Role of Kazachstania humilis and Saccharomyces cerevisiae in the strain-specific assertiveness of Fructilactobacillus sanfranciscensis strains in rye sourdough

Sourdough is a common ingredient for baked goods as it improves their texture, shelf life and flavor. One of the dominant key species in type I sourdoughs is Fructilactobacillus sanfranciscensis (formerly Lactobacillus sanfranciscensis), which occurs with a multitude of different strains. Noticeably, this species often shares its habitat with the yeasts Saccharomyces cerevisiae or Kazachstania humilis. It is still unclear, which relationship exists between these organisms and whether it is characterized by coexistence, interaction, or mutualism. In this study, competitiveness of different F. sanfranciscensis strains in rye sourdough was examined and its dependence of co-existing yeasts was explored. In particular, it was investigated whether competitiveness of F. sanfranciscensis strains depends on the presence/absence of S. cerevisiae or K. humilis when co-inoculated in the sourdough. Competitiveness of strains was monitored in rye sourdough using the CRISPR locus length polymorphism (CLLP)—PCR for strain differentiation. It was found that F. sanfranciscensis TMW 1.1150, TMW 1.1221 and TMW 1.1597 were dominant regardless of the presence/absence of both yeast species. Dominance of F. sanfranciscensis TMW 1.392, TMW 1.907 and TMW 1.2137 was significantly and diversely influenced by the presence of S. cerevisiae or K. humilis. F. sanfranciscensis TMW 1.2138 and TMW 1.726 were not able to compete against the other F. sanfranciscensis strains. It was possible to sort the eight strains into three different groups: 1. Strain competitiveness was independent of the presence/absence of yeasts; 2. Strain competitiveness was dependent on yeast species and 3. Strains were not competitive in the presence of strains belonging to group 1 or 2. Interestingly, in fermentations that were not inoculated with any yeast a spontaneous occurrence of S. cerevisiae or K. humilis was observed depending on the synergistic competitiveness of the respective F. sanfranciscensis used. Thus, the level of competitiveness was strain specific and, in some strains, dependent on the presence/absence of specific yeast species.