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W Soetaert

Publications and source records attributed to W Soetaert.

8 recordsLinked to original sources
PubMed2006

Transport kinetics of ectoine, an osmolyte produced by Brevibacterium epidermis.

Brevibacterium epidermis DSM 20659 is a halotolerant Gram-positive bacterium which can synthesize the osmolyte, ectoine, but prefers to take it up from its environment. The present study revealed that B. epidermis is equipped with at least one transport system for ectoine, with a maximal transport velocity of 15.7 +/- 4.3 nmol/g CDW.min. The transport requires energy (ATP) and is completely inhibited by the proton uncoupler, CCCP. The ectoine uptake system is constitutively expressed at a basal level of activity and its activity is immediately 10-fold increased by hyper-osmotic stress. Initial uptake rates are not influenced by the intensity of the hyper-osmotic shock but the duration of the increased activity of the uptake system could be directly related to the osmotic strength of the assay solution. Competition assays indicate that betaine, but not proline, is also transported by the ectoine uptake system.

Amino Acids, Diamino↗
PubMed2003

Dynamics and optimal conditions of intracellular ectoine accumulation in Brevibacterium sp.

The optimal conditions for the intracellular synthesis of ectoine were determined in a halotolerant Brevibacterium sp. The size of the intracellular ectoine pool in the bacterial cells is shown to depend on the external salt concentrations, type of carbon source and aeration level. In erlenmeyer flasks a maximum concentration of intracellular ectoine of about 0.9 g/l was obtained. Under controlled aeration in a 1.5 l fermentor this level could be increased to 1.2 g/l. Consecutive cell transfers to media with increasingly higher salt concentrations enabled us to reach even higher levels, up to 1.6 g/l on erlenmeyer scale. The ectoine synthesis takes place immediately after the osmotic upshock. Within one generation time, the new corresponding specific intracellular ectoine concentration is reached.

Amino Acids, Diamino↗
PubMed2003

Application of NAD-dependent polyol dehydrogenases for enzymatic mannitol/sorbitol production with coenzyme regeneration.

D-Mannitol and D-sorbitol were produced enzymatically from D-fructose using NAD-dependent polyol dehydrogenases. For the production of D-mannitol the Leuconostoc mesenteroides mannitol dehydrogenase could be used. Gluconobacter oxydans cell extract contained however both mannitol and sorbitol dehydrogenase. When this cell extract was used, the reduction of D-fructose resulted in a mixture of D-sorbitol and D-mannitol. To determine the optimal bioconversion conditions the polyol dehydrogenases were characterized towards pH- and temperature-optimum and -stability. As a compromise between enzyme activity and stability, the bioconversion reactions were performed at pH 6.5 and 25 degrees C. Since the polyol dehydrogenases are NADH-dependent, an efficient coenzyme regeneration was needed. Regeneration of NADH was accomplished by formate dehydrogenase-mediated oxidation of formate into CO2.

Culture Media↗
PubMed2003

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.

Biotransformation↗
PubMed2000

Optimized synthesis of L-sorbose by C(5)-dehydrogenation of D-sorbitol with Gluconobacter oxydans.

The optimization of L-sorbose synthesis by regiospecific dehydrogenation of D-sorbitol using Gluconobacter oxydans is reported. The current L-sorbose production processes that are based on G. oxydans and other bacterial strains are suboptimal as to yield and rate of L-sorbose synthesis. One reason for these problems is the toxicity that is induced by the substrate D-sorbitol when used in concentrations of >10% (w/v). This phenomenon significantly limits the potentials of L-sorbose production from an industrial point of view. The goal of this study was to develop a fast production process that yields L-sorbose in stoichiometric amounts starting from D-sorbitol concentrations that exceed 10% (w/v). A gradual improvement of the inoculum build-up procedure, culture medium composition, and process parameters ultimately led to a theoretically maximal L-sorbose productivity (200 g L(-1) of L-sorbose from 200 g L(-1) of D-sorbitol in 28 h of fermentation) using a Gluconobacter oxydans mutant strain that was selected under conditions of substrate inhibition. Because the D-sorbitol/L‐sorbose bioconversion is used to mass-produce vitamin C, the procedure reported here will contribute to a more efficient and more economic synthesis of vitamin C.

Ascorbic Acid↗