Use of the Chrome Azurol S Agar Plate Technique To Differentiate Strains and Field Isolates of Rhizobium leguminosarum biovar trifolii

Identification of Rhizobium and Bradyrhizobium strains and especially of indigenous isolates continues to be one of the major difficulties associated with competition studies. Because there is no universally accepted method, the method of choice depends on preference, experience, and equipment. Here, an agar plate technique was used to distinguish strains and field isolates of Rhizobium leguminosarum biovar trifolii to provide a basis for identifying nodule occupants in further competition studies. A rapid plate technique, based on differential growth characteristics, complements other techniques such as serological reactions, particularly when antisera cross-react with nonhomologous strains. The technique involves culturing strains and isolates on chrome azurol S agar. Although similar responses were observed among some strains, the response was highly reproducible and was considered an ideal complementary technique used in conjunction with serological procedures. Strains with similar responses could often be differentiated by varying media components, such as the source of carbon.