Because of the scarcity of literature on the successful use of serological methods for differentiation of Rhizobium meliloti isolates, the objectives of this study were to provide a rationale for selecting isolates to which antisera could be raised and to appraise the suitability of published methods of preparing R. meliloti antigens for the serological identification of field isolates. We used one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis to develop protein profiles of eight field isolates and one commercial inoculant strain of R. meliloti in order to choose candidates that were either identical or distinctly different from each other for the production of antisera. The serological methods of tube agglutination and gel immunodiffusion complemented the sodium dodecyl sulfate-polyacrylamide gel electrophoresis method of identification. On the basis of their agglutination titers and gel immunodiffusion analysis, the isolates were placed in five serogroups which were identical to the groupings based on protein profiles. Antigenic characteristics of gel immunodiffusion antigens were influenced by the composition of the growth medium, sonication of whole-cell antigens, and the addition of Formalin. We recommend that careful attention be given to the effects of varying antigen preparation procedures when analyzing R. meliloti so that experimental protocols do not complicate the results. The wide range of homologous-antiserum titers observed for the nine isolates indicates different inherent degrees of immunogenicity of R. meliloti which cannot be predicted before serum production. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis method is a useful tool for screening a collection of R. meliloti isolates to better ensure that strain-specific antisera representative of different types of organisms will be obtained.
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