Assessment of the Roles of LuxS, S -Ribosyl Homocysteine, and Autoinducer 2 in Cell Attachment during Biofilm Formation by Listeria monocytogenes EGD-e

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Abstract
LuxS is responsible for the production of autoinducer 2 (AI-2), which is involved in the quorum-sensing response of Vibrio harveyi . AI-2 is found in several other gram-negative and gram-positive bacteria and is therefore considered a good candidate for an interspecies communication signal molecule. In order to determine if this system is functional in the gastrointestinal pathogen Listeria monocytogenes EGD-e, an AI-2 bioassay was performed with culture supernatants. The results indicated that this bacterium produces AI-2 like molecules. A potential ortholog of V. harveyi luxS , lmo1288, was found by performing sequence similarity searches and complementation experiments with Escherichia coli DH5α, a luxS null strain. lmo1288 was found to be a functional luxS ortholog involved in AI-2 synthesis. Indeed, interruption of lmo1288 resulted in loss of the AI-2 signal. Although no significant differences were observed between Lux1 and EGD-e with regard to planktonic growth (at 10°C, 15°C, 25°C, and 42°C), swimming motility, and phospholipase and hemolytic activity, biofilm culture experiments showed that under batch conditions between 25% and 58% more Lux1 cells than EGD-e cells were attached to the surface depending on the incubation time. During biofilm growth in continuous conditions after 48 h of culture, Lux1 biofilms were 17 times denser than EGD-e biofilms. Finally, our results showed that Lux1 accumulates more S -adenosyl homocysteine (SAH) and S -ribosyl homocysteine (SRH) in culture supernatant than the parental strain accumulates and that SRH, but not SAH or AI-2, is able to modify the number of attached cells.