Antimicrobial activity of oregano oil against antibiotic-resistant Salmonella enterica on organic leafy greens at varying exposure times and storage temperatures Original Research Article

The objective of this study was to evaluate the effectiveness of oregano oil on four organic leafy greens (Iceberg and Romaine lettuces and mature and baby spinaches) inoculated with Salmonella Newport as a function of treatment exposure times as well as storage temperatures. Leaf samples were washed, dip inoculated with S. Newport (6-log CFU/ml) and dried. Oregano oil was prepared at 0.1, 0.3, and 0.5% concentrations in sterile phosphate buffered saline (PBS). Inoculated leaves were immersed in the treatment solution for 1 or 2 min, and individually incubated at 4 or 8 °C. Samples were taken at day 0, 1, and 3 for enumeration of survivors. The results showed that oregano oil was effective against S. Newport at all concentrations. S. Newport showed reductions from the PBS control of 0.7–4.8 log CFU/g (Romaine lettuce), 0.8–4.8 log CFU/g (Iceberg lettuce), 0.8–4.9 log CFU/g (mature spinach), and 0.5–4.7 log CFU/g (baby spinach), respectively. The antibacterial activity also increased with exposure time. Leaf samples treated for 2 min generally showed greater reductions (by 1.4–3.2 log CFU/g), than those samples treated for 1 min; however, there was minimal difference in antimicrobial activity among samples stored under refrigeration and abuse temperatures. This study demonstrates the potential of oregano oil to inactivate S. Newport on organic leafy greens.