Quorum sensing and butanediol fermentation affect colonization and spoilage of carrot slices by Serratia plymuthica.

In this work we investigated the role of quorum sensing and specific quorum-sensing dependent properties in the colonization and spoilage of carrot slices by Serratia plymuthica RVH1, a strain isolated previously from a vegetable washing and cutting machine in an industrial kitchen. Disinfected carrot slices were inoculated by immersion in a bacterial suspension and then placed in a Petri dish with a shallow layer of the same bacterial suspension. Subsequently, visible spoilage of the air-exposed upper side of the slices and the evolution of bacterial numbers and pH of the surrounding suspension were recorded during 19 days. A knockout mutant in the N-acyl-homoserine lactone (AHL) synthase splI was clearly compromised in its ability to colonize the surface of the carrot and cause browning, and the addition of synthetic AHL could restore this phenotype. To examine in more detail which properties contribute to this phenomenon, we isolated mutants deficient in the production of extracellular proteases and in butanediol fermentation, both of which are regulated by quorum sensing in S. plymuthica RVH1. The protease-deficient mutant (lipB) was not affected in the carrot slice spoilage assay. Since RVH1 does not produce pectinolytic enzymes, this suggests that hydrolytic enzymes do not play a major role in produce spoilage by this organism. On the other hand, a budB mutant with inactive butanediol fermentation pathway showed strongly enhanced growth on the carrot slices, in spite of a reduced survival in the surrounding medium. To explain these results, we hypothesize that a response is induced in the carrot slices that suppresses bacterial colonization and outgrowth, similar to the defense response induced by volatile butanediol pathway products in intact plants.