Using the disc diffusion assay, we assessed the extent to which the growth of S. mutans, A. actinomycetemcomitans, and dental plaque bacteria (DPB#1, DPB#2, and DPB#3) was inhibited after EETC treatment. As shown in Fig. 1, ampicillin, which has a broad spectrum of anti-bacterial activity against certain gram-positive, gram-negative, and anaerobic bacteria , inhibited the growth of S. mutans but had negligible effect on the growth of DPBs (DPB#1, DPB#2, or DPB#3) and A. actinomycetemcomitans. EETC effectively suppressed the growth of S. mutans, A. actinomycetemcomitans, and DPB, its antibacterial effect on S. mutans was not as pronounced as ampicillin. The degree of inhibition caused by EETC increased with concentration. Growth inhibition zones were measured on bacterial culture plates after treatment with ampicillin or EETC (Table 1). MIC of EETC against the DPB samples ranged from 32 to 64 μg/ml.Fig. 1EETC suppresses bacterial growth. Inhibitory effect of EETC on the growth of Streptococcus mutans (S. mutans), Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans), and different types of dental plaque bacteria (DPB#1, DPB#2, and DPB#3) compared with ampicillin (Amp). The data shown are representative of three independent experiments Table 1Antibacterial effect of EETC against plaque bacteriaCompoundConcentration (μg/disk)Size of clear zone (disk margin, mm) S. mutants A. actinomycetemcomitans 1DPB#1 1DPB#2 1DPB#3Control−3 -----Ampicillin1015.5 ± 0.21b -0.1 ± 0.11-- 2EETC53.8 ± 0.19b 3.7 ± 0.03a 3.2 ± 0.03a 2.7 ± 0.09b 3.9 ± 0.18b 107.2 ± 0.07b 7.5 ± 0.02b 6.9 ± 0.04b 5.3 ± 0.05b 7.4 ± 0.06b 1DPB; dental plaque bacteria, 2EETC; ethanol extracts of Terminalia chebula, 3No inhibition(a P < 0.05, b P < 0.01)