A strain marker in Rhizobium trifolii based on the absorption of congo-red

It is widely recognized that most rhizobia absorb congo-red weakly when grown on agar media containing this dye. In contrast to this generalized behaviour, an ineffective strain of Rhizobium trifolii (Coryn) has been found which strongly absorbs the dye, producing growth of a very deep-red colour on yeast-extract mannitol agar, supplemented with congo-red (0.025 g/l). Assays for dye absorption characteristics on this medium with 21 strains of R. trifolii showed that Coryn could be readily differentiated from all the other strains examined. Supplementary fluorescent antibody tests with Coryn confirmed the reliability of differential absorption of congo-red as a strain-marker system. Data are presented which demonstrate the potential of this characteristic as a strain identification method in nodulation and competition studies under aseptic growth conditions, using mixed inocula.