Homopolysaccharide metabolism in Lactobacillus reuteri and Pediococcus parvulus
Research output: Thesis › Doctoral Thesis (compilation)
This thesis deals with the central carbohydrate metabolism and exopolysaccharide (EPS) formation in two different lactic acid bacteria (LAB): Lactobacillus reuteri and Pediococcus parvulus. LAB are traditionally used as starter cultures in the food industry. In general, LAB harbours one glycolytic pathway converting carbohydrates into lactic acid. However, L. reuteri ATCC 55730 was proven to use two functional glycolytic pathways by metabolic flux analysis and enzyme measurements. Surprisingly, the main flux was directed through the phosphoketolase pathway, while the Embden-Meyerhof pathway was used merely as a shunt. A combination of the NAD(P)H/NAD(P) ratio and an extra energy gain is speculated to be the controlling mechanism. Furthermore, only a minor fraction of the utilised carbohydrates is directed into EPS formation. Therefore, the volumetric production of EPS is too low to make it feasible as a natural biothickener in the food industry. In this work enhanced production of EPSs was achieved in both Lactobacillus reuteri ATCC 55730 and Pediococcus parvulus 2.6 by optimisation of the bacterial cultivation conditions. The combination of high biomass yield and surplus of sugar had a positive effect on both bacteria, and resulted in continued production after entering the stationary phase. Reuteransucrase, the enzyme responsible for EPS formation in L. reuteri ATCC 55730, was proven to be constitutively expressed and the enzymatic activity profile was independent of the sugar source in the medium. Temperature and pH were found to be the most important factors for EPS formation. The best EPS production was obtained at 37?C, pH 4.5 and 100 g sucrose l-1. A growth model developed in this work indicates that maximal growth yield is maintained under these conditions while sucrose is taken up by facilitated diffusion. At higher sucrose concentrations lower EPS formation might be due to a relatively lower specific biomass yield and a lower expression of the sucrase gene. In P. parvulus 2.6 EPS production was increased by the combination of high manganese, glucose and ethanol levels. Lactate, the main fermentation product, chelates manganese therby inhibiting growth. This inhibition could be overcome by increasing the manganese concentration in the growth medium.
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Subject classification (UKÄ) – MANDATORY
Defence details Date: 2007-03-30 Time: 10:15 Place: Kemicentrum, hörsal A Getingevägen 60 Lunds Tekniska Högskola External reviewer(s) Name: Axelsson, Lars Title: Fil. Dr Affiliation: MATFORSK, Norge ---