Antigenic variants of leptospiras selected by antiserum-complement mediated-killing.

The presence of antigenic variants of leptospiras was confirmed by the leptospiricidal reaction mediated by the antiserum plus complement. More than 99% of the organisms of virulent L. copenhageni Shibaura were destroyed microscopically after treatment with the anti-L. copenhageni Shibaura antiserum (1: 2000) plus guinea pig complement (1: 20). The leptospiras which survived treatment with the antiserum plus complement produced colonies on the solid serum medium at a rate of 0.09%. Eighty-seven percent of the survivors were found to be antigenic variants. The antigenicity of these variants reversed to that of the parent after 3--5 passages in the liquid normal serum medium.