Aims: To assess the possibility that bifidobacteria compete with Porphyromonas gingivalis for their mutual growth factor vitamin K. This study also examined whether salivary Bifidobacterium species decrease vitamin K concentration in the growth medium. Methods and Results: Sixty-five strains of Bifidobacterium were obtained from 20 of 24 periodontally healthy subjects. Bifidobacterium dentium was most frequently detected in the saliva of subjects, followed by Bifidobacterium adolescentis, Bifidobacterium longum, and Bifidobacterium urinalis. The growth of most Bifidobacterium isolates, except that of B. urinalis, was stimulated by vitamin K. Moreover, the isolates were capable of decreasing vitamin K after incubation, which suggests that bifidobacteria compete with P. gingivalis for vitamin K. In a co-culture, a representative strain -B. adolescentis S2-1 -inhibited the growth of P. gingivalis if it was inoculated in the medium before P. gingivalis. Conclusions: B. adolescentis S2-1 decreased vitamin K concentration and inhibited the growth of P. gingivalis by possibly competing for the growth factor. Significance and Impact of the Study: Salivary bifidobacteria may possess the potential to suppress the growth of P. gingivalis by reducing the growth factor(s) in the environment.