<![CDATA[Background: Antimicrobial resistance among Gram-negative bacteria, including Escherichia coli, has increased the need to explore alternative antibacterial agents. Although medicinal plants have demonstrated antibacterial potential, comparative evidence on the activity of Ziziphus spina-christi and Piper betle extracts against E. coli across different concentrations remains limited. Objective: This study aimed to compare the antibacterial activity of ethanolic extracts of Z. spina-christi and P. betle against E. coli and to evaluate the effects of extract type, concentration, and their interaction. Method: A laboratory experimental study using a 2×3 factorial completely randomized design with additional positive and negative control groups was conducted. Ethanolic extracts of Z. spina-christi and P. betle were evaluated at concentrations of 1500, 2000, and 2500 ppm using the disk diffusion method. Inhibition-zone diameters were analyzed using two-way analysis of variance followed by Tukey’s honestly significant difference test. Results: The mean inhibition-zone diameter of Z. spina-christi increased from 8.83±0.76 mm at 1500 ppm to 30.33±2.08 mm at 2500 ppm, whereas that of P. betle increased from 16.33±0.38 mm to 23.08±3.51 mm. Concentration significantly affected antibacterial activity (F(2,12)=105.02; p<0.001), whereas extract type showed no significant main effect (F(1,12)=1.89; p=0.194). A significant interaction between extract type and concentration was observed (F(2,12)=29.34; p<0.001). Conclusion: The antibacterial activity of Z. spina-christi and P. betle extracts against E. coli varied according to concentration, with a significant interaction between extract type and concentration. The greatest inhibition among the extract treatments was observed with Z. spina-christi at 2500 ppm.]]>
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