Salmonella, salads and food poisoning

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.

Salmonella
Salmonella

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.

Food poisoning – Signs, symptoms and treatment

What is food poisoning?

Food poisoning is caused by contamination, usually by bacteria such as salmonella, E. coli or campylobacter.

It can also come from viruses such as the norovirus, which is actually the most common stomach bug in the UK.

It usually takes 1-3 days for food poisoning to develop, and symptoms include feeling sick, vomiting, stomach cramps and diarrhoea.

The most common causes are undercooked meat, unrefrigerated or out-of-date ingredients, and sick or dirty people touching the food.

The Food Standards Agency (FSA) runs a scheme where kitchens are inspected and rated out of five.

They inspect everything from restaurants and pubs to hotels and supermarkets, and you’ll see their green Food Hygiene Rating stickers in many places.

There’s a useful search function on ratings.food.gov.uk, however, which will reveal all about the businesses in your area.

A government study released in June 2014 found that, while 500,000 people are diagnosed with food poisoning from specific bugs each year, there are enough ‘unknown pathogens’ in untested sufferers to more than double that number.

The report also found that campylobacter was the most common cause with 280,000 cases, while salmonella caused the most hospital admissions – around 2500 per year.

The riskiest foods include poultry and veg

Poultry was the runaway riskiest food with 244,000 cases per annum, but more surprisingly vegetables, fruit, nuts and seeds were second as a group with 48,000 victims.

There’s little that can be done to treat food poisoning, though you should rest and drink plenty of water.

Norovirus
Norovirus

It’s best to avoid food until you’ve really improved, and then eat easily digested things such as toast.

The main thing is to avoid dehydration, say the NHS, who recommend Oral Rehydration Salts for the over 65s and anyone with immune system problems, such as sufferers of cancer or HIV.


When you should see the doctor

*Your symptoms are severe, even after a few days

*You have a temperature of 38°C (100.4°F) or over

*You’re severely dehydrated (look for sunken eyes and dark, strong-smelling urine)

*There has been an outbreak of similar cases

*You have a baby with food poisoning.

If you suspect your food poisoning came from a specific shop or restaurant, you should contact your environmental health office.