Enterococcus faecalis V583 LuxS/AI-2 system is devoid of role in intra-species quorum-sensing but contributes to virulence in a Drosophila host model

The AI-2 interspecies quorum-sensing molecule is produced by the LuxS enzyme and has been ascribed a role in virulence in several bacteria. The nosocomial pathogen Enterococcus faecalis inhabits several different environments where multispecies communities are established. However, despite the presence of a luxS gene in this pathogen, its role in E. faecalis pathogenesis has never been assessed. In the present work, we deleted the luxS gene from the vancomycin-resistant clinical isolate E. faecalis V583 and demonstrated the lack of AI-2 production by the mutant strain. Using microarrays and externally added (S)-4,5-dihydroxy-2,3-pentanedione we showed that AI-2 is not sensed by E. faecalis as a canonical quorum-sensing molecule and that the luxS mutation caused pleiotropic effects in gene expression, which could not be complemented by extracellularly added AI-2. These global differences in gene expression affected several gene functional roles, mainly those enrolled in metabolism and transport. Metabolic phenotyping of the luxS mutant, using Biolog plates, showed differences in utilization of galactose. AI-2 production by LuxS was shown to be irrelevant for some phenotypes related to the pathogenic potential of E. faecalis namely biofilm formation, adhesion to Caco-2 cells, resistance to oxidative stress and survival inside J-774 macrophages. However, the luxS mutant was attenuated when tested in the Drosophila septic injury model, as its deletion led to delayed fly death. Overall our findings show that differential gene expression related to the luxS mutation cannot be ascribed to quorum-sensing. Moreover, the role of LuxS appears to be limited to metabolism.