Investigation of the influence of salad leaf damage on the growth and virulence of Listeria monocytogenes

Vegetables and fruits are important for a healthy life due to their mineral and vitamin content. A ready-to-eat green salad is considered a better and easier choice for a snack. However, fresh produce, such as green leafy salads, has recently been recognised as a potential source of foodborne infection. Listeria monocytogenes is the most important pathogenic bacterium for refrigerated, perishable fresh produce because it can grow at low temperatures. The aim of this study is to investigate whether compounds released from damaged edible plant tissue, such as salad leaves, modulate L. monocytogenes growth and virulence. An analysis was done to examine the responsiveness of L. monocytogenes to salad extract and salad bag fluids. This analysis was carried out in water to simulate the environment within a salad bag and in host-like serum media to simulate the intake of both the pathogen and salad leaf together. The presence of salad extract at a concentration of less than 0.5% v/v and salad bag fluids at a concentration of 10% v/v significantly enhanced the growth of L. monocytogenes in water and serum-based medium. The growth increase was more than 10,000 times higher compared to the control samples. This study used light and scanning electron microscopy, as well as the eukaryotic Caco-2 cell line and Galleria Mellonella model of infection, to examine the impact of different leafy green extracts (such as rocket, lettuce, spinach and salad bag fluids) on the growth of L. monocytogenes, as well as their ability to build biofilms and infect host cells. The tendency of L. monocytogenes to attach to surfaces is noteworthy. It is interesting to find that salad leaf extracts stimulate surface attachment and biofilm formation of L. monocytogenes after only a few hours of incubation at low refrigeration temperatures or at 37°C for 18 hours. Even a very diluted exposure to the extract from salad leaves increased the attachment and wider colonisation of L. monocytogenes on the leaves, potentially making them more capable of causing an infection if later consumed. In conclusion, this study has shown that the salad leaf extracts caused marked changes in L. monocytogenes gene expression, leading to increased growth and virulence. This has implications for the safety of bagged salad leaves if they become contaminated with L. monocytogenes.