Variations in the molecular masses of the capsular exopolysaccharides amylovoran, pyrifolan and stewartan.

Erwinia amylovora, causing fire blight of apple, pear and some ornamentals, Erwinia pyrifoliae, causing Asian pear blight, and Pantoea stewartii, causing Stewart's wilt of sweet maize, synthesize capsular extracellular polysaccharides (EPSs) with a high molecular mass. The EPSs are virulence factors and form viscous aggregates, which participate in clogging vessels of infected plants and causing wilting. The sizes of EPSs produced under different environmental growth conditions were determined by analysis with large pore HPLC columns. Their molecular mass of ca. 5 MDa, when isolated from agar plates, decreases to ca. 1 MDa for E. amylovora amylovoran from freeze-dried supernatants from liquid cultures and to 2 MDa for freeze-dried preparations of P. stewartii stewartan. Size changes were also found following growth in various other media and for different strains. Stewartan, amylovoran and E. pyrifoliae pyrifolan were also shown to be completely degraded by a bacteriophage EPS depolymerase.