Salmonella enterica serovar Typhimurium (ST) remains a major infectious agent in the USA, with an increasing antibiotic resistance pattern, which requires the development of novel antimicrobials capable of controlling ST. Polyphenolic compounds found in plant extracts are strong candidates as alternative antimicrobials, particularly phenolic acids such as gallic acid (GA), protocatechuic acid (PA) and vanillic acid (VA). This study evaluates the effectiveness of these compounds in inhibiting ST growth while determining changes to the outer membrane through fluorescent dye uptake and scanning electron microscopy (SEM), in addition to measuring alterations to virulence genes with qRT-PCR. Results showed antimicrobial potential for all compounds, significantly inhibiting the detectable growth of ST. Fluorescent spectrophotometry and microscopy detected an increase in relative fluorescent intensity (RFI) and red-colored bacteria over time, suggesting membrane permeabilization. SEM revealed severe morphological defects at the polar ends of bacteria treated with GA and PA, while VA-treated bacteria were found to be mid-division. Relative gene expression showed significant downregulation in master regulator hilA and invH after GA and PA treatments, while fliC was upregulated in VA. Results suggest that GA, PA and VA have antimicrobial potential that warrants further research into their mechanism of action and the interactions that lead to ST death.

Organic farming, including integrated crop-livestock farms (ICLF) and backyard farming (BF), are gaining popularity in the US, and products from these farms are commonly sold in farmers markets, local stores, or roadside stalls. Due to avoiding use of antibiotics and chemicals, as well as the non-professional harvesting and packaging methods, and the use of composted animal waste, there is an increased risk of cross-contamination with zoonotic pathogens. This study sets out to evaluate the efficiency of new post-harvest disinfection processes using natural berry pomace extracts (BPE) as a means to reduce the bacterial load found in two common leafy-greens, spinach and celery. Both spinach and celery were inoculated with a fixed bacterial load of Salmonella Typhimurium (ST) and later soaked in BPE supplemented water (wBPE) for increasing periods of time, and at two different temperatures (24°C and 4°C). The remaining live bacteria were quantified in Log of CFU/leaf and compared to that of vegetables soaked in only water. The relative expression of virulence genes ( hilA1/C1/D1, invA1/C1/E1 / F1 ) of wBPE treated ST was determined. Results show that in the case of spinach, there was a significant ( p <0.05) reduction of ST from a range of 0.2 to1.2 log CFU/mL and 0.5 to 5 log CFU/mL at 24°C and 4°C, respectively. For celery there was also a significant ( p <0.05) reduction of ST at either 24°C or 4°C. The changes in relative expression of virulence genes of ST isolated from spinach and celery varied depending on the treatment conditions but showed a significant downregulation of inv genes when treated at 24°C for 1440 minutes ( p <0.05). After 7 uses of wBPE, concentration of total polyphenolic compounds remains at the effective concentration. This research suggests that soaking these vegetables with BPE containing water at lower temperatures can still reduce load of ST enough to minimize risk of infection and alter the virulence properties.