Contribution of glutaminase activity in Lactobacillus reuteri to acid resistance and glutamine metabolism in sourdough

Abstract

Sourdough is used as an additive in bread production for proper dough volume (leavening), or for desired dough acidity, or for dough texture and bread flavor improvement, or for bread shelf life extension. Lactobacillus reuteri, an intestinal isolate and a stable member of sourdough, prevails in Type II sourdough fermentation, owing to the elevated fermentation temperatures with long incubation time. The aim of this study was to understand the role of glutaminase activity in L. reuteri to acid resistance and glutamine metabolism throughout sourdough fermentation. An isogenic deletion mutant L. reuteri 100-23 Δgls1-2-3 was generated, by disruption of the three glutaminases (gls) in L. reuteri 100-23, for interpreting the role of glutaminase activity. Acid resistance mechanism was assessed at both pH 2.5 and 3.5 to mimic the gastric and sourdough environments; sourdough fermentation and sensory evaluation of sourdough bread were applied to establish the effect on glutamine metabolism. Glutaminase-mediated deamidation contributes to the survival of L. reuteri 100-23 in both acidic environments (in vitro) and sourdough fermentations (in vivo) by converting glutamine to glutamate. Analysis of the glutamine / glutamate dependent acid resistance indicated that glutamine deamidation increases acid resistance independent of glutamate decarboxylation. Without the ability to convert glutamine, alternative acid tolerance mechanisms compensate (i.e. GAD; ADI; etc.) when L. reuteri can no longer rely on gls-mediated glutamine metabolism. Evaluation of sourdough bread fermented with L. reuteri strains demonstrated a significant difference between the glutamate accumulating L. reuteri 100-23ΔgadB and the γ-aminobutyrate accumulating wild type L. reuteri 100-23. In contrast, bread produced with L. reuteri 100-23 iii Δgls1-2-3, which does not convert glutamine to either glutamate or γ-aminobutyrate, was not different from bread produced with L. reuteri 100-23. Glutamine conversion of sourdough lactobacilli contributes to acid resistance, and enhances the taste of bread. Extended information on acid resistance and glutamine metabolism of L. reuteri furthers the understanding of L. reuteri to cereal ecosystems, and results will contribute to the selection of strains that can be used as starter cultures for baking improvers (i.e. production of salt reduced bread).