Mechanistic Insights Into the Regulation of Tyramine Production by NaCl in Enterococcus From Sufu: Growth, Gene Expression, and Enzymatic Activity

ABSTRACT High salt content in sufu suppresses the formation of harmful biogenic amines by microorganisms; however, excessive salt intake poses health risks. Therefore, understanding how NaCl regulates biogenic amine synthesis is essential for optimizing sufu production. This study investigated the effect and mechanism of NaCl concentration on biogenic amine synthesis using two dominant Enterococcus strains and tyramine as a representative biogenic amine. The investigation focused on three perspectives: bacterial growth capacity, gene cluster expression, and recombinant tyrosine decarboxylase (TDC) activity. Results indicated that NaCl concentrations exceeding 8% significantly inhibited Enterococcus growth, accompanied by the inhibition of tyramine production. In contrast, although the growth of Enterococcus was suppressed at 6%–8% NaCl, tyramine levels notably increased, with the maximum tyramine observed at 8% NaCl. Furthermore, as NaCl concentration increased from 0% to 8%, the transcriptional level of three key tyramine synthesis genes— tyrDC , tyrP , and nhaC —was gradually upregulated. Specifically, they all exhibited the highest expression level, correlating with the highest tyramine production observed at 8% NaCl. Notably, recombinant TDC maintained high enzymatic activity within the 0%–8% NaCl range, suggesting that NaCl stabilized TDC activity, thereby supporting tyramine synthesis in Enterococcus. This study investigated the regulatory role of NaCl in Enterococcus tyramine production from three complementary perspectives, providing a theoretical basis for optimizing sufu manufacturing processes.