Microbiologists from the University of Leicester have warned that just a small amount of damage to salad leaves could massively stimulate the presence of the food poisoning bug Salmonella in ready-prepared salad leaves. They also said that juices released from damaged leaves can enhance the virulence of salmonella bugs.

Dr Primrose Freestone, who led the research, is looking into novel methods of preventing food poisoning pathogens from attaching to the surface of salad leaves, to help producers improve food safety for consumers. The latest findings show that juices from damaged leaves in bagged spinach and mixed salad increased Salmonella pathogen growth 2400-fold over a control group and also enhanced their adherence to surfaces and overall virulence, or capacity to cause disease.

Commenting, Dr Freestone said, “Salad leaves are cut during harvesting and we found that even microliters of the juices (less than 1/200th of a teaspoon) which leach from the cut-ends of the leaves enabled Salmonella to grow in water, even when it was refrigerated. These juices also helped the Salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.

"This strongly emphasises the need for salad leaf 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.

"It also serves as a reminder to consume a bagged salad as soon as possible after it is opened. We found that once opened, the bacteria naturally on the leaves also grew much faster even when kept cold in the fridge.” Dr Freeston did acknowledge, “This research did not look for evidence of salmonella in bagged salads. Instead, it examined how Salmonella grows on salad leaves when they are damaged."

Whilst these greens unquestionably form part of a healthy diet, scientists are increasingly looking at the role of bagged salads and other vegetables, usually consumed uncooked, in outbreaks of food poisoning. In recent years, a number of high-profile outbreaks of salmonella and E.Coli have been linked to fresh produce, including the devastating outbreak of E.Coli in Germany in 2011, which was linked to sprouted seeds, and an E.Coli outbreak that claimed two lives in the UK over the summer, the source of which is suspected to be bagged salad.  

The Leicester researchers said that, despite the growing interest in this area of food safety, and the weight of research on improving the microbiological safety of salad leaf production, very few studies have investigated the behaviour of Salmonella once the leaves have been bagged.

Giannis Koukkidis, who worked with Dr Freestone, said, "Anything which enhances adherence of foodborne pathogens to leaf surfaces also increases their persistence and ability to resist removal, such as during salad washing. Even more worrying for those who might eat a Salmonella contaminated salad was the finding that proteins required for the virulence (capacity to cause infection) of the bacteria were increased when the Salmonella came into contact with the salad leaf juices."

Of more than 500,000 cases of food poisoning in the UK each year, poultry meat remains the most common source of infection, but the Food Standards Agency believes 48,000 cases of food poisoning cases stem from fresh produce: vegetables, fruit, nuts and sprouting seeds. Salmonella, which was the focus of the Leicester study, was the pathogen that caused the greatest number of hospital admissions last year – around 2,500.