Studies on the Streptococcus mutans sortase A inhibitors from medicinal plants

Abstract

Sortase A (SrtA) is a membrane-bound cysteine transpeptidase that is responsible for catalyzing the covalent anchoring of surface proteins to the Gram-positive bacterial cell wall. Streptococcus mutans is known to be the primary pathogen underlying dental caries and utilizes SrtA to anchor surface proteins to the cell wall peptidoglycan and forms a biofilm to facilitate its adhesion to the tooth surface. Studies have confirmed the important role of S. mutans SrtA in bacterial pathogenesis and its potential as a suitable target for pharmacological inhibition. In this study, the coding region of the srtA gene (620 bp) from S. mutans OMZ65 isolated from the human oral cavity was cloned, expressed in Escherichia coli, and purified by metal chelate affinity chromatography. To identify potential inhibitors of S. mutans SrtA, 100 commercially available medicinal plants were used for screening, and five specimens that had the highest scores were chosen for further molecular analysis. Among them, SrtA assay-guided separation of the crude extract of Sophora japonica flowers using various chromatographic techniques yielded a new maltol derivative along with six known maltol derivatives and flavonoids. Based on the results of combined spectroscopic methods, the structure of new compound was determined to be maltol-3-O-(4-O-cis-p-coumaroyl-6-O-(3-hydroxy-3-methylglutaroyl))-β-glucopyranoside. This compound exhibited potent inhibitory activity against SrtA, with an IC50 value of 34 µg/mL, and saliva-induced cell aggregation. In addition, several coumarin compounds including triphasiol were isolated from Poncirus trifoliata and evaluated for their bioactivities. Triphasiol showed strong inhibitory activity against S. mutans SrtA, with an IC50 value of 10 μg/mL. This compound also exhibited significant inhibition of S. mutans adherence to S-HAs (saliva-coated hydroxyapatite beads) and biofilm formation to the surface of resin teeth in a dose-dependent manner. These chemical and biological data revealed the potential of these compounds for the treatment of S. mutans infections via inhibition of SrtA activity and prevention of dental plaque.