Quillaja saponaria Extract to Control the Spread of Escherichia coli O157:H7 and the Emerging Non-O157 Shiga Toxin-producing E. coli

Introduction: Shiga Toxin-producing Escherichia coli (STEC) O157:H7 and the emerging big six STECs are important food safety concerns. Natural plant-based alternatives for control are gaining popularity. Aqueous Quillaja saponaria bark extract (QE) has U.S. Food and Drug Administration approval as a food additive. QE contains bioactive polyphenols, tannins and saponins, with anti-inflammatory and antimicrobial activity.

Purpose: The objective of this study was to determine the effects of QE against E. coli O157:H7 and non-O157 STECs over 16h at 37^oC and RT.

Methods: Overnight cultures of 5 strains of E. coli O157:H7 and 6 non-O157 STECs were grown in Tryptic Soy Broth, washed, resuspended in phosphate-buffered saline (PBS, pH 7.2), and treated with QE, citric acid (pH 3.75), sodium benzoate (0.1% w/w) or PBS (control) for 6 or 16h at RT and 37°C. Treatments were serially diluted and plated on Tryptic Soy Agar and enumerated after 24h. Data from duplicate treatments replicated thrice were statistically analyzed (ANOVA). Scanning electron microscopy (SEM) was used to determine bacterial structural changes.

Results:  Reductions ranging from 0.71 ± 0.09 to 2.95 ± 0.31 log colony forming units (CFU) were obtained for four E. coli O157:H7 strains treated with QE after 6 and 16h at RT. However, O157 strain ATCC 43894 showed at least 4-log reduction after both 6 and 16h, with ~1-log increase at 37°C. For 5 non-O157 STECs, 0.85 ± 0.12 to 1.75 ± 0.02 log reductions were obtained after 6 and 16h, with 1.1 ± 0.33 and 2.95 ± 0.32 log reduction for O145 after 6 and 16h with QE at RT, respectively, and higher reductions at 37°C.  Citric acid and sodium benzoate controls showed no reduction. Overall, reductions were strain-dependent. SEM analysis showed structural deformations of treated bacteria.

Significance: QE shows antibacterial effects for potential application in food systems. Further research involves determination of the mechanism of action and evaluation in food systems.