Cleavage of poly(cis-1,4-isoprene) rubber as solid substrate by cultures of Gordonia polyisoprenivorans

Potential biotechnological recycling processes for rubber products include the bacterial degradation of poly(cis-1,4-isoprene) (IR) in order to achieve its total biodegradation or its biotransformation into useful products. The actinomycete Gordonia polyisoprenivorans strain VH2 catalyzes the degradation of IR and enables its use as a sole carbon source via β-oxidation. The initial cleavage reaction is catalyzed by the extracellular latex clearing protein (Lcp). This dioxygenase is the key enzyme for the formation of oligo(cis-1,4-isoprene) molecules with different lengths, i.e., numbers of isoprene units. For the first time, IR was used as a solid substrate in 2-l fermenters. Two different particle size fractions (63-500 and 500-1000 μm) and three stirring rates (300, 400 and 500 rpm) were evaluated in the process. An increase of the cell concentration was achieved by using smaller particles and by using lower stirring rates, reaching a final biomass concentration of 0.52 g l^-1 at 300 rpm after 12 days of cultivation. In order to enhance the formation of oligo(cis-1,4-isoprene) molecules, a transposon insertion mutant (TH5) of G. polyisoprenivorans strain VH2 that has lost the ability to transport the partial degradation products into the cells was used, thereby allowing the accumulation of the degradation products in the culture supernatants. Propionate, glucose and glycerol were evaluated as additional carbon sources besides IR, and the highest yields were observed on propionate. In 2-l bioreactors with pH control, different feeding regimes were performed during cultivation by the addition of propionate every 24 or 48 h for 16 days. After liquid-liquid extraction and a derivatization with Girard's T reagent, the oligo(cis-1,4-isoprene) molecules were detected by ESI-MS. The mass distribution of the degradation products was affected by the selection of the extraction solvent, but no influence of longer cultivation periods was detected.