**1. Introduction**

More than 700 bacterial species live in the oral cavity [1, 2]. These bacteria form their own indigenous flora in their habitats, such as teeth, gingival sulcus, and tongue dorsum, making the oral environment considerably complicated. Oral indigenous bacteria coexist with humans and are vital for preventing colonization by foreign pathogenic microorganisms in the oral cavity. Such oral indigenous bacteria proliferate with time, and together with the extracellular polymeric substance (EPS) that they produce, form a biofilm visible even to the naked eye known as dental plaque [3–5].

It has recently been clarified that the formation of biofilms is controlled by quorum-sensing (QS) signals in a communication system between microorganisms that sense each other's abundance [6–8]. It has further been shown that microorganisms constituting a biofilm activate the expression of pathogenic factors when QS signaling molecules, so-called "autoinducers (AI)," act as transcription factors [8].
