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Biology of Rickettsia rickettsii infection

Daniel J Sexton, MD
Micah T McClain, MD
Section Editors
Stephen B Calderwood, MD
Morven S Edwards, MD
Deputy Editor
Jennifer Mitty, MD, MPH


Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever (RMSF) and the prototypic member of the genus Rickettsia. The basic biologic features of R. rickettsii and how it produces disease will be reviewed here. The clinical manifestations of RMSF and its treatment are discussed separately. (See "Clinical manifestations and diagnosis of Rocky Mountain spotted fever" and "Treatment of Rocky Mountain spotted fever".)


Like other gram-negative bacteria, R. rickettsii is a member of the alpha-group of purple bacteria. It is a member of the order Rickettsiales and the family Rickettsiaceae. The family Rickettsiaceae, in turn, contains the genera Rickettsia and Orientia. The genus Rickettsia is divided into the typhus and spotted fever groups.

R. rickettsii is the prototype of the spotted fever group, which has more than 12 separate pathogenic species (see "Other spotted fever group rickettsial infections"). Phylogenetic studies utilizing the 16S ribosome have shown that R. rickettsii is closely related to other members of the spotted fever group such as Rickettsia conorii and Rickettsia sibirica, whereas its phylogenetic relationship to other spotted fever members such as Rickettsia akari, Rickettsia australis, and Rickettsia belli is substantially more distant.


R. rickettsii is a weakly gram-negative non-motile coccobacillus measuring 0.3 to 0.7 mcm by 0.8 to 2.0 mcm. R. rickettsii are difficult to see in tissue without special stains, but they can be visualized using Giemsa, Machiavello, and Gimenez staining and by the use of direct fluorescent antibody staining techniques.

Ultrastructure — R. rickettsii has ribosomes and a single circular chromosome located in an amorphous cytosol surrounded by a plasma membrane. In addition, an indistinct microcapsular layer is present on the outer surface of the cell wall. An electron-lucent zone separates this layer from the host cytosol. This zone is thought to represent a slime layer which may be important in pathogenicity [1].

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Literature review current through: Nov 2017. | This topic last updated: Apr 25, 2016.
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