the parasites during feeding. For more than 60 years there was no consensus on the choice of name – both R. orientalis and R. tsutsugamushi were equally used. Akira Tamura and colleagues reported in 1991 the structural differences of the bacterium from Rickettsia species that warranted a separate genus, and proposed the name Orientia tsutsugamushi. Finally, in 1995, they made a new classification based on the morphological and biochemical properties, formally creating the new name O. tsutsugamushi. Biology Orientia tsutsugamushi is a Gram-negative bacterium and is a permanent (obligate) parasite in mites. Within a single host cell, O. tsutsugamushi rapidly divides into many individuals as shown in Figure 1. A unicellular organism, it is oval shaped and measures 0.5 to 0.8 µm wide and 1.2 to 3.0 µm long. Due to similarity, it was previously classified in the genus Rickettsia among other bacteria, but later assigned a separate genus, Orientia, which it shares (as of 2010) only with Candidatus Orientia chuto. It is broader but shorter than other rickettsial bacteria, which are rod shaped and on average measure 0.25 to 0.3 µm wide and 0.8 to 1 µm long. During reproduction, it divides (by binary fission) into two daughter cells by the process of budding. While undergoing budding, it accumulates on the host cell surface, unlike other bacteria. One complete budding cycle takes 9 to 18 hours. The structure of O. tsutsugamushi (revealed by transmission electron microscopy) is shown in Figure 2. The bacterium is enclosed by a cell wall on the outside and cell membrane on the inside. The cell covering takes up stains such as Giemsa and Gimenez stains. Although its cell wall has a classic bacterial double layer, its outer leaflet is much thicker than the inner one, which is just the opposite in Rickettsia species. A capsule layer that forms a spherical halo in other bacteria is missing. The cell wall is less rigid due to the absence of peptidoglycan, which is otherwise characteristic of the rigid cell walls of other bacteria. Classic bacterial lipophosphoglycans such as muramic acid, glucosamine, hydroxy fatty acids, heptose, and 2-keto-3-deoxyoctonic acid are also absent in the cell wall. Due to the absence of peptidoglycan, the bacterium is naturally resistant to all β-lactam antibiotics (such as penicillin), to which Rickettsia species are normally sensitive to. Its genome totally lacks the genes for lipophosphoglycan synthesis, but does contain some for those of