| Summary: | <i>Listeria monocytogenes</i> and <i>Escherichia coli</i> O157:H7 are common causes of foodborne illness worldwide. Ohelo berry (<i>Vaccinium calycinum</i>) juice was found to possess inhibitory activity against <i>L. monocytogenes</i>. This study aimed to determine which constituents of ohelo berry have the most potent antimicrobial effects. The crude extract of ohelo berry was separated into sugar plus organic acids (F1), non-anthocyanin phenolics (F2), and anthocyanins (F3). The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the fractions were determined against <i>L. monocytogenes</i> and <i>E. coli</i> O157:H7. The results demonstrated that F3 contained the highest concentrations of total phenolics and anthocyanins. All fractions caused a significant growth reduction in two bacteria compared to controls. F1 at native pH had the same MIC (1.39/0.36 Bx/acid) and MBC (5.55/1.06 Bx/acid) against the two bacteria, while neutralized F1 did not inhibit the growth of either pathogen. The MIC of F3 against <i>L. monocytogenes</i> was 13.69 mg/L cyanidin-3-glucoside equivalent, which was not affected by neutralization. Besides, <i>L. monocytogenes</i> was more sensitive than <i>E. coli</i> O157:H7 to all fractions. These findings suggest that both phenolics and organic acids contribute to the antimicrobial properties of ohelo berry, which have the potential to be used as natural food preservatives.
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