Study of the internal mechanism of L-glutamate for improving the survival rate of Lactiplantibacillus plantarum LIP-1 after freeze-drying

Lactic acid bacteria are important fermentation strains, and improving their freeze-drying survival rate (FDSR) has become a hot research topic in recent years. In this research, we found that adding 0.1 g/L L-glutamate to culture medium improved the FDSR of four strains of L. plantarum. Using L. plantarum LIP-1 as an example, we explored the internal mechanism of this phenomenon. L-glutamate had this effect through two metabolic pathways. First, it made the strain grow rapidly and produce more acid, resulting in a rapid decline of pH in the culture environment. The acidic environment led to the upregulation of the dnaK gene and secG gene expression, increased DnaK protein levels, promoted its anchoring effect on the cell membrane, and enhanced the integrity of the cell membrane. Second, it upregulated the expression of the murL, murD, and vanY genes, increased peptidoglycan synthesis, and increased cell wall integrity. In sum, the strain obtained a higher FDSR by adjusting the cell membrane and cell wall structure using L-glutamate. This study provides a new method for improving the FDSR of lactic acid bacteria.