Antimicrobial action of the American cranberry constituents; phenolics, anthocyanins, and organic acids, against Escherichia coli O157:H7

摘要

We investigated the antimicrobial effect of constituents of the American cranberry (Vaccinium macrocarpon); sugar plus organic acids, phenolics, and anthocyanins, against Escherichia coli O157:H7. Each fractional component was assayed over a 24-h period with 5-log initial inocula to determine the minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC), and log CFU/ml reductions, at their native pH and neutral pH. Each fraction produced significant reductions (P <0.05) at the native pH: MICs for sugars plus organic, phenolics, and anthocyanins were 5.6/2.6 Brix/acid (citric acid equivalents) 2.70g/L (gallic acid equivalent), and 14.80mg/L (cyanidin-3-glucoside equivalent), respectively. Sugars plus organic acids at native pH (3) produced a reduction below detectable limits (<1 log CFU/ml) compared to the control at 24h for 11.3/5.2 and 5.6/2.6 Brix/acid. Phenolics at native pH (4) produced reductions below detectable limits compared to the control at 24h and initial inocula for treatments of 5.40 and 2.70g/L. Anthocyanins at native pH (2) produced reductions below detectable limits for treatments of 29.15 and 14.80mg/L cyanidin-3-glucoside equivalents. Neutralized phenolics and anthocyanins had the same MIC and MBC as those at their native pH. Neutralized sugars plus organic acids did not inhibit bacterial growth compared to the control. Neutralized phenolics reduced bacteria below detectable limits in treatments of 5.40g/L and 2.70g/L compared to the control. Neutralized anthocyanins reduced bacterial growth below detectable limits at the concentration of 29.15mg/L, but at 14.80mg/L there was no significant reduction. Stationary-phase cells of E. coli O157:H7 were treated with 5% of each fraction in 0.8% NaCl for 20min and viewed under transmission electron microscopy. All fractions caused significant damage compared the control. Sugars plus organic acids caused visible osmotic stress, while phenolics and anthocyanins caused disintegration of the outer membrane.