Survival of Escherichia coli O157:H7 and Listeria monocytogenes on Spinach and Blueberries Treated With Aqueous Chlorine Dioxide and Muscadine (Vitis rotundifolia) Extract.

The antimicrobial effect of various concentrations of chlorine dioxide (ClO) and muscadine extract (ME) was investigated on Escherichia coli O157:H7 and Listeria monocytogenes under in vitro conditions and on treated spinach and blueberries. Tryptic soy broth (TSB) and produce were inoculated with a 3-strain cocktail of each pathogen. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined using either 96-well plates incubated in a microplate reader for ClO or test tubes incubated in a conventional incubator at 37 °C for 24 h for the ME. Pathogen reduction on inoculated produce (inoculation levels were 5.6-7.0 log CFU/g for spinach and 4.5-5.1 log CFU/g for blueberries) was evaluated after exposure to various antimicrobial treatments for 10 min + 30-45 min dry time + storage at 8 °C for up to six days. Under in vitro, L. monocytogenes was more susceptible than E. coli O157:H7 to ClO and ME. On spinach, 300 mg/ml ME and 3 ppm ClO + 300 mg/ml ME treatments resulted in the maximum reduction of E. coli O157:H7 (4.6 log CFU/g reduction at day 1) and L. monocytogenes (5.6 log CFU/g reduction at day 2), respectively. On blueberries, all the treatments significantly decreased E. coli O157:H7 counts (≤1.7 log CFU/g), and no differences (P > 0.05) were detected from day 1 to day 6. The treatment 3 ppm ClO + 300 mg/ml ME led to nondetectable (P ≤ 0.05) L. monocytogenes levels on blueberries at days 1, 2, 4, and 6, leading to surface reductions >4.1 log CFU/g. Based on the study, ME combined with ClO could be a potential candidate as postharvest treatment to reduce the presence of E. coli O157:H7 and L. monocytogenes on blueberries. Both pathogens were less susceptible to these combined treatments on spinach.