ARS | Publication request: Antibacterial activity of oregano oil against antibiotic resistant Salmonella enterica on organic leafy greens at varying exposure times and storage temperatures

Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 2012
Publication Date: February 1, 2013
Citation: Moore-Neibel, K., Gerber, C., Patel, J.R., Friedman, M., Jaroni, D., Ravishankar, S. 2013. Antibacterial activity of oregano oil against antibiotic resistant Salmonella enterica on organic leafy greens at varying exposure times and storage temperatures. Food Microbiology. 34(1):123-129.

Interpretive Summary: Recent increases in foodborne illnesses due to the consumption of fresh produce contaminated with Escherichia coli O157:H7 and Salmonella has prompted the research to find alternatives to chlorine as a produce wash treatment. Chlorine is the standard antimicrobial compound used in produce wash water, however, it dissipates very rapidly from exposure to dirt and/or plant juices. In this study, the antimicrobial effect of oregano oil in reducing Salmonella enterica on four organic leafy vegetables (iceberg and romaine lettuce and natural and baby spinach) was evaluated. Leaf samples were dip inoculated with Salmonella Newport (6 million cells/ml), air dried, and immersed in different concentrations of oregano oil. Surviving S. Newport populations were determined during storage of leafy vegetables. Oregano oil reduced Salmonella on all four organic leafy vegetables; the antibacterial activity increased with exposure time. Leaf samples treated with oregano oil for 2 min resulted in greater reductions (10- to 1000-fold) than those treated for 1 min. Oregano oil should be evaluated further for its antimicrobial activity against other enteric pathogens. This information should be useful to other scientists, the produce industry and to regulatory agencies.

Technical Abstract: 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. Romaine lettuce, iceberg lettuce, mature spinach, and baby spinach showed between 0.7-4.8-log, 0.8-4.8-log, 0.8-4.9-log, and 0.5-4.7-log CFU/g reductions in S. Newport, respectively, when compared to PBS control. 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.