The interaction of F. nucleatum and P. gingivalis appeared to be mediated by an adhesion protein identified as the outer membrane protein FomA on F. nucleatum and a carbohydrate receptor on P. gingivalis [18] and [33], although only a few studies have shown a role for FomA in the pathogenesis of periodontal diseases and halitosis [25]. Our data demonstrate for Nutlin-3 in vivo the first time that F. nucleatum co-opts P. gingivalis via FomA to enhance co-aggregation, biofilm formation, gum inflammation, and VSC production. Co-aggregation

between F. nucleatum and P. gingivalis strains has been previously observed using either a macroscopic visual co-aggregation assay, based on radioactive labeling of bacteria, or using fluorochromes

and confocal microscopy [32]. Although assaying co-aggregation by detecting visible clusters of bacteria is a common method, one main disadvantage of the method is the inability to dynamically quantify the co-aggregation. This method also lacks the capability of verifying the physical interactions among bacteria although bacterial clusters can be observed. On the other hand, the use of Malvern Zetasizer Nano-ZS equipped with DLS provides the ability to detect an increase in particle sizes derived from the physical aggregation of multiple particles [32]. Although F. nucleatum is a spindle-shaped bacterium, a size distribution between 342 and 712 nm is detected by the DLS analysis of Malvern Zetasizer Nano-ZS. Size analysis of the co-aggregation of F. nucleatum and P. gingivalis using Malvern Zetasizer Nano-ZS showed the presence

of larger Decitabine aggregates (712–1281 nm) ( Fig. 1B), verifying the physical interaction between two bacteria. Although we observed larger aggregates in the co-culture of bacteria on nonpyrogenic polystyrene plates ( Fig. 1A), these larger aggregates were undetectable by Malvern Zetasizer Nano-ZS. Possible explanations include that the Malvern Zetasizer Nano-ZS has a limitation that restricts its ability to detect particle sizes greater than 6000 nm. It is also possible that bacteria on the nonpyrogenic polystyrene plates formed larger aggregates Metalloexopeptidase than those bacteria suspended in the bacterial medium during Malvern Zetasizer Nano-ZS analysis. It is worthwhile to note that only few P. gingivalis (103 CFU) are needed to trigger the enhancement of bacterial co-aggregation between F. nucelatum (4 × 108 CFU) and P. gingivalis ( Supplementary Fig. 1). This result is consistent with recent findings that a low dose of P. gingivalis (106 CFU) synergistically enhances the pathogenicity of F. nucleatum (109 CFU) in a murine model using subcutaneously implanted chambers [32] and [34]. Thus, besides the physical interaction among bacteria, bacterial co-aggregation may also be strengthened by quorum sensing mechanisms [35].