Superbug salad: Leafy green vegetables could carry antibiotic-resistant bacteria

How raw leafy greens may be hiding dangerous superbugs: Experts warn ‘hard to wash’ vegetables like lettuce are most likely to spread antibiotic-resistant bacteria

  • Experts in California found antibiotics did not protect mice from E.coli bugs
  • Bacteria were passed into the body on lettuce leaves, which are eaten raw
  • Scientists said people needed to know these bacteria may not only be in meat 

Raw leafy green vegetables such as lettuce are the most likely to carry dangerous superbugs, researchers have warned.

Scientists found bacteria strong enough to survive antibiotic treatment could be transported on lettuce.

Lettuce, in particular, was of concern to the experts because the natural grooves and folds in its leaves make it difficult to wash thoroughly.

And lettuce and other leafy greens are often eaten raw, meaning bacteria haven’t been killed off through heat.

Although farmed meat is known to contain antibiotic-resistant bacteria, researchers have warned the same illness-causing bugs could be transported from farms on salad vegetables like lettuce which are eaten raw and may be difficult to clean (stock image)

Antibiotic-resistance has been named one of the top threats to human health, and farmed meat is considered to be a major cause.

Many animals are fed antibiotics when they’re not even ill in order to make them grow bigger, but bacteria become used to being around the drugs.  

But researchers from the University of Southern California say it’s not just meat that is to blame, and vegetables may also be playing a role.


Antibiotic-resistant bacteria are those which have developed to be strong enough to survive treatment with previously effective medicines.

Exposure to antibiotics in small amounts or when there is no infection  increases the risk of bacteria getting used to the medicine.

Farm animals being kept to produce meat are sometimes fed antibiotics, many of which are the same as those used to treat humans, to make them grow larger faster. 

In a natural environment, animals would be exposed to bacteria and then use energy to fight off infection and build up immunity.

Antibiotics remove the need for this immune reaction by killing bacteria immediately, meaning more of the animal’s energy can be used for the body to grow larger. Therefore, the farmer gets more meat. 

However, bacteria are becoming resistant to these antibiotics because they’re constantly exposed to them, meaning the antibiotic-resistant bacteria – the superbugs – build up inside the animal.

These are then passed into the human food chain when the animals are slaughtered and sold as meat, or in their milk or if their manure is used to fertilise crop farms. 

They added mutant strains of E.coli to lettuce leaves and fed them to mice which had been receiving antibiotic treatment for four days, New Scientist reported.

And the E.coli was able to survive its passage through the body and colonise – take root and multiply – in the mice’s intestines in spite of the antibiotics.

‘We come across people saying because they’re vegetarian now they’re safe,’ said lead researcher Marlene Maeusli.

‘What we’re trying to say is that everyone, regardless of whether you’re a vegetarian, you’re still connected to the larger food chain.’

According to the US’s Centers for Disease Control and Prevention, two million people get antibiotic resistant infections in the country every year and 230,000 die.

And some 400,000 of those infections are believed to come from food – mostly meat.

Ms Maeusli, a PhD student, did not specify where the bacteria might come from in a real-world environment.

But past research has suggested they could pass onto vegetables through soil fertilised with animal manure or from water used to irrigate crop farms.

Although these antibiotic resistant bacteria may not cause an immediate infection, if they are allowed to build up in the body they could cause a worse and hard-to-treat illness further down the line. 

‘The environment and human health – in this context via agriculture and microbiomes – are inextricably linked,’ Ms Maeusli said.

‘Our findings highlight the importance of tackling foodborne antibiotic resistance from a complete food chain perspective that includes plant foods in addition to meat.’

Ms Maeusli and her team presented their research at the American Society for Microbiology conference in California at the weekend.