Transcriptomic analysis was used to investigate the antibacterial mechanism of phenolic compounds from kefir fermented soy whey (FSP) against Escherichia coli 0157:H7 and Listeria monocytogenes. The kefir fermentation increased the concentration of several phenolic aglycones with proven antibacterial efficacy in the FSP. The time-kill curve showed that 2× MICs of the FSP killed >99.9 % of the strains within 2 h of exposure. The checkerboard fractional inhibition concentration (FIC) assay proved that phenolics were the sole antibacterial agent in the FSP. The transmission electron microscope (TEM) photomicrograph corroborated the propidium iodide (PI) uptake, protein, and nucleic acid leakage assays. They demonstrated that the phenolics permeated the cell membrane, disrupted the cytoplasm, and caused cell lysis in the treated cells leading to protein and nucleic acid leakage. The transcriptome analysis revealed that exposure of the cells to MICs of the phenolics induced molecular responses leading to differential expression of 1850 genes in E. coli 0157:H7 and 2090 in L. monocytogenes. The phenolics suppressed the expression of genes crucial for carbohydrate utilization, transmembrane glucose transport, tricarboxylic acid (TCA), and ATP synthesis. The phenolic-induced stress also downregulated the expression of quorum sensing and virulence-related genes, peptidoglycan and phospholipid synthases, and ABC transporters. The cells initiated a resistance response by stimulating the two-component signal transduction systems to trigger the over-expression of a cascade of genes involved in stress resistance, gluconeogenesis, ATPase activity and proton transmembrane transport. Nonetheless, the data indicated that the phenolics suppressed the expression of translational proteins that would have facilitated the resistance and repair of the cell damage caused by the phenolics. The study provides discrete data evidence that FSP could be used to control the pathogenicity and the proliferation of E. coli 0157:H7 and L. monocytogenes in our foods and food systems.
- Gene ontology
- Transmission electron microscope (TEM)
- Virulence factors