During studies to serotype tick-borne rickettsiae from various areas in the United States, particularly those areas associated with the vectors of , we obtained 263 isolates of a rickettsia whose prototype is referred to as strain 369-C. Although rickettsiae of the strain 369-C serotype are distinct from rickettsiae of other serotypes, as determined by cross-micro-immunofluorescence tests of mouse antisera, they do possess one or more antigens that react with convalescent sera from Rocky Mountain spotted fever, epidemic typhus, and murine typhus patients. Because strain 369-C-like isolates have been recovered from eight species of argasid and ixodid ticks from widely separated areas of the United States, we set out to characterize these organisms more precisely, particularly with regard to their relationships to rickettsiae of the spotted fever and typhus groups. The characteristics which we used included morphology as determined by light microscopy and electron microscopy; cytopathogenicity in Vero cell cultures; pathogenicity for embryonated chicken eggs, laboratory mice, guinea pigs, and voles; protein composition as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis; and deoxyribonucleic acid base composition. Our results indicated that strain 369-C-like rickettsiae are significantly different phenotypically from rickettsiae of both the spotted fever and typhus groups. Furthermore, the guanine-plus-cytosine content of the deoxyribonucleic acid of strain 369-C (30 mol%) was not different from the guanine-plus-cytosine content of rickettsiae of the typhus group, but it was lower than the guanine-plus-cytosine content recorded by other workers for rickettsiae of the spotted fever group. We propose that this species be named sp. nov., with strain RML 369-C as the type strain; all serologically related rickettsiae should henceforth be referred to as the serogroup.


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