RESUMEN
Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus are the main species used for yogurt preparation. Glutathione (GSH) can be synthesized by S. thermophilus and plays a crucial role in combating environmental stress. However, the effect of GSH biosynthesis by S. thermophilus on cocultured L. delbrueckii ssp. bulgaricus is still unknown. In this study, a mutant S. thermophilus ΔgshF was constructed by deleting the GSH synthase. The wild strain S. thermophilus ST-1 and ΔgshF mutants were cocultured with L. delbrueckii ssp. bulgaricus ATCC11842 by using Transwell chambers (Guangzhou Shuopu Biotechnology Co., Ltd.), respectively. It was proven that the GSH synthesized by S. thermophilus ST-1 could be absorbed and used by L. delbrueckii ssp. bulgaricus ATCC11842, and promote growth ability and stress tolerance of L. delbrueckii ssp. bulgaricus ATCC11842. The biomass of L. delbrueckii ssp. bulgaricus ATCC11842 cocultured with S. thermophilus ST-1 or ΔgshF (adding exogenous GSH) increased by 1.8 and 1.4 times compared with the biomass of L. delbrueckii ssp. bulgaricus ATCC11842 cocultured with S. thermophilus ΔgshF. Meanwhile, after H2O2 and low-temperature treatments, the bacterial viability of L. delbrueckii ssp. bulgaricus cocultured with S. thermophilus ΔgshF, with or without GSH, was decreased by 41 and 15% compared with that of L. delbrueckii ssp. bulgaricus cocultured with S. thermophilus ST-1. Furthermore, transcriptome analysis showed that the expression levels of genes involved in purine nucleotide and pyrimidine nucleotide metabolism in L. delbrueckii ssp. bulgaricus ATCC11842 were at least 3 times increased when cocultured with S. thermophilus (fold change > 3.0). Moreover, compared with the mutant strain ΔgshF, the wild-type strain ST-1 could shorten the fermented curd time by 5.3 hours during yogurt preparation. These results indicated that the GSH synthesized by S. thermophilus during cocultivation effectively enhanced the activity of L. delbrueckii ssp. bulgaricus and significantly improved the quality of fermented milk.
