Uses and limitations of green fluorescent protein as a viability marker in Enterococcus faecalis: An observational investigation

Enterococci are capable of producing biofilms that are notoriously difficult to treat and remove, for instance in root canal infections. The tenacious nature of these organisms makes screening of known and novel antimicrobial compounds necessary. While traditionally growth and fluorescence-based screening methods have proven useful, these methods have their limitations when applied to enterococci (e.g. time consuming, no kinetic data, diffusion properties of the fluorescent dyes). The aim of this study was to develop and validate a GFP-based high-throughput screening system to assess the bactericidal activity of a broad range of antimicrobial agents on Enterococcus faecalis and its biofilms. The effect of antimicrobial compounds on cell viability and GFP fluorescence of enterococcal planktonic and biofilm cells was determined using colony forming unit counts, fluorescence spectrophotometry and real-time imaging devices. There was a linear correlation between cell viability and GFP fluorescence. The intensity of the GFP signal was effected by the extracellular pH. For a range of antimicrobials however, there was no correlation between these two parameters. In contrast, for oxidizing agents such as sodium hypochlorite, the antimicrobial of choice for root canal disinfection, there was a correlation between loss of fluorescence and loss of viability. To conclude, the use of a GFP-based system to monitor the antimicrobial activity of compounds on E. faecalis is possible despite significant limitations. This approach is useful for analysis of susceptibility to oxidizing agents. Using real-time measuring devices to follow GFP fluorescence it should be possible to investigate the mode of action and rate of diffusion of oxidizing agents in E. faecalis biofilm.