<![CDATA[Background: Escherichia coli is an important indicator of food contamination and a foodborne pathogen capable of causing serious infections. Naturally produced antimicrobial metabolites in yoghurt, including bacteriocins, have attracted increasing attention as potential alternatives to synthetic food preservatives. This study aimed to evaluate the antibacterial activity of yoghurt against E. coli ATCC 25922 at different concentrations. Methods: A true experimental study with a posttest-only control group design was conducted. Antibacterial activity was evaluated using the agar well diffusion method against E. coli ATCC 25922. Yoghurt was tested at concentrations ranging from 5% to 50% with 5% intervals. Data were analyzed using the Shapiro–Wilk normality test, followed by the Kruskal–Wallis test and Mann–Whitney post hoc analysis. Results: Chemical analysis estimated bacteriocin, lactic acid, and hydrogen peroxide levels in yoghurt at 148.00 mg/L, 35.12 mg/L, and 11.75 mg/L, respectively. No antibacterial activity was observed at yoghurt concentrations below 25%. Inhibition zones first appeared at a concentration of 25% and increased progressively with increasing yoghurt concentration, reaching the largest inhibition zone at 50%. Significant differences in inhibition zone diameters were observed among treatment groups (p < 0.05), indicating a concentration-dependent antibacterial effect. Conclusions: Yoghurt exhibited concentration-dependent antibacterial activity against E. coli ATCC 25922, with 25% representing the minimum concentration capable of inhibiting bacterial growth. The observed antibacterial activity is likely attributable to the synergistic effects of bacteriocins, lactic acid, and hydrogen peroxide produced during yoghurt fermentation. These findings support the potential application of yoghurt-derived antimicrobial metabolites as natural biopreservatives for improving food safety.]]>
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