Closeup on fresh wet lettuce in the garden

Bioscientists have revealed that just a small amount of damage could massively stimulate the presence of the food poisoning bug Salmonella in ready-prepared salad leaves.

Microbiologists from the University of Leicester have discovered that juices released from damaged leaves also had the effect of enhancing the virulence of the pathogen, potentially increasing its ability to cause infection in the consumer.

The research was led by Dr Primrose Freestone of the university’s Department of Infection, Immunity and Inflammation and PhD student Giannis Koukkidis, who has been funded by a Biotechnology and Biological Sciences Research Council (BBSRC) i-case Studentship.

They investigated methods of preventing food poisoning pathogens from attaching to the surface of salad leaves to help producers improve food safety. The findings suggested that juices from damaged leaves in bagged spinach and mixed salad increased Salmonella pathogen growth 2,400-fold over a control group.

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.”

The research did not look for evidence of salmonella in bagged salads. Instead, it examined how Salmonella grows on salad leaves when they are damaged.

Giannis Koukkidis said: “Anything which enhances adherence of food-borne pathogens to leaf surfaces also increases their persistence and ability to resist removal, such as during salad washing procedures. Even more worrying for those who might eat a Salmonella-contaminated salad was the finding that proteins required for the virulence of the bacteria were increased when the Salmonella came into contact with the salad leaf juices.

“Preventing enteric pathogen contamination of fresh salad produce would not only reassure consumers but will also benefit the economy due to fewer days lost through food poisoning. We are now working hard to find ways of preventing salad-based infections.”

Professor Melanie Welham, Chief Executive, BBSRC, said: “Food-borne pathogens like Salmonella are serious bacterial threats that affect our health which is why BBSRC invests in research to understand and combat food poisoning.”

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, 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 – about 2,500 per year.