ABSTRACT
Bacteria of the order Rickettsiales are Gram-negative, small, rod-shaped, and coccoid, with all described species existing as obligate intracellular parasites of a wide range of eukaryotic organisms. This chapter primarily focuses on the family Rickettsiaceae , which is currently comprised well-studied species in the genera Rickettsia and Orientia, as well as candidate species in several provisional genera that are poorly understood. The genus Rickettsiella , originally described as a member of Rickettsiales, comprises a group of intracellular pathogens of diverse arthropod species. Family Holosporaceae was first added to the Rickettsiales in 2001. The first sequenced genome of a rickettsial species, R. prowazekii , revealed numerous pseudogenes and a highly reduced genome relative to facultative intracellular and free-living bacterial species. Throughout modern history, the morbidity and mortality associated with rickettsial infections has been underestimated, primarily due to misdiagnosis. In the United States, a major concern is the changing ecology of endemic typhus, hereafter referred to as murine typhus.
