['The aim of this study was to investigate the antibacterial effect of Coptidis Rhizoma (CR), a traditional medicinal plant, on oral bacteria.', 'CR extract was prepared by boiling CR in water for 2 h. Alkaloids contained in CR extract were assayed by high performance liquid chromatography (HPLC). Antibacterial activity of CR extract was estimated from the lowest concentration that did not permit bacterial growth (minimum inhibitory concentration, MIC) and the concentrations that inhibited 50% of bacterial proteolytic activity (IC50).', 'CR extract inhibited the growth of Actinomyces naeslundii, Porphyromonas gingivalis, Prevotella intermedia, Prevotella nigrescens and Actinobacillus actinomycetemcomitans at MIC of 0.031-0.25 mg ml(-1), whereas it had less inhibitory effect (MIC: 0.5-2 mg ml(-1)) on the growth of Streptococcus and Lactobacillus. The major active component of CR extract was berberine (Ber), an alkaloid, and its inhibiting specificity to bacterial growth was similar to that of CR extract. CR extract and Ber were bacteriostatic at the MICs against most of the bacteria, and bacteriocidal at the concentrations higher than the MICs. Ber inhibited the activities of collagenase from P. gingivalis and A. actinomycetemcomitans.'] Does coptidis rhizoma inhibit growth and proteases of oral bacteria?