Exogenous nucleotides enhance acid stress resistance in Lactobacillus plantarum FQR: Metabolome and transcriptome

Exogenous nucleotides have garnered significant attention as a novel class of protective agents against acid stress in bacteria; however, their protective mechanism in Lactic Acid Bacteria (LAB) remains unclear. In this study, Lactobacillus plantarum FQR was used to investigate the protective effect of exogenous nucleotides (represented by 5'-GMP-2Na) against acid stress and to elucidate the underlying mechanism. The results showed that exogenous nucleotides significantly mitigated cell wall damage induced by acid stress. They helped maintain the secondary structure proportions of cell membrane proteins, with the α-helix and β-turn contents increasing by approximately 5% and 4%, respectively, compared with the control (CK) group. This preservation supported membrane protein function and fluidity, stabilized membrane protein structure, promoted protein embedding, regulated protein-protein interactions, enhanced the adaptability of membrane proteins to environmental changes, and restored bacterial growth capacity under acid stress. Integrated metabolomic and transcriptomic analyses revealed that 5'-GMP-2Na activated the glycolytic pathway (ATP-dependent phosphofructokinase activity increased about 2.28-fold) to promote the tricarboxylic acid (TCA) cycle, thereby ensuring energy supply. It also upregulated the expression of DNA repair genes such as nucleoside diphosphate kinase (ndk) (approximately 3.53-fold) and enhanced the synthesis and metabolic pathways of nucleotides, amino acids, and fatty acids, collectively improving the acid stress tolerance of the strain. Based on the optimized process, a direct-vat starter culture of FQR was prepared, with an average viable count of 6.8 × 10¹¹ cfu/mL.