Quercetin repressed the stress response factor (sigB) and virulence genes (prfA, actA, inlA, and inlC), lower the adhesion, and biofilm development of L. monocytogenes.

The present study explored the effect of quercetin on the expression of virulence genes actA, inlA, inlC, and their regulatory components, sigB and prfA, in L. monocytogenes. Furthermore, the physicochemical changes on the surface, membrane permeability, and biofilm formation of quercetin-treated bacteria were evaluated. An inhibitory dose-dependent effect of quercetin (0.1-0.8 mM) was observed on the cell attachment on stainless steel at 2 and 6 h at 37 °C. Quercetin at 0.8 mM prevented the biofilm formation on stainless steel surfaces after 6 h of incubation at 37 °C, while the untreated bacteria formed biofilms with a cell density of 5.1 Log CFU/cm2. The microscopic analysis evidenced that quercetin at 0.2 mM decreased the biovolume and covered area of the attached micro-colonies. Also, sigB, prfA, inlA, inlC, and actA genes were downregulated by 7-29 times lower compared to untreated bacteria. In addition, quercetin decreased the superficial cell charge, increased the membrane permeability, and its surface hydrophobicity. These results demonstrated that quercetin prevented biofilm formation, repressed the genes of stress and virulence of L. monocytogenes and also altered the physicochemical cell properties.

Antibacterial and antioxidant properties of grape stem extract applied as disinfectant in fresh leafy vegetables.

In the present study total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity and antimicrobial properties of grape (Vitis vinifera var. Red Globe) stem extract is reported. Also, the identification of main phenolic compounds was carried out by UPLC-PAD analysis. TPC and TFC of extract were 37.25 g GAE kg-1 and 98.07 g QE kg-1, respectively. Extract showed an antioxidant capacity of 132.60 and 317 g TE kg-1 for DPPH and ABTS radical scavenging capacity, respectively. The main phenolic compounds identified were rutin, gallic acid, chlorogenic acid, caffeic acid, catechin and ferulic acid. Extract inhibited the growth of Listeria monocytogenes, Staphylococcus aureus, Salmonella enterica subsp. enterica serovar Typhimurium, and Escherichia coli O157: H7 at MIC range 16-18 g L-1. Extract affected the different phases of bacterial growth. In addition, application of Extract (25 g L-1) as a sanitizer was effective to reduce the populations of all bacteria inoculated in lettuce (0.859-1.884 log reduction) and spinach (0.843-2.605 log reduction). This study emphasizes the potential of grape processing byproducts as an emergent and attractive source of bioactive compounds with antioxidant properties and antimicrobial activity against important foodborne pathogens. The study demonstrated that stem extract could be used to control the presence of human pathogenic bacteria in fresh leafy vegetables.