Survival of Listeria monocytogenes on the surface of basil, cilantro, dill, and parsley plants

Fresh herbs are popular commodities that generally undergo minimal processing. Salmonella and Escherichia coli O157:H7 survival has been observed on herbs; however, little data exists on the survival of Listeria monocytogenes on herbs. The purpose of this study was to evaluate the survival of L. monocytogenes on the surface of basil, cilantro, dill, and parsley plants grown in a greenhouse. Greenhouse production continues to increase worldwide due to a year-round growing season and controllable conditions. Herb plants were grown in a greenhouse with average temperature of 21 ± 5 °C and relative humidity of 62 ± 10%. Each herb plant was inoculated with a five-strain nalidixic acid-resistant cocktail of L. monocytogenes. Samples were enumerated using standard methods at time-points: 0, 0.21, 1, 2, 3, 7, 14, 21, and 28 d. Population levels in log CFU/g of each herb plant were calculated. The initial inoculum was approximately 4-5 log CFU/g for each herb plant. Overall, L. monocytogenes populations did not grow on the studied herb plants; however, L. monocytogenes was able to survive on the surface of the studied herb plants for up to 28 d, except parsley plants, which fell below the limit of detection 7 d post-inoculation. L. monocytogenes populations demonstrated a similar biphasic survival curve on each of the four different herb plants. L. monocytogenes population decline was most evident within the day post-inoculation for all herb plants. Between 0 and 1 d, L. monocytogenes populations significantly decreased by 3.3, 2.4, 2.6, and 3.2 log CFU/g for basil, cilantro, dill, and parsley plants, respectively. Additionally, within that first 24 h, L. monocytogenes populations significantly decreased on the surface of all herb plants at 5 h post-inoculation (approximately 2.4, 1.6, 2.7, and 2.0 log CFU/g for basil, cilantro, dill, and parsley plants, respectively). No significant difference was observed in L. monocytogenes populations between each of the four herb plants after 7 d post-inoculation. Since L. monocytogenes exhibited survival on the herb plants studied, the adoption of pre-harvest best practices (e.g., sanitation, Good Agricultural Practices) is fundamental to limiting the introduction of contamination, especially in greenhouse environments.