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Abstract
Vegetable seeds are potential vehicles of human pathogens and a likely source of contamination in most sprout-associated outbreaks. The objectives of this study were to observe 1) the fate of two important human pathogens, Salmonella enterica and enterohemorrhagic Escherichia coli (EHEC), artificially inoculated on seed-producing flowers or internalized into vegetable seeds, during pilot-scale seed production or germination processes, and 2) the competition of the two human pathogens with a group of selected bacterial strains for growth in microbiological media and attachment to vegetable seeds. Salmonella and EHEC cells were vacuum-infiltrated into seeds of alfalfa, fenugreek, tomato, and lettuce before germination at 25 C. Sections of sprout and seedling tissues were collected for microbiological analysis during the 9-day germination process. Results showed that the growth and distribution of the two human pathogens on sprout/seedling tissues were affected by bacterial species and strains as well as vegetable seed types. Open flowers of alfalfa, fenugreek, tomato, and lettuce were inoculated with different strains of Salmonella and EHEC. Seeds from the inoculated flowers were germinated at 25 C for 7 days, and sprout or seedling tissues were analyzed subsequently for Salmonella or EHEC. An average of 2.7% of the sprouts or seedlings developed from seeds of contaminated flowers tested positive for Salmonella whereas none of the samples tested positive for EHEC. Salmonella or EHEC were set for competition with a group of selected bacterial competitors, including Lactobacillus. rhamnosus GG, for growth in nutrient broth and attachment to sprout seeds. The inhibitory effect of 72 h cell-free supernatants of the bacterial competitors on Salmonella and EHEC growth was also evaluated. The presence of competitive bacteria, especially L. rhamnosus GG significantly inhibited the growth of Salmonella and EHEC. Cell-free supernatants of L. rhamnosus GG spent cultures also inhibited the two pathogens. Pseudomonas. fluorescens A506 showed strong ability to reduce attachment of Salmonella and EHEC to seed `surfaces. Results of this study provided a better understanding on the fate of human pathogens during seed production and germination, highlighting the importance of following Good Agricultural Practice for vegetable seed and seed sprout production.