Rickettsia rickettsii-rickettsia rickettsii (abbreviated as r. rickettsii) is a unicellular, gram-negative coccobacillus (plural coccobacilli) that is native to the new world. it belongs to the spotted fever group (sfg) of rickettsia and is most commonly known as the causative agent of rocky mountain spotted fever (rmsf). by nature, r. rickettsii is an obligate intracellular parasite that survive by an endosymbiotic relationship with other cells.
r. rickettsii is a non-motile, non-spore forming aerobic organism. cells are typically 0.3-0.5 x 0.8-2.0 um in size. they lack a distinct nucleus and membrane bound organelles. their outer membrane is composed mostly lipopolysaccharides.
rmsf is transmitted by the bite of an infected tick while feeding on warm-blooded animals, including humans. humans are considered to be accidental hosts in the rickettsiatick life cycle and are not required to maintain the rickettsiae in nature.