peptidoglycan. Important genes essential for peptidoglycan synthesis such as alr, dapF and PBP1 are missing: alr encodes an enzyme L-alanine racemase, which converts L-alanine to D-alanine in the first step of peptidoglycan synthesis pathway; dapF encodes diaminopimelate epimerase, which convert LL-2,6-diaminoheptanedioate (L,L-DAP) to meso-diaminoheptanedioate (meso-DAP); and PBP1 encodes penicillin-binding protein-1 (PBP1), which converts periplasmic lipid II to peptidoglycan. Thus, the bacterium cannot synthesise a typical peptidoglycan cell wall, and instead makes a peptidoglycan-like structure on its surface. The cell membrane is also chemically different in its protein composition, and this difference gives rise to strain variations within the species itself. The cytoplasm is clear and shows distinct DNA and ribosomes. The bacterium is highly virulent, such that its isolation and cell culture are done only in a laboratory facility with biosafety level 3. Unlike other bacteria which can easily grow on different culture media, rickettsiales can be cultured only in living cells. O. tsutsugamushi specifically can be grown only in the yolk sacs of developing chicken embryos and in cultured cell lines such as HeLa, BHK, Vero, and L929. In contrast to Rickettsia species which reside in the nucleus of the host cell, O. tsutsugamushi mostly grows within the cytoplasm of the host cell. Genetically, it differs from other Rickettsia by only 9%. Even though adaptation to obligate intracellular parasitism among bacteria generally results in a reduced genome, it has a genome size of about 2.0–2.7 Mb depending on the strains (Figure 3), which is comparatively larger than those of other rickettsiales – two times larger than that of Rickettsia prowazekii, the most well-known member. The entire genome is distributed in a single circular chromosome. Whole genome sequences are available only for Ikeda and Boryong strains, both from the Republic of Korea. The genome of the Ikeda strain is 2,008,987 base pairs (bp) long, and contains 1,967 protein-coding genes. The Boryong strain is larger with 2,127,051 bp and 2,179 protein-coding genes. Genome comparison shows only 657 core genes among the different strains. With about 42-47% of repetitive sequences, O. tsutsugamushi has the most highly repeated bacterial genome sequenced as of 2013. The repeated DNA sequence includes short repetitive sequences, transposable elements (including insertion sequence elements, miniature inverted-repeat transposable elements, a Group II intron), and a greatly amplified integrative and conjugative element (ICE) called the rickettsial amplified genetic element (RAGE). RAGE is also found in other rickettsial bacteria. In O. tsutsugamushi,