Adenine acts as a growth promoter to promote the growth of the lactic acid bacteria (LAB), but the effect on the viability of freeze-dried strains has rarely been studied. In this study, adding 0.01 g/L of adenine to medium increased the growth and freeze-dried viability of Lactiplantibacillus plantarum LIP-1. Further research has found that L. plantarum LIP-1 synthesized large amounts of adenosine triphosphate (ATP) by metabolizing adenine. Elevated intracellular ATP content caused feedback inhibition on the conversion pathway of pyruvate to lactic acid, while promoting the conversion of pyruvate to acetyl coenzyme A (acetyl-CoA). After a large accumulation of acetyl-CoA in the cells, there was sufficient substrate for the synthesis of cell membrane fatty acids. Elevated intracellular ATP content also activated the acyl-CoA thioesterase activity to catalyse the conversion of saturated fatty acids to unsaturated fatty acids, thereby improving the integrity of the cell membrane and reducing damage to the cell membrane during the freeze-drying process. Additionally, a reduction in the amount of pyruvate converted into lactate prevented the decrease in intracellular pH (pH_in), which alleviated the degree of acid stress on the strain, resulting in less DNA damage and improved DNA stability. It is concluded that L. plantarum LIP-1 reduced the degree of cell membrane and DNA damage by metabolizing adenine and improved the freeze-dried viability of the strain.

Amino acids are often used as probiotic growth factors. Their addition to the growth medium is found to effectively enhance the resistance of the strain to adverse environments. In this research, we found that adding 0.05 g/L L-cysteine to culture medium improved the freeze-drying survival rate of the strain. We investigated the internal mechanism behind this phenomenon and found that the addition of L-cysteine can reduce DNA damage to bacterial cells during the freeze-drying process. In comparison to the control group without L-cysteine, the treatment group with the addition of 0.05 g/L of L-cysteine exhibited an up-regulation of the metC gene, leading to the metabolism of L-cysteine into pyruvate and NH₃, which raised the intracellular pH, reduced DNA damage, and consequently enhanced the resistance of Lactiplantibacillus plantarum LIP-1 to freeze-drying.