Fusobacterium nucleatum Increases Growth of Motile Bacterium Treponema denticola without Contact In Vitro

More than 700 types of bacteria coexist in the human oral cavity, including motile and non-motile taxa. Fusobacterium nucleatum plays an important role in this environment as a bridge between early- and late-colonizing bacteria. We hypothesized that Treponema denticola, a motile bacterium, receives a non-contact benefit from coexisting F. nucleatum. To evaluate this hypothesis, the growth and gene expression of T. denticola were investigated in an environment where F. nucleatum was present but not in contact with it. The results showed that proliferation of T. denticola increased in this environment, and that this was attenuated by autoinducer-2 (AI-2) inhibitor _D-ribose. The results of RNA-seq and qRT-PCR showed that the presence of F. nucleatum increased the gene expression levels of TDE_0039 and TDE_0040. These findings revealed that proliferation of T. denticola increased via a substance produced by F. nucleatum. Importantly, this phenomenon occurs even without direct contact between the two species. In other words, T. denticola may gain a survival advantage by moving through the oral environment, and F. nucleatum may play a crucial role in facilitating this.