Sinorhizobium meliloti cells require biotin and either cobalt or methionine for growth

Sinorhizobium meliloti is usually cultured in rich media containing yeast e xtract. It has been suggested that some components of yeast extract are als o required for growth in minimal medium. We tested 27 strains of this bacte rium and found that none were able to grow in minimal medium when methods t o limit carryover of yeast extract were used during inoculation. By fractio nation of yeast extract, two required growth factors were identified. Bioti n was found to be absolutely required for growth, whereas previously the ne ed for this vitamin,was considered to be strain specific. All strains also required supplementation with cobalt or methionine, consistent with the req uirement for a vitamin B-12-dependent homocysteine methyltransferase for me thionine biosynthesis.