Gram-negative quorum sensing signalling enhances biofilm formation and virulence traits in gram-positive pathogen <i>Enterococcus faecalis</i>

Abstract

Acyl-homoserine lactones (AHLs) are typical quorum-sensing molecules of gram-negative bacteria. Recent evidence suggests that AHLs may also affect gram-positives, although knowledge of these interactions remains scarce. Here, we assessed the effect of AHLs on biofilm formation and transcriptional regulations in the gram-positive <i>Enterococcus faecalis</i>. Five <i>E. faecalis</i> strains were investigated herein. Crystal violet was employed to quantify the biomass formed, and confocal microscopy in combination with SYTO9/PI allowed the visualisation of biofilms' structure. The differential expression of 10 genes involved in quorum-sensing, biofilm formation and stress responses was evaluated using reverse-transcription-qPCR. The AHL exposure significantly increased biofilm production in strain ATCC 29212 and two isolates from infected dental roots, UmID4 and UmID5. In strains ATCC 29212 and UmID7, AHLs up-regulated the quorum-sensing genes (<i>fsrC</i>, <i>cylA</i>), the adhesins <i>ace</i>, <i>efaA</i> and <i>asa1</i>, together with the glycosyltransferase <i>epaQ</i>. In strain UmID7, AHL exposure additionally up-regulated two membrane-stress response genes (σ^V, <i>groEL</i>) associated with increased stress-tolerance and virulence. Altogether, our results demonstrate that AHLs promote biofilm formation and up-regulate a transcriptional network involved in virulence and stress tolerance in several <i>E. faecalis</i> strains. These data provide yet-unreported insights into <i>E. faecalis</i> biofilm responses to AHLs, a family of molecules long-considered the monopole of gram-negative signalling.