsugamushi (4), while these substances are generally found in the other species belonging to the genus Rickettsia (1, 8,15, 24,35). R. tsutsugamushi is very soft and fragile (39, 44), which reflects the lack of peptidoglycan in this microorganism, and the growth of R tsutsugamushi is more resistant to penicillin than the growth of other rickettsiae (21, 32). The protein composition of R. tsutsugamushi as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis is very different from the protein compositions of the other rickettsiae, especially in the envelopes (43). In R. tsutsugamushi, a 54-to 58-kDa protein (designated the 56-kDa protein) is most abundant and is located on the cell surface (16, 41). Other major proteins (80, 46, 43,39, 35,28, and 25 kDa) are also located on the rickettsial surface (41); three of these proteins, the 25-,28-, and also 56-kDa proteins, are heat modifiable (16, 22, 46). Another abundant protein, the 60-kDa protein, which is located inside the rickettsial cells, has been shown to exhibit homology to the GroEL family of proteins (37). On the other hand, rickettsiae belonging to the typhus and spotted fever groups resemble each other in their protein compositions. The major antigenic proteins located on the surfaces of these organisms have been estimated to have the molecular sizes of 150 to 180, 110 to 130,49,32,27.5, and 16.5 to 17.5 kDa (51,53). The two proteins larger than 110 kDa are immunodominant antigens and have both heat-labile and heatstable antigenic sites (5-7, 12, 15). The 16.5-to 17.5-kDa antigen is a lipoprotein and e...

Rickettsia tsutsugamushi strains from three recent patients of Tsutsugamushi disease in Niigata Prefecture were isolated primarily in mice and then in L cell cultures. By this procedure, low virulent strains to mice, as well as high virulent ones, could be isolated and cultivated serially in L cell cultures, suggesting the usefulness of L cells for isolation of this species of rickettsia. Each newly isolated strain was identified as a member of R. tsutsugamushi from the results of cross immunological tests and morphological observation. On the other hand, it was recognized that one of these rickettsiae showed immunological properties distinguishable from the prototype strains of Kato, Karp, and Gilliam by the cross complement fixation test, and also had low virulence in mice.

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