Commercial Bacteriophage Cocktail Treatment Reduces Salmonella Infantis Levels in Pre-harvest Agricultural Water

Abstract

Contaminated irrigation water in the preharvest environment can lead to outbreaks associated with fruits and vegetables. The potential of microbial contamination in preharvest waters by bacterial pathogens has created an ongoing demand for effective water treatment methods to mitigate this risk. This study sought to evaluate the efficacy of a commercial bacteriophage cocktail against Salmonella Infantis in agricultural water. A test agricultural water (TAW) medium prepared at different pH (6.5 and 8.4) and turbidity levels (4, 20, 50, and 100 NTU) was inoculated with a nalidixic acid resistant strain of S. Infantis (∼6 log CFU/mL), in triplicate trials, and treated with a bacteriophage cocktail (SalmoFresh™) at a phage titer of 8 log PFU/mL. Water samples were taken after a 5-minute contact time, with shaking (250 rpm), at 25 °C. Additionally, collected pond water, natural (nonsterile) and autoclaved, was inoculated with S. Infantis (∼4 log CFU/mL) and treated with bacteriophage cocktail (7 log PFU/mL or 8 log PFU/mL) at either 12 °C or 32 °C for 5, 10, or 30 min in triplicate trials. Samples were enumerated by plating onto XLD or TSA supplemented with 50 μg/mL Nalidixic acid. In TAW, S. Infantis levels were reduced by an average of 1.0 log CFU/mL after the 5-minute phage treatment, with no significant differences in reductions across all pH and turbidity levels tested (p > 0.05). In pond water (natural and autoclaved), S. Infantis reductions only occurred when the phage titer was 8 log PFU/mL, with average reductions of 1.04, 1.50, and 1.67 log CFU/mL after 5, 10, and 30 min, respectively, at 32 °C. At 12 °C, average reductions were 0.90, 1.15, and 1.36 log CFU/mL after 5, 10, and 30 min, respectively. These results demonstrate that commercial lytic phage cocktail specific for Salmonella are effective in water across various conditions (pH, turbidity, temperature) and may be considered with other technologies to reduce Salmonella levels in agricultural water.