Antimicrobial Efficacy of Fulvic Acid Formulations against Escherichia coli O157:H7 on Bagged Organic Leafy Greens at Refrigeration Temperatures

Introduction: Escherichia coli O157:H7 is a public health problem worldwide, with leafy greens as a major source of recent outbreaks. Standard organic-produce-industry washes have been ineffective in reducing this problem. Need for effective antimicrobials for organic produce is therefore clearly evident. Fulvic acid is an organic acid reported to have antibacterial and antifungal properties. Previous investigations have shown its effectiveness in vitro against a range of bacteria: E. coli, Staphylococcus aureus, Streptococcus faecalis, Klebsiella pneumoniae, and Candida albicans. To our knowledge, no studies have been conducted to determine its efficacy against E. coli O157:H7 in organic leafy greens.

Purpose: Determine the antimicrobial efficacy of fulvic acid against E. coli O157:H7 in organic leafy greens at refrigerated temperatures.

Methods: Organic baby and adult spinach, and romaine and iceberg lettuce were inoculated with a cocktail of three E. coli O157:H7 strains (10⁶ CFU/ml). Fulvic acid I, II, III and IV at different concentrations (1, 2 and 3%) were prepared in sterile deionized water. The inoculated greens were washed in each treatment solution for 2 minutes, bagged and refrigerated at 4°C. Hydrogen peroxide and water were used as the controls. Surviving bacterial populations were enumerated on days 0, 1, and 3 of storage.

Results: Significant log reductions (P < 0.01) were observed in pathogen populations on leafy greens treated with all fulvic acid concentrations. Fulvic acid III was most effective, with a 3-log reduction for each leafy green by day 3. Fulvic acid II (3%) and III (1%) showed a 2.8 and 2.5-log CFU/g reduction, respectively, in romaine lettuce and 1.6 to 2.0-log reduction for the rest of the leafy greens. Fulvic acid I and IV resulted in 1.4 to 1.8-log CFU/g reductions, respectively.

Significance: Fulvic acid is an effective natural antimicrobial and can be used as a wash treatment for organic leafy greens stored at refrigerated temperatures.