Growth of typhus group and spotted fever group rickettsiae in insect cells.

To analyze the host dependency of rickettsial growth, NIAS-AeAl-2 insect cells (AeAl2) derived from mosquito were first used in this study. It was demonstrated that typhus group rickettsiae (TGR) grew well in AeAl2 cells, but spotted fever group rickettsiae (SFGR) failed. To elucidate the inhibitory process of the growth of SFGR in AeAl2 cells, the adherence and invasion were first analyzed. SFGR possessed abilities to adhere to and invade AeAl2 cells as well as TGR in contrast to their inability of the growth in the cells. Morphologically, generation of microvilli could not be observed on AeAl2 cells inoculated with either group of rickettsiae. On the contrary, Vero cells inoculated with rickettsiae generated a great number of microvilli that adhered to rickettsiae and engulfed them into the cells. The roles of rickettsial major outer membrane protein A and B (rOmpA and rOmpB) were later investigated using E. coli expressing either rOmpA or rOmpB on their surface. Bacteria expressing either one of the major outer membrane proteins of rickettsiae as well as bacteria not expressing these proteins showed adherence to and invasion of AeAl2 cells. Thus, it is yet to be elucidated whether these major outer membrane proteins have any roles in these steps.