Rickettsia is a genus of small, obligate intracellular Gram‑negative bacteria transmitted by ticks, lice and fleas. They infect endothelial cells and cause diseases such as typhus and spotted fevers.
Biology and Pathogenesis
Rickettsia species are rod‑shaped or pleomorphic Alphaproteobacteria that have a thin peptidoglycan layer and an outer membrane with lipopolysaccharide. Because they lack many metabolic pathways, including a complete glycolytic pathway and the ability to synthesize nucleotides, they rely on the host cell for energy and biosynthetic precursors. Rickettsiae enter host cells via induced phagocytosis and escape into the cytosol or multiply within the phagosome depending on the species. They use host ATP through an ATP/ADP translocase and replicate by binary fission. Infection of endothelial cells leads to vascular injury, vasculitis and increased vascular permeability, which explain the characteristic fever and rash. The genus is divided into two major groups: the typhus group, which includes Rickettsia prowazekii and Rickettsia typhi, and the spotted fever group, which includes Rickettsia rickettsii and Rickettsia conorii. A transitional group contains species such as Rickettsia akari and Rickettsia australis.
Key Species and Diseases
Rickettsia rickettsii causes Rocky Mountain spotted fever, a severe tick‑borne illness characterized by fever, headache, rash and multiorgan involvement. Rickettsia prowazekii is the agent of epidemic typhus, transmitted by body lice and historically associated with crowded conditions; recrudescent infection causes Brill–Zinsser disease. Rickettsia typhi produces endemic or murine typhus, transmitted by fleas from rodents. Other spotted fever group pathogens include R. conorii (Mediterranean spotted fever), R. parkeri and R. africae. These infections are typically treated with doxycycline, and preventive measures focus on vector control and minimizing exposure to arthropods.
Rickettsia are important vector‑borne pathogens that depend on host cells for survival. Understanding their intracellular lifestyle and vector relationships is essential for diagnosing and controlling rickettsial diseases.
Related Terms: Rickettsial, Rickettsia rickettsii, Typhus, Rocky Mountain spotted fever, Tick‑borne disease