Salmonella/Salad Interactions

Introduction: Fresh produce such as salad leaves are an important part of a healthy diet but in recent years have been associated with infection by enteric pathogens such as Salmonella enterica. So far, studies have concentrated on improving the hygiene of production and have not considered what happens to the behaviour of Salmonella when they enter the actual salad bag. It is known that salad leaves become damaged during processing and that juices are released, so bacteria residing in a salad bag will be bathed in leaf juice.

Purpose: The intention of our research is to investigate the effect of juices released from damaged salad leaves on the growth, virulence and salad leaf colonisation of S. enterica. Our aim is to use this information to develop ways of preventing enteric pathogen attachment to fresh salad produce.

Methods: Salmonella responsiveness to salad juices was analysed in water, to reflect the salad bag environment, and in serum- media to model the co-consumption of pathogen and salad leaf. We used assays that measured the effect of salad leaf juice on S. enterica growth, motility and biofilm formation. Light and scanning electron microscopy were used to visualise juice effects on Salmonella colonisation of salad leaves and the salad container.

Results: Salad juices at more than 1:50 dilutions stimulated Salmonella growth in all media tested. In serum-media, juices enhanced growth by several logs via provision of host iron from serum-transferrin. In water, leaf juices from all salad leaves tested significantly increased Salmonella biofilm formation and its capacity to colonise and persist on salad leaves, and the salad bag container. 

Significance: Our study shows that even very dilute salad juice can contribute to Salmonella colonisation of salad leaves and re-emphasises the importance of preventing enteric pathogen of fresh produce.