Variant strains, induced by co-cultivation of Leptospira interrogans serovar copenhageni strain Shibaura with its homologous antiserum in a liquid medium, showed reduced agglutinability to the antiserum in comparison with that of the original parent Shibaura strain. The reduction was inversely correlated to the concentration of the antiserum in the medium. Furthermore, these variants showed reduced agglutinability to anti-leptospiral lipopolysaccharide (LPS) serum and antileptospiral LPS monoclonal antibodies. The variants became resistant to complement-mediated and macrophage-mediated bactericidal effects in the presence of antibody to the LPS. The level of resistance was related to the degree of the reduced agglutinability. The clearance time of these variants from the blood of the mice was observed to be longer than that of the parent Shibaura strain when these bacteria were inoculated into the mice immunized with heatinactivated parent Shibaura strain. It was concluded, therefore, that these variants acquired the ability to survive for a longer period in vivo than that of the parent organism. These results suggest that variation of Leptospires, which has been considered as a temporary modification of LPS, is one of the ways it escapes from the host defense mechanisms
