Salmonella sticks to salad with help of finger-like flagella, scientists say

Wednesday, September 3, 2008

CBC News


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Scientists need to know how salmonella bacteria causes food poisoning in order to find ways to prevent health risks, British researchers told a food conference in Scotland on Wednesday.

The researchers, Gadi Frankel from Imperial College in London and Rob Shaw from the University of Birmingham, said the process by which salmonella bacteria contaminate vegetable products is not rocket science, but it is important.

Frankel and Shaw, who presented their findings to the Food Micro 2008 symposium in Aberdeen, focused on salmonella bacteria and salad leaves.

They said salmonella bacteria attach themselves to salad leaves by using stringy appendages that work like long thin fingers.

They said only a few cases of food poisonings are linked to bagged salads, but the numbers are likely to increase in the 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 than ever before," Frankel said.

"All of these factors, together with the globalization 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."

The appendages used in the binding process mostly help the bacteria swim and move about, they said.

The researchers said the number of food poisoning cases involving bagged salads is growing. In 2007, a salmonella outbreak in the United Kingdom was traced to imported basil, and in 2006, an E. coli outbreak in the U.S. was traced to bagged baby spinach.

The researchers said between 1996 and 2000, 23 per cent of all infectious disease outbreaks in the U.K. were caused by contaminated food, and of these, four per cent were linked to bagged salad.

Frankel and Shaw focused on one particular form of salmonella known as salmonella enterica serovar senftenberg.

They said scientists know that salmonella can spread to salads and vegetables if they are fertilized with contaminated manure, irrigated with contaminated water or come into contact with contaminated products during cutting, washing or packing. But they said scientists did not know how the pathogens bind themselves to the leaves.

The long stringy appendages, known as flagella, flatten out beneath the bacteria and cling. To test their observations, the scientists genetically engineered salmonella without flagella in the lab and found they could not attach themselves to the leaves.

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," Frankel said.

In a previous study, the researchers discovered how E. coli 0157, a strain of E. coli that causes serious sickness in humans, binds to salad leaves. It uses short, needle-like filaments, which are normally used to inject bacterial proteins into human cells, to attach themselves, causing contamination.



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