Structural studies of an exopolysaccharide produced by Gluconacetobacter diazotrophicus Pal5

Gluconacetobacter diazotrophicus is a nitrogen-fixing bacterium that has been found colonizing several plants. This acid-tolerant bacterium produces phytohormones that promote plant growth and is also able to grow in high-sugar concentrations. It has been demonstrated that exopolysaccharides (EPS), which are produced by strain Pal5 of G. diazotrophicus, play an important role in plant infection. We have investigated the structure of the EPS, which was produced by a strain of Pal5 grown in liquid medium containing mannitol as the sole carbon source. The results reveal an EPS with Glc, Gal, Man in a molar ratio of 6:3:1, respectively. NMR spectroscopy and chemical derivatization have revealed that the EPS structure has 4-O-substituted units of β-glucose, 3-O-substituted units of β-galactose and 2-O-substituted units of α-mannose. Glucose and galactose units linked at C6 were also found. The structure proposed herein is different from EPS produced by other species of Gluconacetobacter published to date.