Enhancement of the antigenic activity and virulence of the vaccine strain E of Rickettsia prow azeki by passages in cell culture.

Changes in the biological properties of the vaccine strain E of Rickettsia prowazeki occurred upon cultivation of A1 (human amnion) cells infected with this strain. In the course of passages of these cells the antigenic activity and virulence of the rickettsia increased. The changes were observed in 10 out of 22 cell cultures examined: in 6 cultures there was an increase in the antigenic activity and in 4 both in the antigenic activity and in virulence. The time of the occurrence of these changes in the rickettsial populations varied from 12-18 to 53-102 days of passage of the infected cells.