Regulation of Protein Expression in Alkaline-Induced Fusobacterium nucleatum Biofilms

Background: Biofilm formation, often associated with altered physiology and increased virulence, is proposed to be a survival strategy for bacteria exposed to adverse conditions. Fusobacterium nucleatum, a putative periodontopathogen, formed biofilms and exhibited altered morphology when exposed to growth pH at 8.2 in continuous culture. This pH has been recorded in dental plaque in individuals with periodontal disease. Objective: The aim of this study was to investigate physiological changes and adaptive mechanisms in F. nucleatum associated with these alkaline induced biofilms. Methods: The protein expression of planktonic (pH 7.4) and biofilm (pH 8.2) cells was assessed using a proteomic approach and validated using alternative methods. Results: Fifty five proteins were differentially expressed (p<0.05) in F. nucleatum biofilm cells and these proteins were classified according to functional class, including metabolic, transport and stress proteins. At pH 8.2, F. nucleatum significantly regulated protein expression in an attempt to increase available energy and to maintain intracellular pH. Biofilm cells appeared to be more metabolically efficient as pathways involved in amino acid and glucose metabolism were altered to generate additional energy required to maintain cellular functions. Shifts in metabolic patterns were associated with the changed expression in solute-transporters in order to maintain proton gradients and intracellular pH. The change to biofilm formation may be attributed to an increased expression of FomA, an adhesin that is known for its ability to bind a vast number of bacterial species and human epithelial cells. Conclusion: This investigation successfully identified F. nucleatum proteins that were regulated in response to alkaline conditions, similar to those that are thought to exist at sites in diseased periodontal pockets. These results provide insight into the pH adaptation mechanisms this bacterium employs in response to changes in its natural habitat.