Salad leaf juices enhance Salmonella growth, fresh produce colonisation and virulence

Abstract

Abstract: We show in this report that traces of juices released from salad leaves as they became damaged can significantly enhance Salmonella enterica salad leaf colonisation. Salad juices in water increased Salmonella growth by 110% over the un-supplemented control, and in host-like serum based media by more than 2400-fold over controls. In serum based media salad juices induced growth of Salmonella via provision of Fe from transferrin, and siderophore production was found to be integral to the growth induction process. Other aspects relevant to salad leaf colonisation and retention were enhanced, such as motility and biofilm formation, which increased over controls by >220% and 250% respectively; direct attachment to salad leaves increased by >350% when a salad leaf juice was present. In terms of growth and biofilm formation the endogenous salad leaf microbiota was largely unresponsive to leaf juice, suggesting that Salmonella gains a marked advantage from fluids released from salad leaf damage. Salad leaf juices also enhanced pathogen attachment to the salad bag plastic. Over 5 days refrigeration (a typical storage time for bagged salad leaves) even traces of juice within the salad bag fluids increased Salmonella growth in water by up to 280-fold over control cultures, as well as enhancing salad bag colonisation, which could be an unappreciated factor in pathogen fresh produce retention. Collectively, this study shows that exposure to salad leaf juice may contribute to the persistence of Salmonella on salad leaves, and strongly emphasizes the importance of ensuring the microbiological safety of fresh produce. Importance: Salad leaves are an important part of a healthy diet, but in recent years have been associated with a growing risk of food poisoning from bacterial pathogens such as Salmonella enterica. Although this is considered a significant public health problem, very little is known about what happens to the behaviour of the Salmonella when in the actual salad bag. We show that juices released from the cut-ends of the salad leaves enabled the Salmonella to grow in water, even when it was refrigerated. Salad juice exposure also helped the Salmonella cells to attach to the salad leaves so strongly that washing could not remove them. Collectively, this study shows that exposure to even traces of salad leaf juice may contribute to the persistence of Salmonella on salad leaves as well as priming it for establishing an infection in the consumer