Dynamics of Enterobacteriaceae and lactobacilli in model sourdoughs are driven by pH and concentrations of sucrose and ferulic acid

This study aimed to assess, in model sourdough fermentations, the relevance of physico-chemical features of flour for the interactions between Enterobacteriaceae and lactobacilli, and for the interactions within the latter microbial group. Initially, model sourdoughs made with white wheat flour, whole wheat flour or wheat bran were inoculated with Cronobacter sakazakii, Klebsiella pneumoniae, Lactobacillus plantarum and Lactobacillus sanfranciscensis. Subsequently, white wheat sourdoughs were prepared with phosphate buffer, ferulic acid, maltose, sucrose or baker's yeast. During sourdough propagation, C. sakazakii and K. pneumoniae disappeared after few fermentation cycles in white wheat and whole wheat sourdoughs, but persisted in phosphate buffered or in wheat bran sourdoughs. Sucrose, maltose or ferulic acid did not impact on the ecological fitness of Enterobacteriaceae, whereas baker's yeast inhibited these bacteria. In white wheat and in whole wheat sourdoughs, L. sanfranciscensis outcompeted L. plantarum. A variation of sucrose level and the presence of ferulic acid reduced the competitiveness of L. sanfranciscensis, thus favoring L. plantarum.This study demonstrated that the pH is key contributor to the elimination of Enterobacteriaceae in cereal fermentation. In addition, L. sanfranciscensis prevails in wheat sourdoughs, but minor perturbations of the ecosystem reduce its competitiveness

Isomer-Specific Consumption of Galactooligosaccharides by Bifidobacterial Species

Prebiotics are non-digestible substrates that stimulate the growth of beneficial microbes in the human intestine. Galactooligosaccharides (GOS) are food ingredients that possess prebiotic properties, in particular, promoting the growth of bifidobacteria in situ. However precise mechanistic details of GOS consumption by bifidobacteria remains poorly understood. Because GOS are mixtures of polymers of different lengths and linkages, there is interest to determine which specific structures provide prebiotic effects in order to potentially create better supplements. We here present a method comprising porous graphitic carbon separation, isotopic labeling and mass spectrometry analysis for the structure specific analysis of GOS isomers and their bacterial consumption rate. Using this strategy, the differential bacterial consumption of GOS by the bifidobacteria species B. longum subsp. infantis, B. animalis subsp. lactis and B. adolescentis was determined, indicating that the use of specific GOS isomers in infant formula may provide enrichment of distinct species