Cellular glycogen, β-1,2-glucan, poly-β-hydroxybutyric acid and extracellular polysaccharides in fast-growing species of Rhizobium

Abstract

Synthesis of acidic exopolysaccharides, neutral cellular polysaccharides and poly-β-hydroxybutyric acid (PHB) by Rhizobium is strongly dependent on cultural conditions and the strains used. Exopolysaccharide production by R. leguminosarum, R. Phaseoli and R. trifolii closely parallels growth, whereas R. meliloti mainly excretes (low mol wt) polysaccharides when cell propagation is limited by lack of a necessary growth element (nitrogen) and an excess of carbon source is still present in the medium.

In all strains, accumulation of cellular glycogen, β-1,2-glucan and PHB is initiated only under growth-limiting conditions. When the external carbon source is exhausted, glycogen and PHB are metabolized by the cells, sustaining their longevity and thus act as true reserve materials; on the other hand, β-1,2-glucan and excreted polysaccharides are not utilized on further incubation of the culture.

Differences exist in the nature and relative amounts of the products synthesized by strains of different species of Rhizobium. R. leguminosarum, R. phaseoli and R. trifolii synthesize a uronic acid-containing exopolysaccharide, PHB and/or glycogen, non-metabolizable capsular polysaccharide and low amounts of β-1,2-glucan. R. meliloti synthesizes a uronic acid-free exopolysaccharide, PHB and/or glycogen and high concentrations of β-1,2-glucan.

Exopolysaccharides, β-1,2-glucan and glycogen preparations were obtained by isolation and purification from cells of fast-growing species of Rhizobium and chemically characterized.