Phytochemicals in Lowbush Wild Blueberry Inactivate Escherichia coli O157:H7 by Damaging Its Cell Membrane

The antimicrobial activity and model of action of polyphenolic compounds extracted from lowbush wild blueberries (LWB) were studied against Escherichia coli O157:H7. Polyphenols in LWB were extracted using 80% vol/vol methanol and designated as total blueberry phenolics (TBP). The fraction was further separated by a C-18 Sep-Pak cartridge into monomeric phenolics acids (MPA) and anthocyanins plus proanthocyanidins (A&P). The A&P fraction was further separated into anthocyanins and proanthocyanidins using a LH-20 Sephadex column. Each fraction was diluted in 0.85% wt/vol NaCl, inoculated with E. coli O157:H7 to achieve 8 log colony-forming units (CFU)/mL, and incubated at 25 degrees C for 1h. The survival populations of E. coli O157:H7 in the phenolic fractions were determined by a viable cell counts method. The permeability of the cell membrane of E. coli O157:H7 was determined using LIVE/DEAD viability assay, and the damage was visualized by using transmission electron microscopy (TEM). Significant (p<0.05) reductions of 5 log CFU/mL of E. coli O157:H7 were observed for MPA at 0.4g/L gallic acid equivalents (GAE), A&P at 0.9g/L GAE, and anthocyanins at 0.65g/L GAE. Reductions of 6-7CFU/mL were observed for MPA at 0.8g/L GAE, A&P at 1.8g/L GAE, and anthocyanins at 1.3g/L GAE compared to the control. The cell membrane of E. coli O157:H7 exhibited a significantly increased permeability when treated with proanthocyanidins (0.15g/L GAE), A&P (0.45g/L GAE), anthocyanins (0.65g/L GAE), and TBP (0.14g/L GAE). TEM confirmed the inactivation and increased membrane permeability of E. coli O157:H7. This study demonstrated the antimicrobial effect of polyphenols from LWB against E. coli O157:H7 and the probable mode of action.