Essential Oil Microemulsions Inactivate Antibiotic-Resistant Bacteria on Iceberg Lettuce during 28-Day Storage at 4 °C

• Published in: Molecules
• Main Authors: Stephanie Arellano, Libin Zhu, Govindaraj Dev Kumar, Bibiana Law, Mendel Friedman, Sadhana Ravishankar
• Format: Article
• Language: English
• Published: MDPI AG 2022-10-01
• Subjects: antimicrobials; microemulsions; Iceberg lettuce; <i>Escherichia coli</i> O157:H7; <i>Pseudomonas fluorescens</i>; oregano oil
• Online Access: https://www.mdpi.com/1420-3049/27/19/6699

The objective of this study was to investigate the antimicrobial activities of essential oil-based microemulsions in the wash water against <i>Escherichia coli</i> O157:H7 and <i>Pseudomonas fluorescens</i> on Iceberg lettuce. Evaluated wash microemulsions included oregano oil, lemongrass oil, and cinnamon oil, along with a plant-based emulsifier for improved solubility. Iceberg lettuce was inoculated for 2 min with <i>E. coli</i> O157:H7 (6.0 log CFU/g) or <i>P. fluorescens</i> (6.0 log CFU/g) and then dip-treated in a phosphate buffered saline (PBS) control, 50 ppm chlorine, 3% hydrogen peroxide treatment or a 0.1%, 0.3%, or 0.5% microemulsion solution. Treated leaves were stored at 4 °C, and analyzed for surviving bacteria on days 0, 3, 7, 10, 14, 21, and 28. Efficacies of the antimicrobials were concentration and storage-time dependent. There was a 1.26–4.86 log CFU/g reduction in <i>E. coli</i> O157:H7 and significant reductions (0.32–2.35 log CFU/g) in <i>P. fluorescens</i> during storage at days 0–28 (<i>p</i> < 0.05). The 0.1% oregano oil microemulsion resulted in the best visual appeal in Iceberg leaves inoculated with <i>E. coli</i> O157:H7 and showed better improvement in the quality of the Iceberg leaves inoculated with spoilage bacteria <i>P. fluorescens</i>. The results suggest that 0.5% cinnamon and 0.3% oregano oil treatments have the potential to provide natural, eco-friendly, and effective alternatives to chemicals for the decontamination of leafy greens, eliminating <i>E. coli</i> O157:H7 and <i>P. fluorescens</i>.