New research from my colleagues at the University of Leicester shows that “salad juice” from damaged leaved in bagged salads can stimulate the growth of Salmonella, even at refrigerator temperatures. Although this research did not look for evidence of Salmonella in bagged salads, it does show how Salmonella grows on salad leaves when they are damaged. Research published recently by the Food Standards Agency reported that annually there are more than 500,000 cases of food poisoning in the UK. While poultry meat was the most common source of infection, some 48,000 of food poisoning cases were from fresh produce: vegetables, fruit, nuts and sprouting seeds. Salmonella was the pathogen that caused the greatest number of hospital admissions – around 2,500 per year.
This work strongly emphasises the need for salad growers to maintain high food safety standards as even a few Salmonella cells in a salad bag at the time of purchase could be become many thousands by the time a bag of salad leaves reaches its use by date, even if kept refrigerated. Even small traces of juices released from damaged leaves can make the pathogen grow better and become more able to cause disease.
Salad leaf juices enhance Salmonella growth, fresh produce colonisation and virulence. Applied and Environmental Microbiology, 18 November 2016, doi: 10.1128/AEM.02416-16.
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.