Production of B-group vitamins by two Rhizobium strains in chemically defined media

Twenty-eight strains of Rhizobium spp. were tested for their ability to gro w in chemically-defined medium lacking growth factors. Two strains, R. meli loti GR4B and Rhizobium spp. (Acacia) GRH28, were selected, on the basis of their good growth under the conditions imposed, for further quantification of the production of mater-soluble vitamins (thiamine, niacin, riboflavin, pantothenic acid and biotin) in chemically defined media amended with diff erent compounds (mannitol, glucose or sodium succinate) as sole carbon sour ces. Qualitative and quantitative production of vitamins in chemically-defi ned media was significantly affected by the use of C sources of a different nature and the age of the cultures. Strain GRH28 produced all the vitamins analysed, and high biological levels of biotin (14 ng ml(-1) culture) were detected after 6 d of culture in mineral medium amended with mannitol. Pan tothenic acid was the vitamin detected in the highest amounts (up to 1 mu g ml(-1) of culture) in culture supernatant fluids of strain GR4B grown for 6 d with succinate as sole carbon source.