The mechanism of curcumin inhibits Bacillus cereus protease production by modulating the LuxS/AI-2 system.

Bacillus cereus (B. cereus) is a food-borne pathogen that produces proteases in raw milk, causing dairy products to deteriorate in quality through protein precipitation and bitterness. Quorum sensing (QS) is a bacterial communication mechanism where autoinducers (AI) are produced and accumulated to regulate gene expression, including that of extracellular enzymes. In this study, curcumin could act as a natural QS inhibitor (QSI), hindering the AI -2 production and reducing proteolytic activity. Whole genome sequencing results showed that B. cereus D8 contained luxS and other genes related to the transport of AI-2, indicating that the LuxS/AI-2 QS system regulated physiological activity. In addition, the presence of nprB demonstrated the protease-producing ability of B. cereus D8. Transcriptome and molecular docking analyses revealed that curcumin may bind to LuxS, leading to upregulation of the luxS gene for AI-2 synthesis and downregulation of the lsrK, lsrA, and lsrR genes involved in AI-2 transport. Curcumin addition also led to the inhibition of various physiological activities of B. cereus D8 including biosynthesis of various amino acids, as well as the downregulation of nprB gene expression, thereby reducing protease production. The exogenous addition of AI-2 also confirmed that curcumin did not alter the expression of luxS gene and upregulated lsrK, lsrR, and nprB genes, indicating that curcumin did not degrade AI-2. This study reveals the molecular mechanism that curcumin inhibited protease production in B. cereus by regulating the LuxS/AI-2 QS system, providing a theoretical basis for biological inhibitor development.