EXOPOLYSACCHARIDE AND POLY-BETA-HYDROXYBUTYRATE COPRODUCTION IN 2 RHIZOBIUM-MELILOTI STRAINS

The effects of different nitrogen and carbon sources on cell growth, p H, and exopolysaccharide (EPS) and poly-beta-hydroxybutyrate (PHB) pro duction by two strains of Rhizobium meliloti (M5N1 and Su47) are repor ted. Differences in the behavior of glucose- and fructose-grown cells were shown, in particular with the M5N1 strain. Growth in a glucose-co ntaining medium was accompanied by acidification of the culture medium , which leads to cell death. On fructose, acidification was detected o nly in the medium with a mineral nitrogen supply. A lag phase in EPS p roduction was observed with cells grown with glucose, probably related to an initial extracellular conversion of the carbohydrate into an ac id. No lag phase was observed in EPS production from fructose or in PH B synthesis whatever the carbon source. A decrease in PHB content was noticed for both strains under conditions where acidification of media occurred. The extent of production, emphasized by the use of a coprod uction index, indicates that the M5N1 strain is a more promising organ ism than is the Su47 strain for polymer production. Such a strain, put in rich medium (containing yeast extract) supplemented,vith fructose, accumulated PHB up to 85% of dry cell weight and excreted about 1.5 g of EPS per liter in the medium. Regulation of the coproduction of EPS and PHB by these cells is suggested.