Virulence conversion of Burrelia duttonii by subculturing in tissue culture system and by passage in mice

Borrelia duttonii strain 406K which has been maintained in ddy mice did not grow in Kelly or BSK II medium, althought these media were reported as the growth media of B. duttonii. K. Sugiyama in this laboratory established an in vitro cultivation system of the strain on conventional monolayer cultures of SflEp cells. In the present study, the author estimated the virulence of B. duttonii cultivated in the system for ddy mice when administered by the intraperitoneal route. The results were as follows: 1. LD_<50> and MID of B.duttonii strain 406K passaged in mice were 10^3-10^6 organisms/mouse and a single organism/mouse, respectively. 2. Five passages in the system resulted in extreme reduction of the virulence. Cultured borreliae, however, have retained infectivity for mice for at least 36 successive cultures. 3. Mice injected with cultured borreliae showed atypical course of borrelemia. 4. The growth of the organisms in mice was repressed by simultaneous injection with a high dosage of heatkilled borreliae 5. When attenuated borreliae of 11th oassage in the system were passed 4 times in ddy mice, fully virulent organisms were recovered.