Purpose: This study was undertaken to determine whether EHEC on artificially-inoculated and accompanying-contaminated vegetable seeds could migrate to different tissues of sprouts/seedlings.
Methods: Alfalfa, fenugreek, tomato, and lettuce seeds were inoculated with EHEC F4546, K4499, H1730, or ATCC BAA-2326 by artificial-inoculation (IN) or accompanying-contamination (AC). For IN, overnight EHEC cultures were coincubated with vegetable seeds at 20°C for 5 h, whereas for AC, freeze-dried EHEC cells in sterile, sandy soil were coincubated with vegetable seeds at 20°C for one day. Contaminated vegetable seeds were subsequently germinated on 1% water agar at 25°C, in the dark. Populations of EHEC on different sprout/seedling tissues were determined every other day for five days.
Results: Approximately 91% (n=1,024) and 66% (n=1,024) of the sprout/seedling samples inoculated by IN and AC tested positive for EHEC, respectively. For IN, seed coats had the highest EHEC population (5.40 log), followed by radicle (5.18 log), cotyledon (4.80 log), and stem (4.41 log) tissues. For AC, however, EHEC populations associated with seed coat (3.63 log) and radical (3.48 log) tissues were similar, but significantly higher than those associated with cotyledon (3.16 log) and stem (2.94 log) tissues. IN fenugreek samples had the highest number of EHEC cells (5.79 log), followed by alfalfa (5.49 log), lettuce (5.26 log), and tomato (1.80 log) samples. For AC, a similar trend was observed, except that alfalfa samples had lower EHEC population (1.09 log) than lettuce samples. For both inoculation methods, the highest number of F4546 and lowest number of ATCC BAA-2326 cells were recovered from contaminated sprout/seedling tissues.
Significance: These data suggested that EHEC migrated from IN and AC vegetable seeds to various sprouts/seedlings tissues during germination. This indicated the importance of using pathogen-free seeds for vegetable sprout/seedling production.