Strain-specific serological techniques for the identification of Rhizobium meliloti in commercial alfalfa inoculants

Antisera were prepared against five Rhizobium meliloti strains including three strains adapted to Canadian soil and climate conditions and recently released for use in the production of commercial alfalfa (Medicago spp.) inoculants. The antisera were highly cross-reactive, but specificity was obtained by repeated massive adsorptions of the antisera with cells of the cross-reacting strains. The adsorbed antisera were used in microagglutination tests and the enzyme-linked immunosorbent assay (ELISA) to demonstrate the presence of the Canadian strains in their respective commercial peat-base inoculants at 10 8 –10 9 viable cells/g. A plant infection technique which was also used to evaluate inoculant quality was specific only at the Rhizobium species level and required up to 4 weeks for completion. Serological techniques made it possible to identify and quantitate inoculant strains of R. meliloti while reducing the time required for testing. The agglutination analysis of inoculants was simple, but required regrowth of each colony isolate to generate sufficient cell numbers. The ELISA technique was used directly with colonies picked from plate counts, and the results were available within 5 days, including the time for colony growth on the plate media.