Microbial extracellular polysaccharides (EPS), produced by a wide range of bacteria, are high molecular weight biopolymers, presenting an extreme diversity in terms of chemical structure and composition. They may be used in many applications, depending on their chemical and physical properties. A rather unexplored aspect is the presence of rare sugars in the composition of some EPS. Rare sugars, such as rhamnose or fucose, may provide EPS with additional biological properties compared to those composed of more common sugar monomers. This review gives a brief overview of these specific EPS and their producing bacteria. Cultivation conditions are summarized, demonstrating their impact on the EPS composition, together with downstream processing. Finally, their use in different areas, including cosmetics, food products, pharmaceuticals, and biomedical applications, are discussed.

Cofactor imbalance impedes xylose assimilation in Saccharomyces cerevisiae that has been metabolically engineered for xylose utilization. To improve cofactor use, we modified ammonia assimilation in recombinant S. cerevisiae by deleting GDH1, which encodes an NADPH-dependent glutamate dehydrogenase, and by overexpressing either GDH2, which encodes an NADH-dependent glutamate dehydrogenase, or GLT1 and GLN1, which encode the GS-GOGAT complex. Overexpression of GDH2 increased ethanol yield from 0.43 to 0.51 mol of carbon (Cmol) Cmol ؊1 , mainly by reducing xylitol excretion by 44%. Overexpression of the GS-GOGAT complex did not improve conversion of xylose to ethanol during batch cultivation, but it increased ethanol yield by 16% in carbon-limited continuous cultivation at a low dilution rate.In order to develop an efficient process for the production of bioethanol from lignocellulosic material, there have been many attempts to improve the conversion of xylose to ethanol by construction of recombinant Saccharomyces cerevisiae strains.Even though the open reading frames encoding the three first enzymes involved in xylose metabolism, i.e., xylose reductase (XR), xylitol dehydrogenase (XDH), and xylulose kinase (XK), have been identified in the genome of S. cerevisiae (10, 15), the genes are poorly expressed (or not expressed at all), and xylose cannot be metabolized without introduction of heterologous genes from naturally xylose-fermenting microorganisms and overexpression of endogenous XK (3,6,17). In the resulting strains, there is a low rate of xylose consumption and substantial xylitol secretion.Since xylose-metabolizing strains of S. cerevisiae harbor a NAD(P)H-dependent xylose reductase and a NAD ϩ -dependent xylitol dehydrogenase from Pichia stipitis, one of the main reasons for xylitol excretion can be explained by the redox imbalance occurring at the level of XR and XDH. Metabolic engineering of redox metabolism may therefore be a strategy for improving the conversion of xylose to ethanol. This has been illustrated by deletion of the zwf1 gene encoding glucose-6-phosphate dehydrogenase in a xylose-metabolizing strain of S. cerevisiae (7). The main source of NADPH originating from the oxidative part of the pentose phosphate pathway has thereby been reduced, and during growth on xylose, this has resulted in a significant improvement of ethanol yield, from 0.31 g g Ϫ1 (yield on consumed sugar) to 0.41 g g Ϫ1 with a concomitant reduction in the xylitol yield.In this study, we have taken a different approach to modulating the redox metabolism to favor xylose metabolism, namely through metabolic engineering of the ammonia assimilation. Ammonium assimilation in S. cerevisiae involves glutamate dehydrogenase and glutamine synthetase. Glutamate dehydrogenase catalyzes the synthesis of glutamate from ammonium and 2-ketoglutarate. The most important enzyme for ammonia assimilation is the NADPH-dependent glutamate dehydrogenase 1, encoded by GDH1. The other glutamate dehydrogenase that is present in S. cerevisiae is N...

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