Stability and stabilization of biocatalysts

Abstract

The problems encountered when trying to stabilize whole-cell biocatalysts are very different from those addressed above. Genetic engineering provides improved strains that overproduce valuable metabolites and proteins, but recombinant cells have the tendency to lose foreign plasmids, especially in continuous processes. Combining both genetic- and enzyme-engineering techniques may be useful to overcome the difficulties.Jean-Noel Barbotin (Universite de Picardie Jules Verne, Amiens, France) described the state of knowledge of plasmid stability in different immobilized recombinant microorganism systems; he also addressed the potential spread of foreign genetic elements in Nature.In order to exploit the potential of metabolic engineering fully, suitable expression systems that can be activated by low-cost inducers are required. Marcel Wubbolts (ETH, Zurich, Switzerland) has succeeded in constructing expression cassettes using the alk and xyl promoters that induce transcription when activated by octane and xylene, respectively; these expression cassettes were very stable under the harsh conditions of industrial applications.Plasmid stability in recombinant Saccharomyces cerevisiae expressing the exo-β-glucanase genes was higher in cultures immobilized in alginate than in free cell cultures (Enrique Roca, Universidad de Santiago de Compostela, Santiago, Spain). Martin Bekers (University of Latvia, Riga, Latvia) described the stabilization of Zymomonas mobilis cells immobilized onto porous stainless steel spheres and used to produce levan. Finally, the reduction of acids to flavouring aldehydes and alcohols catalysed by Pyrococcus furiosus at 95°C was demonstrated by Colja Laane (Wageningen Agricultural University). This growth temperature allows a cheap, sterile production process in which volatile aldehydes and alcohols can be distilled in situ.In summary, this symposium was an outstanding success at updating the topic of stability and stabilization of biocatalysts. Important new discoveries are expected in the near future and we should look forward to them with increasing enthusiasm.