Purpose: The objective 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.
Methods: Basil, cilantro, dill, and parsley plants were grown in a greenhouse with average temperature of 25±8°C and relative humidity of 63±20%. Upon maturity, basil, cilantro, dill, and parsley plants were 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 days. JMP was used to perform statistical analysis using Tukey's multiple comparison test with a P-value <0.05.
Results: Listeria monocytogenes populations demonstrated a similar biphasic survival curve on each of the four different herb plants. No significant difference was observed in L. monocytogenes survival over 28 days, for each of the four herb plants. Listeria monocytogenes populations decreased by ≥ three log CFU/g on each herb plant by seven days postinoculation. The largest L. monocytogenes population reduction (basil: 3.6 log CFU/g, cilantro: 2.2 log CFU/g, dill: 2.1 log CFU/g, and parsley: 2.3 log CFU/g) was observed between zero and one day. While L. monocytogenes did not grow on herbs plants grown in a greenhouse environment, L. monocytogenes was able to survive up to 28 days post-inoculation for all the herb plants, excluding parsley, which fell below the limit of detection on seven days.
Significance: Since L. monocytogenes exhibited long-term survival on the herb plants studied, the need for greenhouses to implement best practices (e.g., sanitation, good agricultural practices) is critical to minimize the introduction of contamination.