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Warning over food bugs in packed salad

Scientists say pre-washed doesn't mean safe to eat

pizza and salad

Share a pizza and side salad

The growing popularity of pre-packed salads is likely to lead to an increase in food poisoning cases, scientists have warned.

They said the increased uptake in the salads in particular, but also in fruit and vegetables, is likely to be reflected in a future rise in food poisoning.

They also warned that while scientists have a role to play in developing technology to protect food from contamination, consumers should be aware of the risks.

Food contamination

The comments were made before research into food contamination was discussed at a conference in Aberdeen yesterday.

Led by Imperial College London, the research details how salmonella bacteria are able to contaminate salads and vegetables.

Professor Gadi Frankel, from Imperial College, said a greater understanding of how salads are contaminated is important because cases of food poisoning caused by salads are ‘likely to rise in the future’.


He said there had been recent outbreaks that could specifically be related to pre-packed salads, including a salmonella outbreak in the UK last year traced to imported basil and an E.coli outbreak in the US in 2006 traced to pre-packed baby spinach.

He said: ‘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.

‘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 that a label stating food was pre-washed did not necessarily mean it was safe to eat and, although the risk of poisoning remained low, consumers should make ‘informed decisions’.

‘There are many kinds of salad bags being marketed as washed and ready-to-eat,’ he said. ‘It is about individual choice but people should be aware of the risks so they can make informed decisions about whether they want to wash their food or not.’

Food poisoning from salmonella and E.coli is commonly associated with eating contaminated bovine or chicken products.

Food poisoning

The germs live in the guts of cows and egg-ducts of chickens, and contamination of meat can occur during slaughtering but recent outbreaks of food poisoning highlight the dangers associated with contaminated salad or vegetables.

Prof Frankel’s research, carried out with the University of Birmingham, found that salmonella bacteria are able to use their flagella – the stringy ‘propellors’ designed to aid their movement – to grip salad leaves and contaminate them.

Scientists know that salmonella and E.coli 0157 – a strain that can cause serious sickness in humans – can spread to salads and vegetables if they are fertilised with contaminated manure or come into contact with contaminated products.

Bound to the leaves

Until now they did not understand how the germs managed to bind to the leaves.

Professor Frankel said they would try to identify the factors that made some leaves less susceptible to salmonella to protect others from contamination in future.

He said: ‘Discovering that the flagella play a key role in salmonella’s ability to contaminate salad leaves gives us a better understanding than ever before of how this contamination process occurs.

‘Once we understand it, we can begin to work on ways of fighting it.’

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