Orientia tsutsugamushi (O. tsutsugamushi), the causative agent of scrub typhus, is an obligate intracellular pathogen. Recent studies have demonstrated the complete genome of O. tsutsugamushi. However, the route and detailed molecular mechanism for O. tsutsugamushi to get accessed into mammalian cells remains unclear. In this study, we demonstrated different adhesive properties of three major outer membrane proteins of O. tsutsugamushi, TSA56, TSA47 and TSA22. TSA56 showed higher antibody responses against patient serum samples compared with those of TSA47 and TSA22. In the adhesion assay, TSA56 exhibited a relative higher adhesion to host cells than TSA47 and TSA22, suggesting that TSA56 is the major outer membrane protein required for O. tsutsugamushi adhesion. Furthermore, the antigen domain (AD) I (residues 19-114) corresponding to the extracellular domain of TSA56 demonstrated a relative high antibody response against the patients' sera than the previously reported ADIII (residues 237-366), which has been suggested to facilitate the invasion of O. tsutsugamushi through interaction with fibronectin. Taken together, our results consistently showed that TSA56 of O. tsutsugamushi is important in the adhesion of Escherichia coli (E. coli) transformants to Vero cells. Moreover, in contrast to known ADIII-fibronectin interactions, TSA56-ADI may also play a role in the adhesion and/or invasion of O. tsutsugamushi to its host cells through unidentified receptors. A further study aimed at delineating the receptor of TSA56-ADI during O. tsutsugamushi infection is warranted.

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