Contribution of Thiamine to Lactococcus lactis Tolerance toward Acid Stress

Lactococcus lactis is used extensively in cheese and fermented milk production. Acid stress is often encountered when L. lactis is used as a starter for the production of fermented foods. The effects of thiamine on the acid-stress tolerance of L. lactis were explored in this study. Exogenous thiamine improved the acid-stress tolerance of L. lactis during acid stress. When cultivated with 100 μM thiamine, the survival rate of cells was 14.2-fold higher than the control after 1.5 h of acid exposure at pH 3.8. The acid-stress responses of L. lactis mediated by thiamine were investigated by transcription level approaches. The results indicated a role for thiamine in increasing the transcription level of genes associated with acetoin biosynthesis during acid stress, which could consume more protons and maintain the relative homeostasis of intracellular pH. In addition, the transcription levels of thiamine-diphosphate-dependent enzymes were also upregulated, which provided more energy for cells to withstand acid stress. Furthermore, metabolite profiling highlighted the increased abundance of amino acid, especially glutamate, in thiamine-added cells during acid stress, and the intracellular ATP level was also improved. Finally, the production of thiamine in L. lactis was engineered, suggesting an increased acid-stress tolerance. These findings suggest that thiamine results in more excellent resistance to acid stress and could be employed as a promising protectant during probiotic fermentation.