Enzymatic conversion of the clavan exopolysaccharide by Streptomyces sp. YSDL-20

A screening programme was set up to isolate microorganisms able to hydrolyse the complex biopolymer clavan produced by Clavibacter michiganensis subsp. michiganensis LMG 5604. This valuable exopolysaccharide is very rich in L-fucose (37.5% w/w), a rare sugar, used in the medical field (Vanhooren, 1999). A microorganism capable of depolymerizing the polymer may decrease the high viscosity during clavan batch fermentations and remove the limitations of the oxygen transfer and consequently increase the clavan yield. It could also release free L-fucose or L-fucose rich oligosaccharides. An actinomycete, designated YSDL-20, isolated from a soil sample, was able to depolymerize this biopolymer. Based on its morphology and molecular characteristics, this strain could only be identified as Streptomyces sp.. On clavan, this strain displays good growth (17.5 g DCW/l after 96 h of cultivation) characterized by filamentous growth during the earlier days of cultivation followed by sporulation after 4 days. The flow behaviour of the Clavibacter broth was characterized, the fermentation culture broth behaves as a pseudoplastic fluid. The viscosity of the culture broth as well as of the purified clavan EPS, decreases when lyophilised supernatant of Streptomyces sp. YSDL-20 was added, indicating clavanase action. The viscosity decreases by 26% when the Clavibacter culture broth was incubated during 18 h with the crude Streptomyces enzyme source, whereas a 82% viscosity drop was observed, when the purified clavan EPS (10 g/l) was incubated with the lyophilised Streptomyces supernatant for 5 h.