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Consequences of Disrupting Salmonella AI-2 Signaling on Interactions Within Soft Rots

First and third authors: Interdisciplinary Ecology Program, Department of Soil and Water Science, University of Florida-Institute of Food and Agricultural Sciences, Gainesville 32611; and second author: Department of Pathology, University of California, Irvine 92697-4800.

Within soft rots, Salmonella spp. reach population densities 10- to 100-fold higher than within intact plants. The hypothesis that Salmonella spp. exchange AI-2 signals with Pectobacterium carotovorum to increase its competitive fitness was tested using mutants involved in AI-2 production (luxS) or perception (lsrACDBF or lsrG). Co-infections of a wild-type Salmonella sp. and its AI-2 mutants (at ≈3 to 10⁴) were established in green or red tomato (‘FL 47’ or ‘Campari’ for 3 or 5 days) as well as tomato co-infected with Pectobacterium (at 10⁹) or its luxS mutant. There were no significant differences in the competitive fitness of Salmonella, indicating that AI-2 signaling is not a major input in the interactions between these organisms under the tested conditions. A Salmonella lsrG::tnpR-lacZ resolvase in vivo expression technology (RIVET) reporter, constructed to monitor AI-2-related gene expression, responded strongly to the luxS deletion but only weakly to external sources of AI-2. Growth in soft rots generally decreased RIVET resolution; however, the effect was not correlated to the luxS genotype of the Pectobacterium sp. The results of this study show that AI-2 signaling offers no significant benefit to Salmonella spp. in this model of colonization of tomato or soft rots.

Additional keywords: produce safety, quorum sensing.