Enterococcus faecalis : biofilms, quorum sensing and a novel antibiofilm compound

Abstract

The formation of sessile communities (biofilms) that are highly tolerant to antimicrobials results in recalcitrance and consequently, the high public health care burden of bacterial infections. Enterococcus faecalis as a nosocomial pathogen is implicated in the pathogenesis of endocarditis, urinary tract, surgical wound and persistent dental root canal infections. There is a constant quest to discover new antibiofilm agents that can suppress the virulence traits of this bacterium. What has been overlooked is the influence of various environment-related and bacterium-related factors, which influence its biofilm development. This knowledge is instrumental in the discovery of compounds that can intricately manipulate biofilm formation mechanisms in E. faecalis. This speaks to the unappreciated level of understanding biofilms prior to drug development. Motivated by this grand challenge, we first interrogated if conditioning the substrate with different organic molecules and body fluids relevant to the dental root canal environment influences the composition of E. faecalis biofilms. For the first time, we discovered that substrate conditioning has a definitive influence on bacterial viability and modulates the exopolysaccharide and protein fractions of E. faecalis biofilms in a temporal manner. Bacteria communicate with each other by quorum sensing (QS). The Fsr-QS system of E. faecalis is a specifically important communication system that coordinates its virulence traits such as biofilm formation and production of extracellular proteases. We asked if inactivation of the Fsr system can cause changes in E. faecalis biofilm composition. Significantly, Fsr inactivation diminished biofilm development by reducing the quantity of biofilm matrix components, without adversely affecting biofilm viability. Finally, to identify safe therapeutics to thwart E. faecalis biofilms, we chose the plant kingdom as a potential source of novel antibiofilm compounds. Our systematically conducted literature review revealed that several phytocompounds remain unexplored against E. faecalis. We selected trans-cinnamaldehyde (TC) as our compound of choice and discovered that TC inhibited biofilm formation and reduced exopolysaccharides production by E. faecalis. Notably, it reduced the production of extracellular proteases and hemolytic activities of E. faecalis, demonstrating potential to reduce its pathogenicity. This pioneering study also discovered that TC downregulated the expression of the genes of the Fsr system. To further strengthen the translational potential of this compound, we asked if TC could eradicate established biofilms of E. faecalis, in comparison with compounds that are commonly used in root canal treatment as irrigants and intracanal medicaments. This study revealed that TC was as effective as sodium hypochlorite and chlorhexidine (CHX) in diminishing biofilm viability after just 15 min treatment. The most notable finding from this work was that TC, but not CHX prevented the recovery of E. faecalis up to 10 days after treatment, indicating potential tremendous benefits in root canal disinfection. Taken together, our series of cogent studies unravelled 1) the role of the environment and the Fsr quorum sensing system in biofilm development in E. faecalis. 2) that trans-cinnamaldehyde thwarts E. faecalis biofilms by inhibiting the Fsr quorum sensing system, kills bacteria in established biofilms and prevents biofilm recovery.