How Salmonella Bacteria Contaminate Salad Leaves

The new study shows how some Salmonella bacteria use the long stringy fingers to help attach themselves to salad leaves and other vegetables, causing contamination and a health risk.

Some recent outbreaks of food poisoning have been associated with contaminated salad or vegetable products, and more specifically, pre-bagged salads.

Understanding the method that pathogens such as salmonella use to bind themselves to salad leaves is important if scientists are to develop new methods of preventing this kind of contamination and the sickness it causes.
Scientists know that Salmonella and E. coli O157 – a strain of E. coli that can cause serious sickness in humans - can spread to salads and vegetables if they are fertilised with contaminated manure, irrigated with contaminated water, or if they come into contact with contaminated products during cutting, washing, packing and preparation processes.

However, until now, scientists did not understand how the pathogens managed to bind to the leaves. Professor Frankel and his colleagues at the University of Birmingham found that Salmonella bacteria have a secondary use for their flagella propellers- (those long stringy fingers) they use to move around. The flagella flatten out beneath the bacteria and cling onto salad leaves and vegetables like long thin fingers. To test this observation the scientists genetically engineered salmonella without flagella in the lab and found that they could not attach themselves to the leaves, and the salad remained uncontaminated.
Professor Frankel says: "Discovering that the flagella play a key role in Salmonella's ability to contaminate salad leaves gives us a better understanding then ever before of how this contamination process occurs.

Once we understand it, we can begin to work on ways of fighting it."
The team's next steps will involve looking at the extent to which different types of salad leaves are affected by salmonella. Professor Frankel explains that some types of leaves are less susceptible to salmonella contamination that others: "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," he says.

However, Professor Frankel says that even though such a small minority of cases are currently linked to salads, 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, then ever before. 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 in the future. This is why it's important to get a head start with understanding how contamination occurs now," he said.