Homologous serological analysis of Rhizobium meliloti strains by immunodiffusion

The homologous titers of antisera prepared against 24 Rhizobium meliloti strains ranged from 8 to 64 in immunodiffusion tests when intact cells were used as test antigens. The antisera titers against a number of strains were higher when heated or ultrasonicated cell preparations were used as sources of antigens. The minimum concentration of intact cells required to produce a positive reaction varied between strains and the heat treatment of the cells of some strains increased the detectability of surface antigens. Titers of antigens in sonicated preparations of several strains were higher than those of either intact or heated cells. The minimum concentration of antigens detectable for different strains was independent of the antisera titers. Serological reactions of strains using intact cells were categorized in several groups based on the shape, position, and number of precipitin bands formed at different cell densities. The sonicated cell preparations produced additional bands and most strains contained both heat-stable and heat-labile antigens in such preparations. Some of these antigens were similar but not identical to the surface antigens whereas others appeared to be unrelated.