Abstract: Heyndrickxia coagulans finds wide applications in the food, pharmaceutical, and aquaculture industries due to its high stress resistance. In this study, we investigated the acid tolerance mechanism of H. coagulans by analyzing the effect of acid stress on its intracellular pH (pHin), intracellular ATP content, H+-ATPase activity, cell membrane fatty acid composition, and gene expression. The results showed that under acid stress (pH 3.0), both high acid-tolerant (C6) and low acid-tolerant strains (B8) maintained stable pHin and exhibited increased intracellular ATP content and decreased H+-ATPase activity. The unsaturation degree and average carbon chain length of fatty acids in their cell membranes increased, with C6 exhibiting more prominent performance in these aspects. Transcriptome sequencing analysis identified a total of 1 430 differentially expressed genes (DEGs; 639 upregulated and 791 downregulated) in strain C6 under acid stress, which were involved in six functional categories: amino acid metabolism, fatty acid synthesis and metabolism, energy metabolism, carbohydrate metabolism, macromolecular protection and repair, and transcription and translation regulation. Under acid stress, C6 enhanced cell membrane fluidity by reducing the content of saturated fatty acids, thereby preventing the influx of hydrogen ions. Meanwhile, it strengthened the synthesis of branched-chain amino acids to reduce the conversion of pyruvate to lactic acid via glycolysis, thus avoiding intracellular acidification and maintaining pHin stability. Additionally, C6 was able to increase intracellular energy production by enhancing the oxidative phase of the pentose phosphate pathway and the tricarboxylic acid cycle, leading to elevated intracellular ATP content. It also promoted hydrogen ion consumption and energy synthesis by upregulating the expression of genes related to the respiratory chain in the oxidative phosphorylation pathway, so as to maintain pHin stability while synthesizing more ATP. Furthermore, we found that the expression of genes encoding molecular chaperones, stress proteins, and DNA repair-related proteins were upregulated, thereby protecting against and repairing damage to biological macromolecules and proteins caused by acid stress. The downregulated expression of ribosomal subunit genes inhibited ribosome synthesis, reduced intracellular protein synthesis, and decreased intracellular ATP consumption, which was consistent with the accumulation of intracellular ATP under acid stress.

Key words: Heyndrickxia coagulans; acid stress; cell membrane fatty acids; intracellular pH; transcriptome