Exploration of the mechanisms of biofilm formation by Enterococcus faecalis in glucose starvation environments

Background: Enterococcus faecalis is the most common single species present in teeth after failed root canal therapy. This is mainly due to its ability to maintain viability for a long time in filled root canals where nutrients are normally sparse. The aim of this study was to explore the mechanism of E. faecalis survival and biofilm formation in glucose-starved environments.

Methods: Enterococcus faecalis ATCC 33186 was inoculated in energy starvation media for biofilm formation. Confocal laser scanning microscopy and fluorescent DNA-binding agents were employed to assess biofilm-forming ability. The physiochemical properties of the biofilm cell wall were investigated by measuring the hydrophobicity, extracellular polysaccharide and ATPase activity. The expression of stress and virulence genes was quantified by real-time quantitative polymerase chain reaction.

Results: The ability for biofilm formation decreased with a decreasing concentration of glucose. The cell surface hydrophobicity increased dramatically with decreasing glucose concentration. Water-soluble exopolysaccharide (WSE) synthesis decreased in glucose starvation media, whereas water-insoluble exopolysaccharide (WIE) synthesis increased. A marked decrease in ATPase activity was observed only in a no glucose medium. In addition, transcription of ace, fsrB and gelE genes increased under glucose starvation stress while atpE, salB and esp genes were down-regulated.

Conclusions: Enterococcus faecalis survival and biofilm formation under glucose starvation stress may be attributed to an increase in cell-surface hydrophobicity and additionally to the up-regulation of some genes involved in stress response and biofilm formation. These characteristics may explain why E. faecalis can maintain viability for a long time in an energy-starved environment and why it is frequently isolated from persistently infected root canals.