| 1. |
- Svensson, Malin, et al.
(författare)
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Altered nucleotide sugar metabolism in Streptococcus thermophilus interferes with nitrogen metabolism
- 2007
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Ingår i: International Journal of Food Microbiology. - : Elsevier BV. - 0168-1605. ; 113:2, s. 195-200
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Tidskriftsartikel (refereegranskat)abstract
- Exopolysaccharide (EPS)-producing Streptococcus thermophilus strains have attracted interest recently, since the EPSs act as natural viscosifiers and texture enhancers of fermented foods. We have previously reported that the low level of EPS production by S. thermophilus LY03 could be improved by altering the activities of enzymes in the central carbon metabolism involved in the nucleotide sugar metabolism. In this study, we observed a reduced growth in milk for the strains with increased UDP-glucose pyrophosphorylase (GalU) activity together with either enhanced phosphoglucomutase activity, and/or enhanced activity of the Leloir enzymes. Rapid growth of these mutants in milk could be restored by the addition of four specific amino acids, i.e. Glu, His, Met, and Val. This amino acid requirement was confirmed in a defined medium. Furthermore, the P-31 NMR spectra showed higher levels of the GalU reactants pyrophosphate (PPi) and UDP-glucose in the engineered strain, TMB 6013, compared to the parent strain, LY03. These products plus Glu and the GalU reactant UTP are known to be involved in the nitrogen regulatory system in many bacteria. Thus, these results suggest that the reaction catalyzed by GalU is connected to the nitrogen demand of these engineered strains. (c) 2006 Elsevier B.V. All rights reserved.
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| 2. |
- Årsköld, Emma, et al.
(författare)
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Environmental influences on exopolysaccharide formation in Lactobacillus reuteri ATCC 55730.
- 2007
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Ingår i: International Journal of Food Microbiology. - : Elsevier BV. - 0168-1605. ; 116:1, s. 159-167
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Tidskriftsartikel (refereegranskat)abstract
- Lactobacillus reuteri is known to produce exopolysaccharides (EPS), which have the potential to be used as an alternative biothickener in the food industry. In this study, the effect of several environmental conditions on the growth and EPS production in the L. reuteri strain ATCC 55730 was determined. The expression of the corresponding reuteransucrase gene, gtfO, was investigated over time and the results indicated that the expression increased with growth during the exponential phase and subsequently decreased in the stationary phase. Fermentation with glucose and/or sucrose as carbon and energy source revealed that gtfO was constitutively expressed and that the activity profile was independent of the sugar source. In the applied ranges of parameter values, temperature and pH were the most important factors for EPS formation and only temperature for growth. The best EPS yield, 1.4 g g(-1) CDW, was obtained at the conditions 37 degrees C, pH 4.5 and 100 g l(-1) sucrose, which were close to the estimated optimal conditions: pH 4.56 and 100 g l(-1) sucrose. No EPS formation could be detected with glucose. In addition, no direct connection between the expression and the activity of reuteransucrase could be established. Finally, the strain ATCC 55730 was benchmarked against 14 other L. reuteri strains with respect to EPS production from sucrose and abilities to metabolise sucrose, glucose and fructose. Eight strains were able to produce glucan and a corresponding glucansucrase gene was confirmed for each of them. (c) 2007 Elsevier B.V. All rights reserved.
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