Transmission of human enteric pathogens from artificially-inoculated flowers to vegetable sprouts/seedlings developed via contaminated seeds

Seeds contaminated with bacterial pathogens were found to be the primary cause of sprout-associated outbreaks of human gastrointestinal infections. This study was undertaken to determine if cells of selected Salmonella enterica and enterohemorrhagic Escherichia coli (EHEC) strains, artificially inoculated onto the flowers of vegetable plants, will result in contamination of sprouts/seedlings that develop from seeds produced by the inoculated flowers. Pistils of alfalfa, fenugreek, lettuce, and tomato flowers were inoculated with cells of selected S. enterica or EHEC strains. A total of 906, 715, 1236, and 1276 mature seeds, produced by lettuce, tomato, alfalfa or fenugreek flowers inoculated with Salmonella were collected as 48, 94, 109, and 116 composite samples (367 in total), respectively. Correspondingly, 934, 640, 1827 and 1027 seeds, produced by the four respective types of flowers after inoculation with E. coli were divided into 42, 81, 162, and 107 composite samples (392 in total), respectively. Seeds in each composite sample were surface-decontaminated with NaOCl solution and germinated at 25 °C in the dark for 5 days. Subsequently, pathogen populations on the sprouts/seedlings developed from each composite seed sample were determined by the plate count assay. The overall Salmonella recovery rate from vegetable sprouts/seedlings developed from the 367 composite seed samples was 2.7%, while none of the sprouts/seedlings grown from the 392 composite seed samples with prior E. coli inoculation tested positive for the pathogen. One of the 94 tomato seedling samples contained 4 CFU of Salmonella and five additional samples tested positive by enrichment (6.4%). Two out of 109 (1.8%) alfalfa and 116 (1.7%) fenugreek composite samples tested positive for Salmonella by enrichment. However, none of the lettuce seedlings tested positive for Salmonella or E. coli even after enrichment. This study suggests that under controlled environmental conditions, human pathogens inoculated onto flowers of vegetable plants can result in the contamination of sprouts/seedlings via seeds produced by the inoculated flowers. However, the frequency of sprout/seedling contamination was low and could be affected by characteristics of the pathogens and plant species tested.