Fusobacterium nucleatum is a Gram-negative anaerobic organism considered to play an important role in the progression of periodontal disease and is commonly found in clinical infections of other body sites. Apart from its metabolic versatility, its cell-surface properties enable it to attach to epithelial cells, collagen, gingival epithelial cells and other bacterial genera, but not with other Fusobacteria. The development of periodontitis is associated with a rise in pH in the gingival sulcus to around 8.5, and this is thought to occur by the catabolism of proteins supplied by gingival crevicular fluid. F. nucleatum is commonly isolated from diseased sites and has also been shown to survive in root canal systems at pH 9.0 after Ca(OH)(2) treatment. In order to survive hostile environmental conditions, such as nutrient deprivation and fluctuating temperature and pH, bacteria form biofilms, which are usually made up of multi-species co-aggregates. We have grown F. nucleatum in a chemostat at a growth rate consistent with that of oral bacteria in vivo and report that, at a growth pH of 8.2, F. nucleatum co-adheres and forms a homogeneous biofilm. Cell-surface hydrophobicity was determined in planktonic and co-adhering cells to characterise the interfacial interactions associated with the response to pH. Cell-surface hydrophobicity was found to increase at pH 8.2 and this was also associated with a decrease in the levels of intracellular polyglucose (IP) and an observed change in the bacterial cell morphology. To our knowledge, these results represent the first study in which F. nucleatum has been shown to co-adhere and form a biofilm, which may be important in the organism's persistence during the transition from health to disease in vivo.