Effects of different initial pH values on freeze-drying resistance of Lactiplantibacillus plantarum LIP-1 based on transcriptomics and proteomics

The growth and the resistance to adverse environments of lactic acid bacteria would be affected by adjusting the initial pH of the medium. In order to explore the effect of changing the initial pH of culture medium on the freeze-drying survival rate of the Lactiplantibacillus plantarum LIP-1, the effect of initial pH on cell membrane fatty acid composition and key enzyme activity were mainly determined, and the internal mechanism was studied by transcriptomics and proteomics methods. We found that compared with initial pH 7.4 group, initial pH 6.8 group could improve the freeze-drying survival rate of the L. plantarum LIP-1. It was possibly due to the lactate dehydrogenase (LDH) was upregulated in the initial pH6.8 group, which led to a rapid decrease in culture pH. To reduce the inhibitory effect of long-term acid environment on growth, the strain upregulated the expression of fatty acid synthesis-related genes and proteins, promoted the relative content of cyclopropane and unsaturated fatty acids, improved integrity of the cell membranes. The adjustment of fatty acid composition maintained the integrity of the cell membrane in a freeze-drying environment to improve the freeze-drying survival rate of the initial pH6.8 group. In addition, the long-term acid environment stimulated a cross-stress tolerance mechanism that significantly upregulated the expression of a cold stress protein. The results indicated that the optimal initial pH of the medium could improve the ability of L. plantarum LIP-1 to resist freeze-drying.