How Salmonella bacteria infect salad leaves

Washington, Sept 3 : A new study has revealed how Salmonella bacteria can cause food poisoning by attaching to salad leaves.

The study shows how some Salmonella bacteria use the long stringy appendages they normally use to help them 'swim' and move about to attach themselves to salad leaves and other vegetables, causing contamination and a health risk.

Usually food poisoning from Salmonella and E. coli is linked with eating contaminated bovine or chicken products, as the pathogens live in the guts of cows and the guts and egg-ducts of chickens, and contamination of meat can occur during the slaughtering process.

But, many of the recent outbreaks of food poisoning have been attributed to contaminated salad or vegetable products, and more specifically, pre-bagged salads.

Led by Professor Gadi Frankel from Imperial College London, the new research has uncovered the mechanism used by one particular form of Salmonella called Salmonella enterica serovar Senftenberg, to infect salad leaves, causing a health risk to people who eat them.

This breakthrough would help scientists in developing 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, they do not understand how the pathogens managed to bind to the leaves, until now.

The researchers have now found that Salmonella enterica serovar Senftenberg bacteria have a secondary use for their flagella - the long stringy 'propellers' they use to move around. The flagella flatten out beneath the bacteria and cling onto salad leaves and vegetables like long thin fingers.

For testing 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.

"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," said Frankel.

The team will now look at the extent to which different types of salad leaves are affected by salmonella. Frankel explained 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 said.

However, Frankel said that even though such a small minority of cases are currently linked to salads, the numbers are likely to increase in coming years.

In a previous study, the same team of researchers 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 them to salad leaves, causing contamination and a risk of transmission via the food chain to humans.

The study is presented at the 21st International ICFMH Symposium 'Food Micro 2008' conference in Aberdeen.

--ANI