Antibacterial Effects of Olive, Apple and Grapeseed Extracts against Escherichia coli O157:H7 on Organic Leafy Greens Stored at 4°C

Introduction: There has been an increase in foodborne outbreaks due to contaminated fresh produce in the past two decades. Escherichia coli O157:H7 has been implicated in many such outbreaks. Due to growing consumer preference for natural products and to limited availability of approved sanitizers for organic produce, there is a need to test the effectiveness of plant-derived extracts against pathogens on organic leafy greens.

Purpose: Various plant extract formulations were evaluated on four types of organic leafy greens contaminated with E. coli O157:H7.

Methods: Organic baby and adult spinach, and romaine and iceberg lettuce were washed in sterile water and inoculated with a cocktail of E. coli O157:H7 strains (10⁶ CFU/ml). Apple, olive, and grapeseed formulations were prepared in phosphate buffered saline (PBS) at 1, 3, and 5% concentrations. The inoculated greens were washed in each treatment solution, bagged and refrigerated at 4ºC. Surviving bacteria were enumerated on days 0, 1, and 3 of storage.

Results: Of all the extracts tested, olive extract was the most effective, resulting in a 4-5 log reduction for each leafy green by day 3. Olive extract (5%) showed complete bacterial inhibition in romaine lettuce by day 3. For iceberg lettuce, no E. coli O157:H7 was recovered on day 1 with 3 and 5% olive extract treatment. No E. coli O157:H7 was recovered on day 3 from adult spinach treated with 3 and 5% olive extract. By day 3, apple extract showed reductions of 3.4 and 3.6-logs in romaine and iceberg lettuce respectively, and 1.4- to 2.2-logs for the rest of the greens whereas grapeseed extract resulted in 4.4-log reduction for romaine lettuce, and about 2-log reduction for the rest of the greens.

Significance: This study demonstrates the potential of natural plant extract formulations to inactivate Escherichia coli O157:H7 strains on organic leafy greens.