INTERACTIONS BETWEEN THE FOODBORNE PATHOGEN (SALMONELLA ENTERICA SUBSP. ENTERICA SEROVAR ORANIENBURG AND ESCHERICHIA COLI O157:H7) AND SOFT-ROT PECTOBACTERIA (DICKEYA SPP.) ON ONIONS (ALLIUM CEPA): IMPLICATIONS FOR FOOD SAFETY

Abstract

Onions, one of the most consumed vegetables worldwide, is quite popular in United States, with an annual per capita consumption of 19.18 pounds and a total 2023 production of 51 million CWT (hundredweight). Despite their nutritional value and year-round availability, onions have been associated with multiple outbreaks of foodborne illnesses. The WHO estimates that contaminated food causes 600 million cases of foodborne diseases and 420,000 deaths globally each year. In the U.S., pathogens like Salmonella, Shiga toxin-producing Escherichia coli (STEC), and Listeria cause over 1.49 million illnesses annually, leading to thousands of hospitalizations and deaths. The irrigation water plays important role in causing foodborne outbreaks through the consumption of contaminated food including onions, where the foodborne pathogens get contaminated with the fresh produce when it comes in contact with irrigation water carrying foodborne pathogens such as Salmonella, E. coli.This research investigates the interplay between foodborne pathogens (FBP) such as Salmonella Oranienburg and E. coli O157:H7 and soft rot-causing pectinolytic bacteria (SRP), specifically Dickeya fangzhongdai and Dickeya dianthicola. Experiments were conducted to understand how SRP influences the survival, colonization, and population dynamics of these pathogens on onion slices (white, red, and yellow varieties) and onion plants.Key findings from the experiments indicate that SRP significantly enhances the proliferation of FBPs as significant difference was observed between the populations of S. Oranienburg in the presence and absence of SRP recovered from onion slices and white onion plants. For instance, at 7 dpi, the mean populations of S. Oranienburg were 4.24 ± 0.53 log10 CFU/g when inoculated alone, 6.70 ± 1.05 log10 CFU/g when co-inoculated with D. fangzhongdai, and 5.22 ± 0.50 log10 CFU/g when co-inoculated with D. dianthicola in white onion slices. Similarly, in yellow onion slices, the populations were 4.83 ± 0.78 log10 CFU/g, 7.39 ± 0.91 log10 CFU/g, and 5.96 ± 0.65 log10 CFU/g, respectively. In red onion slices, the population was significantly higher (5.98 ± 1.62 log10CFU/g) when co-inoculated with D. fangzhongdai at 7 dpi (p < 0.05) compared to inoculated alone (5.15 ± 0.40 log10CFU/g).In white onion plants, S. Oranienburg was not detected in most sections (roots, middle, crown, 50% plant height), except at the point of inoculation in a few plants. The mean population of S. Oranienburg was 8.52 ± 0.79 log10CFU/g and 9.40 ± 1.72 log10CFU/g in the middle section, and 8.52 ± 0.89 log10 CFU/g and 9.34 ± 1.49 log10 CFU/g in the crown region at 7 and 14 dpi, respectively. A significant difference was observed between the populations of S. Oranienburg in the presence and absence of SRP recovered from onion slices and white onion plants. At 7 dpi, the mean populations of S. Oranienburg were 4.24 ± 0.53 log10 CFU/g when inoculated alone, 6.70 ± 1.05 log10 CFU/g when co-inoculated with D. fangzhongdai, and 5.22 ± 0.50 log10 CFU/g when co-inoculated with D. dianthicola in white onion slices. Similarly, in yellow onion slices, the populations were 4.83 ± 0.78 log10 CFU/g, 7.39 ± 0.91 log10 CFU/g, and 5.96 ± 0.65 log10 CFU/g, respectively. In red onion slices, the population was significantly higher (5.98 ± 1.62 log10 CFU/g) when co-inoculated with D. fangzhongdai at 7 dpi (p < 0.05). In onion plants, S. Oranienburg was not detected in most sections (roots, middle, crown, 50% plant height), except at the point of inoculation in a few plants. The mean population of S. Oranienburg was 8.52 ± 0.79 log10 CFU/g and 9.40 ± 1.72 log10 CFU/g in the middle section, and 8.52 ± 0.89 log10 CFU/g and 9.34 ± 1.49 log10 CFU/g in the crown region at 7 and 14 dpi, respectively.Additionally, there was significant population of E. coli O157:H7 present in different sections (middle, crown and inoculation point) at 14 dpi in yellow (7.97±0.37 log10CFU/g, 8.12±0.57 log10CFU/g and 8.23±0.36 log10CFU/g), red (8.26 ± 0.29 log10CFU/g, 7.88 ± 0.85 log10CFU/g and 8.04 ± 0.68 log10CFU/g) and green onions (6.44 ± 0.45 log10CFU/g, 7.0 ± 0.36 log10CFU/g and 8.50 ± 0.18 log10CFU/g) when co-inoculated with D. fangzhongdai. Contrastingly, E. coli O157:H7 was not detected in any sections of yellow, red and green onion plants in absence of D. fangzhongdai from at 7 and 14 dpi. These findings highlight the role of SRP in facilitating the growth and internalization of foodborne pathogens in onions, raising concerns about their impact on food safety. Understanding these interactions is crucial for developing effective strategies to mitigate contamination risks and ensure public health. This study underscores the complex dynamics between plant and foodborne pathogens and their implications for agricultural practices and food safety.