Cultivation Conditions of Spinach and Rocket Influence Epiphytic Growth of Listeria monocytogenes

Leafy vegetables are associated with Listeriosis outbreaks due to contamination with Listeria monocytogenes. To date, contradictory findings were reported on spinach, rocket, and kale, where some studies reported growth of L. monocytogenes, while others did not. Thus, the current study investigated the reason for conflicting findings by producing leafy vegetables, where cultivation factors were known for growth potential studies. Of all polytunnel produce, kale Nero di Toscana demonstrated the highest growth potential (2.56 log cfu g⁻¹), followed by spinach F1 Cello (1.84 log cfu g⁻¹), rocket Buzz (1.41 log cfu g⁻¹), spinach F1 Trumpet (1.37 log cfu g⁻¹), and finally rocket Esmee (1.23 log cfu g⁻¹). Thus, plant species and variety influenced L. monocytogenes growth potentials. Moreover, significantly lower growth potentials of 0.3 log cfu g⁻¹ were identified when rocket Buzz was cultivated in open fields (1.11 log cfu g⁻¹) instead of a polytunnel. The opposite effect was observed for spinach F1 Trumpet, where growth potentials increased significantly by 0.84 log cfu g⁻¹ when cultivated in open fields (2.21 log cfu g⁻¹). Furthermore, a significant seasonality effect between batches was found (p < 0.05). This study revealed that spinach and rocket cultivation conditions are at least co-factors in the reporting of differing growth potentials of L. monocytogenes across literature and should be considered when conducting future growth potential studies.