Cultural reactions of rhizobia with special reference to strains isolated fromSesbania species

Summary

• 1. All the sesbania strains employed in these studies were strikingly similar to other well-known rhizobia as shown by a wide variety of tests involving a diverse collection of mineral salts media containing a variety of carbon and nitrogen sources.

• 2. In comparative tests the 39 strains of sesbania rhizobia are divided into two well-defined groups. Those fromS. macrocarpa andS. longifolia, designated as Group 1, were culturally similar to the fast-growing rhizobia from clover, peas and beans. These strains produced mucilaginous colonies on all agar media, did not grow inHofer's alkaline media, veal infusion broth,Clark andLub's, Koser's uric acid andZoBell's media and did not reduce nitrate. Others fromS. grandiflora, S. tomentosa, S. aculeata andS. sesban, comprising Group 2, produced compact opaque colonies, grew in the above media and reduced nitrate slowly.

• 3. Question is raised that Congo red agar is a completely reliable medium for the separation of rhizobia from closely allied bacteria, as judged by dye absorption.

• 4. The strains of sesbania rhizobia showed considerable variation in their abilities to tolerate crystal violet in a peptone-sucrose medium. The majority of fast-growing strains of the designated rhizobial species did not grow in concentrations greater than 1∶10,000 crystal violet; none of the slow-growing strains tolerated concentrations above 1∶30,000. All the strains of species in the genusAgrobacterium resisted a 1∶2,500 concentration.

• 5. Evidence is presented that a relationship exists between tolerance of rhizobial strains to crystal violet dye in media and their abilities to produce gum.

• 6. Various cultural tests are described which permit reliable differentiations between rhizobia and related bacteria, exemplified by diverse strains ofA. tumefacines andA. radiobacter.