Studies suggest half of outbreaks of E. coli, which can prove fatal in humans, are caused by leafy greens. We take a look at the links between the bacteria and lettuce
It is the bacteria that lives naturally in the intestines of cattle, poultry and other animals.
But while most strains of E. coli are harmless to humans, others can cause serious illness. The recent outbreak of Shiga-toxin-producing E. coli – or STEC O145 – can attack the gut’s lining and is thought to have been caused by some supermarket sandwiches containing salad leaves.
“Earlier this month, we confirmed that several sandwich manufacturers had taken precautionary action to withdraw and recall various sandwiches, wraps, subs and rolls after food chain,” said Darren Whitby, head of incidents at the Food Safety Authority. “Epidemiological links enabled us to narrow down a wide range of foods to a type of lettuce used in sandwich products as the likely cause of the outbreak.”
Food safety bosses are ‘confident’ the outbreak has been narrowed down to a type of butterhead lettuce called Apollo but it is not known how exactly the salad vegetable became contaminated. One person with underlying health conditions has sadly died, according to the UK Health Security Agency and a total of 122 people have been admitted to hospital for treatment due to the strain.
With studies suggesting leafy greens are linked to half of all E. coli outbreaks, we take a closer look at the reasons why…
How are salad leaves contaminated?
Pathogenic bacteria, the type of E. coli that can cause human illness, can occur when produce in the field makes contact with animal faeces. Sources of this can include contaminated water, bird faeces and improperly composted manure.
Wet, warm weather could have been a factor when it comes to the recent strain caused by lettuce, Nicola Holden, a professor of food safety at Scotland’s Rural College (SRUC), told Mail Online. “The rainy weather means that for any field grown crops, humidity will be higher within the leaf canopy,” she said. “We know that STEC favours high humid conditions.”
The texture of lettuce could mean the leafy green is more susceptible to contamination also. Professor Paul Hunter, an infectious diseases expert from the University of East Anglia, told the publication: “Watering during growth allows the potentially contaminated water to rest on the lettuce leaf. The rough and waxy surface of the leaf also makes it difficult to wash off E. coli.
“‘The main risk comes from bacteria attached to the leaf surface. Most other vegetables are either peeled or cooked before eating. Things like cucumbers, tomatoes and peppers may not be cooked but grow some way off the ground so less likely to be contaminated.”
Even after produce is picked and sent to the processing plant, equipment used to cut up leafy greens and water used to wash produce can cause contamination. At the shop the salad is sold in, cross-contamination can occur from raw foods if it isn’t pre-bagged.
Is bagged salad a particular problem?
Bagged lettuce leaves have also been called ‘breeding grounds’ for both E. coli and salmonella, a different germ that can cause food poisoning. This is because the juice from cut leaves hugely increases the growth of the strain in water – ‘bacterial soup’.
Doctor Primrose Freestone, Senior Lecturer in Clinical Microbiology, told Channel 5 documentary Secrets of Supermarket Food back in 2019: “What’s now being realised is that there is a potential infection risk associated with bagged salads. The European Union Foods Standards Agency now actually regards bagged salads as the second most frequent cause of food poisoning.
“Salad leaves are natural, fresh produce. That means they are in contact with everything that might be out there – insects, birds and so on. Basically, they are contaminated with poo and any potential pathogen – such as e-coli or listeria – that might be present within that particular waste matter.”
How can I prevent getting E. coli from preparing salad at home?
Regularly washing hands with warm water and soap — alcohol gels do not kill all bugs that cause diarrhoea – is advised to avoid infection. Cutting boards should be washed before use and leafy greens need to be kept in the fridge at the right temperature and eaten before their best-before dates. Lettuce should always be washed in cold water before consumption.
How can I spot if E. coli is in my lunch?
It is difficult to tell if a food is contaminated with E. coli because you can’t see, smell or taste it, but there are key warning signs that your salad bag has gone bad. Look out for water pooling in the bottom of your bag, as spoilage tends to occur at the bottom of the bag first. On the leaves themselves, you may notice wilting, moisture, and a change in colour or smell. Symptoms usually take a few days to develop and can include diarrhoea that can be bloody, stomach cramping, fever and vomiting.
