Research reveals how Salmonella bacteria contaminate salad leaves

It shows how some Salmonella bacteria use their flagella – normally used for locomotion – to attach themselves to salad leaves and other vegetables, causing contamination and a health risk.

Food poisoning from Salmonella and E. coli is commonly associated with eating contaminated bovine or chicken products, but some recent outbreaks have been associated with contaminated salad or vegetable products, and more specifically with pre-bagged salads.

The new research, led by Professor Gadi Frankel from Imperial College London and carried out with Dr Rob Shaw and colleagues at the University of Birmingham, has uncovered the mechanism used by Salmonella enterica serovar Senftenberg to infect salad leaves.

To test their observation the scientists genetically engineered salmonella without flagella and found that they could not attach themselves to the leaves, and the salad remained uncontaminated.

Understanding the mechanism that pathogens such as salmonella use to bind themselves to salad leaves is important in developing new methods of preventing this kind of contamination.

The team's next steps will involve looking at the extent to which different types of salad leaves are affected by salmonella. "If we can find out what factors affect susceptibility, we may be able to develop new technologies to harness the 'immunity' found in some salad leaves to protect others from contamination," said Professor Frankel.

Even though only a very small proportion of the UK's infectious intestinal disease outbreaks are linked to prepared salad, Professor Frankel believes the numbers are likely to increase in coming years.

"In their efforts to eat healthily, people are eating more salad products, choosing to buy organic brands, and preferring the ease of 'pre-washed' bagged salads from supermarkets. All of these factors, together with the globalisation of the food market, mean that cases of Salmonella and E. coli poisoning caused by salads are likely to rise," he said.

In a previous study, Professor Frankel and his colleagues discovered the mechanism by which E. coli 0157 binds to salad leaves. They have shown that E. coli O157 bacteria use short needle-like filaments, which are normally used to inject bacterial proteins into human cells, to attach to salad leaves, causing contamination and a risk of transmission via the food chain to humans.