Internal Colonization of Salmonella enterica Serovar Typhimurium in Tomato Plants

Abstract

Several Salmonella enterica outbreaks have been traced back to contaminated tomatoes. In this study, the internalization of S. enterica Typhimurium via tomato leaves was investigated as affected by surfactants and bacterial rdar morphotype, which was reported to be important for the environmental persistence and attachment of Salmonella to plants. Surfactants, especially Silwet L-77, promoted ingress and survival of S. enterica Typhimurium in tomato leaves. In each of two experiments, 84 tomato plants were inoculated two to four times before fruiting with GFP-labeled S. enterica Typhimurium strain MAE110 (with rdar morphotype) or MAE119 (without rdar). For each inoculation, single leaflets were dipped in 109 CFU/ml Salmonella suspension with Silwet L-77. Inoculated and adjacent leaflets were tested for Salmonella survival for 3 weeks after each inoculation. The surface and pulp of ripe fruits produced on these plants were also examined for Salmonella. Populations of both Salmonella strains in inoculated leaflets decreased during 2 weeks after inoculation but remained unchanged (at about 104 CFU/g) in week 3. Populations of MAE110 were significantly higher (P<0.05) than those of MAE119 from day 3 after inoculation. In the first year, nine fruits collected from one of the 42 MAE119 inoculated plants were positive for S. enterica Typhimurium. In the second year, Salmonella was detected in adjacent non-inoculated leaves of eight tomato plants (five inoculated with strain MAE110). The pulp of 12 fruits from two plants inoculated with MAE110 was Salmonella positive (about 106 CFU/g). Internalization was confirmed by fluorescence and confocal laser microscopy. For the first time, convincing evidence is presented that S. enterica can move inside tomato plants grown in natural field soil and colonize fruits at high levels without inducing any symptoms, except for a slight reduction in plant growth.

Diguanylate Cyclases AdrA and STM1987 Regulate Salmonella enterica Exopolysaccharide Production during Plant Colonization in an Environment-Dependent Manner

“…In addition, bacterial aggregation or formation of biofilms can provide a protective layer, trap nutrients, and generally aid in stress tolerance (37)(38)(39). S. enterica biofilm mutants are significantly impaired in persistence on produce leaves (51,52). In support of the idea that biofilm formation is vital to S. enterica survival on leaves, we found that adrA and the production of exopolysaccharides, such as cellulose and colanic acid, contributed to persistence on alfalfa leaves (Fig.…”

Colonization and Internalization of Salmonella enterica in Tomato Plants

“…At the preharvest stage, several potential routes for S. enterica colonization and internalization to contaminate tomato fruits have been examined previously (4)(5)(6)(7)(8). Some of the findings point to irrigation with contaminated water as a potential source of fruit contamination (4, 7); however, evidence that S. enterica is able to enter tomato plants through contaminated irrigation water remains inconsistent.…”

Robust cropping systems to tackle pests under climate change. A review

“…Similarly, Salmonella enteric typhimurium internally colonizes tomato (Gu et al 2013; as well as lettuce (Klerks et al 2007) plants. On tomato, S. enterica has been reported to move systemically reaching fruits and seeds from leaves (Gu et al 2011). Once within the fruits, the pathogen can multiply to high densities (Gu et al 2011;Noel et al 2010).…”

“…On tomato, S. enterica has been reported to move systemically reaching fruits and seeds from leaves (Gu et al 2011). Once within the fruits, the pathogen can multiply to high densities (Gu et al 2011;Noel et al 2010). A recent study demonstrated that Escherichia coli can internally colonize both leaves and roots of lettuce and spinach (Wright et al 2013).…”