Growth Inhibition and Alternation of Virulence Genes of Salmonella on Produce Products Treated with Polyphenolic Extracts from Berry Pomace

Abstract

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.