Epitope identification for a panel of anti-Sinorhizobium meliloti monoclonal antibodies and application to the analysis of K antigens and lipopolysaccharides from bacteroids

In two published reports using monoclonal antibodies (MAbs) generated again st whole cells, Olsen et al, showed that strain-specific antigens on the su rface of cultured cells of Sinorhizobium meliloti were diminished or absent in the endophytic cells (bacteroids) recovered from alfalfa nodules, where as two common antigens were not affected by bacterial differentiation (P. O lsen, M. Collins, and W. Rice, Can. J. Microbiol. 38:506-509, 1992; P, Olse n, S, Wright, M. Collins, and W, Rice, Appl, Environ. Microbiol, 60:654-661 , 1994), The nature of the antigens (i.e., the MAb epitopes), however, were not determined in those studies. For this report, the epitopes for five of the anti-S. meliloti MAbs were identified by polyacrylamide gel electropho resis-immunoblot analyses of the polysaccharides extracted from S. meliloti and Sinorhizobium fredii, This showed that the strain-specific MAbs recogn ized K antigens, whereas the strain-cross-reactive MAbs recognized the lipo polysaccharide (LPS) core. The MAbs were then used in the analysis of the L PS and K antigens extracted from S, meliloti bacteroids, which had been rec overed from the root nodules of alfalfa, and the results supported the find ings of Olsen et al. The size range of the K antigens from bacteroids of S. meliloti NRG247 on polyacrylamide gels was altered, and the epitope was gr eatly diminished in abundance compared to those from the cultured cells, an d no K antigens were detected in the S, meliloti NRG185 bacteroid extract. In contrast to the K antigens, the LPS core appeared to be similar in both cultured cells and bacteroids, although a higher proportion of the LPS frac tionated into the organic phase during the phenol-water extraction of the b acteroid polysaccharides. Importantly, immunoblot analysis with an anti-LPS MAb showed that smooth LPS production was modified in the bacteroids.