Effect of Lineage-Specific Metabolic Traits of Lactobacillus reuteri on Sourdough Microbial Ecology

ABSTRACTThis study determined the effects of specific metabolic traits ofLactobacillus reuterion its competitiveness in sourdoughs. The competitiveness of lactobacilli in sourdough generally depends on their growth rate; acid resistance additionally contributes to competitiveness in sourdoughs with long fermentation times. Glycerol metabolism via glycerol dehydratase (gupCDE) accelerates growth by the regeneration of reduced cofactors; glutamate metabolism via glutamate decarboxylase (gadB) increases acid resistance by generating a proton motive force. Glycerol and glutamate metabolisms are lineage-specific traits inL. reuteri; therefore, this study employed glycerol dehydratase-positive sourdough isolates of human-adaptedL. reuterilineage I, glutamate decarboxylase-positive strains of rodent-adaptedL. reuterilineage II, as well as mutants with deletions ingadBorgupCDE. The competitivenesses of the strains were quantified by inoculation of wheat and sorghum sourdoughs with defined strains, followed by propagation of doughs with a 10% inoculum and 12-h or 72-h fermentation cycles. Lineage IL. reuteristrains dominated sourdoughs propagated with 12-h fermentation cycles; lineage IIL. reuteristrains dominated sourdoughs propagated with 72-h fermentation cycles.L. reuteri100-23ΔgadBwas outcompeted by its wild-type strain in sourdoughs fermented with 72-h fermentation cycles;L. reuteriFUA3400ΔgupCDEwas outcompeted by its wild-type strain in sourdoughs fermented with both 12-h and 72-h fermentation cycles. Competition experiments with isogenic pairs of strains resulted in a constant rate of strain displacement of the less competitive mutant strain. In conclusion, lineage-specific traits ofL. reuteridetermine the competitiveness of this species in sourdough fermentations.