Transcriptome Analysis of the Molecular Mechanism of Bacteriocin Synthesis in Lactobacillus plantarum KLDS1.0391 Under Nanoparticle NaCl Stress

Abstract

Lactobacillus plantarum is an important industrial lactic acid bacteria, which can be used as a starter culture for fermented milk and meat products. Many Lactobacillus plantarum strains can produce bacteriocins with broad-spectrum antibacterial properties, heat stability, and easy to be hydrolyzed by protease in the process of food fermentation and metabolism, and their potential as biopreservative starter culture in the preservation of fermented food has been recognized. However, the high salt environment in the food matrix can affect bacteriocin production. Hence, the objective of this research is to reveal how salt stress affects the production of bacteriocin and the expression of related genes in this strain by transcriptome sequencing and to further analyze possible regulatory mechanisms, to provide references for the use of bacteriocin as a natural biological preservative in salt-containing foods. The number of viable counts and the antibacterial activity of bacteriocin over the whole growth stages were determined under the stress of 0%, 2%, 3%, 4%, and 6% NaCl. When the strain was cultured at low nanoparticle NaCl concentration (2% or 3%), the growth of the experimental group had no significant difference compared with the control group; however, bacteriocin antibacterial activity increased significantly in the stable phase. When the target strain was cultured under 2% NaCl stress for 24 h, the antimicrobial activity reached the maximum. Subsequently, based on the transcriptome sequencing results obtained by Illumina HiSEq 2500 sequencing system, the differentially expressed genes under 0% and 2% NaCl stress were compared, and the enrichment pathways of these genes were analyzed. A total of 260 genes displayed significant differential expression induced by NaCl: Among them, 159 genes were significantly up-regulated, and 101 genes were down-regulated. Bioinformatic analysis revealed that differentially expressed genes related to bacteriocin synthesis were mainly enriched in bacterial secretion pathway, amino acid synthesis pathway, proteolytic enzyme regulation pathway, purine metabolism pathway, two-component reg- ulation pathway, etc. It is preliminarily speculated that nanoparticle NaCl stress can regulate the synthesis and release of bacteriocin by affecting the expression of secY and ftsY in the cell membrane secretion pathway. We also speculate that nanoparticle NaCl stress can provide raw materials for bacteriocin by affecting the expression levels of genes hisH, cysE, cysM, metB, metA, lysA, and argH in the amino acid synthesis pathway. In addition, our research signified that the expression levels of sat and rpoB in the purine metabolism pathway were up-regulated under nanoparticle NaCl stress, which is beneficial to provide energy for bacteriocin production. The results will be helpful to understand how salt stress regulates bacteriocin synthesis of Lactobacillus plantarum. Furthermore, this study also provides guidance for using bacteriocin-producing strains as biocontrol bacteria in the salt-containing food matrix.