How salmonella contaminates salads

  • Deborah Condon

Scientists have discovered how Salmonella bacteria can cause food poisoning by attaching to salad leaves.

According to the team, food poisoning from Salmonella and E.coli is often associated with eating contaminated beef or chicken products, as the pathogens live in the guts of cows, and the guts and egg ducts of chickens. Contamination of meat can then occur during the slaughtering process.

However the UK team noted that some recent outbreaks of food poisoning have been associated with contaminated salad or vegetable products, particularly pre-bagged salads. For example, in 2007 a Salmonella outbreak in the UK was traced back to imported basil, while in 2006, an E. coli outbreak in the USA was traced to contaminated pre-packed baby spinach.

The research team from London and Birmingham uncovered the mechanism used by one particular form of Salmonella – called Salmonella enterica serovar Senftenberg – to infect salad leaves, causing a health risk to the people who eat them.

They found that some of the 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, resulting in contamination and potential health risk.

Understanding the mechanism 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 already knew 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.

The research team 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.

To test this observation, they 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,” explained lead researcher, Prof Gadi Frankel of Imperial College London.

The next step will involve looking at the extent to which different types of salad leaves are affected by salmonella. Professor Frankel explained that some types of leaves are less susceptible to salmonella contamination than 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.

While acknowledging that only a minority of Salmonella cases are linked to salads, Prof Frankel insisted that these 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. 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 added.

Details of these findings were presented at a meeting of the the International Committee on Food Microbiology and Hygiene in Aberdeen.


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