E. coli/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 E. coli and Salmonella enterica. In July 2016 a UK E. coli-rocket outbreak made>150 people ill and killed 2. However, despite calls for improvements in salad leaf hygiene until our recent publication, few studies had investigated enteric pathogen behaviour when actually within a bagged salad. Salad leaves become damaged during processing and 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 E. coli. Our aim is to use this information to develop ways of preventing enteric pathogen attachment to fresh salad produce.

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

Results:  Salad juices at>1/50 dilutions significantly stimulated E. coli 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 as well as the fluid within the bag salad tested significantly increased E. coli 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 E. coli colonisation of salad leaves and re-emphasises the importance of preventing enteric pathogen of fresh produce.