Colonization and internalization of Salmonella enterica in tomato plants

Abstract

The consumption of fresh tomatoes has been linked to numerous food-borne outbreaks involving various serovars of Salmonella enterica. Recent advances in our understanding of plant-microbe interactions have shown that human enteric pathogenic bacteria, including S. enterica, are adapted to survive in the plant environment. In this study, tomato plants (Solanum lycopersicum cv. Micro-Tom) grown in sandy loam soil from Virginia's eastern shore (VES) were inoculated with S. enterica serovars to evaluate plausible internalization routes and to determine if there is any niche fitness for certain serovars. Both infested soil and contaminated blossoms can lead to low internal levels of fruit contamination with Salmonella. Salmonella serovars demonstrated a great ability to survive in environments under tomato cultivation, not only in soil but also on different parts of the tomato plant. Of the five serovars investigated, Salmonella enterica serovars Newport and Javiana were dominant in sandy loam soil, while Salmonella enterica serovars Montevideo and Newport were more prevalent on leaves and blossoms. It was also observed that Salmonella enterica serovar Typhimurium had a poor rate of survival in all the plant parts examined here, suggesting that postharvest contamination routes are more likely in S. Typhimurium contamination of tomato fruit. Conversely, S. Newport was the most prevalent serovar recovered in both the tomato rhizosphere and phyllosphere. Plants that were recently transplanted (within 3 days) had an increase in observable internalized bacteria, suggesting that plants were more susceptible to internalization right after transplant. These findings suggest that the particular Salmonella serovar and the growth stage of the plant were important factors for internalization through the root system.

Fig 1

Salmonella enterica populations in soil (CFU/g), on leaves (CFU/leaflet), and blossoms (CFU/blossom) of tomato plants after inoculation. Average S. enterica populations are shown as lines: soil (– –), leaves (- - -) and blossoms (––).

Fig 2

Molecular serology prevalence per Salmonella enterica serovars Saintpaul, Typhimurium, Javiana, Montevideo, and Newport in the rhizosphere, on leaves, and on blossoms of tomato plants after inoculation. A five-strain cocktail was inoculated into soil and onto leaves and flowers of tomato plants in the corresponding experimental groups. At day 8 (A) and day 23 (B) after soil inoculation, day 8 (C) and day 23 (D) after leaf inoculation, or day 7 after blossom inoculation (E), around 100 Salmonella colonies, with 6 to 10 colonies from each Salmonella-positive sample, were randomly picked for serological surveillance. Each dot represents the estimated fraction for each of the five serovars in around 100 Salmonella colonies isolated from each sampling, and each line represents the lower and upper values of the 95% confidence interval (CI).

Fig 3

Recovery of each Salmonella serovar on and within tomato fruit derived from inoculated blossoms. A five-strain cocktail was inoculated onto individually labeled blossom of tomato plants. All the tomato fruits derived from inoculated and emergent flowers in the experimental group were harvested and screened for surface and internal populations of Salmonella. Molecular serotyping was used to determine the serovar of isolated Salmonella colonies. Each bar represents the serovar-associated number of Salmonella-positive tomatoes. Serovars with different letters denote significant differences (P < 0.05) as determined by Tukey-Kramer honestly significant difference (HSD) testing on two-way analysis of variance (ANOVA) results. Uppercase letters refer to differences on tomato surfaces; lowercase letters refer to differences inside tomato fruit.