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Introduction
Streptococcus mutans (S. mutans) is the most well recognized causative agent implicated in the pathogenesis of dental caries (tooth decay), which is an infectious disease of human dentition still exhibiting high global prevalence (1–3). Virulence of this bacterium arises from its ability to form a biofilm on teeth and produce organic acids (acidogenicity) from dietary sucrose within the biofilm, causing tooth decay (4,5). The structural matrix of the biofilm consists of water-insoluble glucans synthesized from sucrose by several isoforms of the glucosyltransferase (Gtf) enzyme present in S. mutans bacteria (5). In this respect, S. mutans produces water-insoluble and partly water-soluble glucans using GtfB and GtfC enzymes encoded by gtfB and gtfC genes, respectively. The synthesized insoluble glucans possess a capacity to concentrate protons generated by the proton-extruding F-type ATPase (F-ATPase), thereby retaining acidogenicity of S. mutans biofilm (6). In addition, it has been reported that biofilm bacteria have up to a 1,000-fold more tolerance to antimicrobials than planktonic bacteria (7,8). Therefore, it is important to develop novel pharmaceuticals in order to inhibit S. mutans biofilm formation and its acidogenicity.
Natural products have been optimized to interact with biological systems through a long natural selection process (9), and because of this, nature is considered the best concocter of medicines and has been a source of medicines for millennia (10–13). During the last two decades, various plants have been tested for their antimicrobial activity and many of them exhibit significant antibacterial activity against Streptococcus species (14). In this context, Rhus coriaria L. (sumac) fruits contain many of the bioactive compounds that were characterized in detail by using high-performance liquid chromatography-diode array detector-hyphenated with tandem mass spectrometry (HPLC-DAD-ESI-MS/MS) in the investigation performed by Abu-Reidah et al (15). Among these bioactive components, methyl gallate (MG) has antibacterial and anti-biofilm effects for S. mutans bacteria, as reported by Kang et al (16). However, currently available studies did not provide sufficient data whether MG can suppress the development of S. mutans biofilm on polystyrene and glass surfaces as well as inhibit acidogenicity. In the scientific report of Kang et al (16), colorimetric assay was applied for testing the effect of MG on S. mutans adherence. Colorimetric assay is considered a well-established method for quantification of the...