For in vivo neutralization, F. nucleatum (4 × 108 CFU) was neutralized with anti-FomA or anti-GFP serum, co-incubated with P. gingivalis (1 × 103 CFU) for 3 h, and then resuspended in an aliquot of 100 μl PBS. After neutralization, co-aggregated bacteria were inoculated into mice to induce gum swelling as described above. The experiments were performed in triplicate at four mice per group. Data are presented as mean ± SE. Student t-test was used to assess the significance of independent experiments. The criterion (*p < 0.05, **p < 0.005, ***p < 0.0005) was used to determine statistical significance. As shown in Supplementary Fig. 1, biofilm enhancement by F. nucleatum

reached the maximal level when F. nucleatum learn more (4 × 108 CFU) was co-cultured with P. gingivalis (103 CFU). Light microscopy and the Zetasizer Nano-ZS were employed to examine the bacterial association. The spindle-shaped F. nucleatum [6] and rod-shaped P. gingivalis [26] were clearly observed using light microscopy ( Fig. 1A). Many bacterial aggregates were found when F. nucleatum was co-cultured with P. gingivalis for 3 h on a nonpyrogenic polystyrene plate, indicating bacterial co-aggregation occurred. HDAC inhibitor To validate that inter-species co-aggregation is mediated by a physical interaction between two bacteria, the Zetasizer Nano-ZS

with dynamic light scattering was utilized to detect the changes in the sizes of bacterial particles or aggregates. F. nucleatum (4 × 108 CFU) alone, P. gingivalis (103 CFU) alone,

or F. nucleatum plus P. gingivalis (4 × 108 CFU/103 CFU) were resuspended in TSB medium for 3 h. The particle sizes of F. nucleatum and P. gingivalis ranged from 342 to 712 nm and 220 to 615 nm, respectively, as detected by the Zetasizer Nano-ZS ( Fig. 1B), are consistent with previous observations using electron microscopy (EM) [18] and [27]. Larger particles ranging from 712 to 1281 nm were detected when F. nucleatum was mixed with P. gingivalis, supporting the hypothesis that F. nucleatum physically interacts with P. gingivalis to form aggregates. Bacterial co-aggregation is an early event of biofilm formation [28]. To investigate if upstream co-aggregation to of F. nucleatum with P. gingivalis can further boost the development of biofilms, F. nucleatum alone, P. gingivalis alone, and F. nucleatum plus P. gingivalis at a ratio of 4 × 105:1 CFU were cultured on nonpyrogenic polystyrene plates for 36 h. Biofilms formed on the plates were stained with 0.4% (v/v) crystal violet. Biofilm formation by F. nucleatum was tremendously enhanced by the presence of P. gingivalis ( Fig. 1C), in agreement with the previous finding that P. gingivalis enhances biofilm formation by F. nucleatum [29]. Notably, the results above support the concept that P. gingivalis co-aggregates with F. nucleatum which leads to an increase in biofilm growth.