Fate of 43 Salmonella Strains on Lettuce and Tomato Seedlings

Multiple outbreaks of foodborne illness caused by Salmonella sp. have been linked to fresh produce. Understanding variability in colonization potential is critical for the selection of experimental strains suitable for research on the ecology of the species in growing food plants. The fate of 43 Salmonella strains from 29 serovars was examined on seedlings from two cultivars of lettuce ('Winter Density' and 'Parris Island Cos') and tomato ('Amish Paste' and 'Manitoba'). Salmonella populations were measured on xylose lysine deoxycholate agar immediately after inoculation and after 5 days of incubation at 21 degrees C. Laser scanning confocal microscopy was also performed to examine the distribution of cells on seedling leaf surfaces. Laser scanning confocal microscopy showed that cells or cellular aggregates were located within stomata, in surface depressions adjacent to stomata, or on random surface locations on seedlings that were successfully colonized. Populations of 26 strains (60.5%) increased on seedlings from all plant species and cultivars, although there were significant differences (P < 0.05) in the extent of population increase achieved by different strains on the same plant species-cultivar combinations. The remaining strains displayed differential ability to colonize seedlings depending on plant species or cultivar. The results of the present study indicate that the colonization potential of Salmonella is highly variable and should be carefully considered in the selection of experimental strains. HIGHLIGHTS Salmonella primarily colonized stomata on lettuce and tomato seedlings. 43 Salmonella strains either grew, remained unchanged, or declined on the seedlings. The fate of Salmonella was affected by strain, plant species, and cultivar. Research on the ecology of Salmonella on plants requires carefully selected strains.