Organic leafy vegetables could be harbouring harmful bacteria such as salmonella, study warns

They’re often seen as healthier alternatives to standard fruits and vegetables, but a new study may put you off buying organic.

Researchers from the Universitat Politècnica de València tested organic lettuce and spinach samples and found that some contained harmful bacteria including Pseudomonas, Salmonella, and Helicobacter.

Worryingly, if consumed, these bacteria can cause a range of nasty infections, including pneumonia and gastrointestinal illness, the researchers warned.

The term organic refers to food that is produced under strict guidance.

‘In Europe the standards are set in EU law and restrict the use of pesticides and prevent the use of herbicides in the production of food,’ the NHS explains.

‘Some people choose to eat organic foods because they are worried about residues of these pesticides in food or they may be concerned about the impact that farming has on the environment.’

While organic foods are not exposed to pesticides or herbicides, they can be contaminated with pathogens through contact with soil, irrigation water, air, rain, insects and during industrial washing.

In particular, vegetables can become contaminated with certain protozoa (single-celled organisms) such as free-living amoeba (FLA), which can act as hosts to pathogenic bacteria.

‘Food and food-related environments create an ideal meeting place for free-living amoebae and pathogenic bacteria’, explained Dr Yolanda Moreno, who led the study.

‘However, comparatively little is known about the occurrence and diversity of free-living amoebae on organic vegetables and their role in transmitting human pathogens.’

In their study, the team analysed 17 samples of organic lettuce and spinach collected from local supermarkets in Valencia between November 2020 and May 2021.

In their study, the team analysed 17 samples of organic lettuce and spinach collected from local supermarkets in Valencia between November 2020 and May 2021. Their analysis revealed that a third of the samples contained 52 potentially disease-casing types of bacteria, including Legionella, Salmonella and Arcobacter, which can cause pneumonia and gastrointestinal illness

Their analysis revealed that a third of the samples contained 52 potentially disease-causing types of bacteria, including Legionella, Salmonella and Arcobacter, which can cause pneumonia and gastrointestinal illness.

A fifth were found to contain Vermamoeba vermiformis, which causes severe infections, while almost two thirds contained Acanthamoeba castellanii, which can cause blindness and encephalitis.

‘The presence of bacteria of public health concern contained inside the free-living amoebae suggests that they are vehicles that can easily transmit pathogens capable of reaching humans and causing health problems through contaminated organic vegetables’, said Dr Moreno.

‘Contamination can arise as a consequence of treating soil with organic fertilisers such as manure and sewage sludge and from irrigation water.

‘Leafy greens are particularly susceptible to faecal contamination due to their proximity to the ground and the likelihood of humans consuming them without cooking. 

‘Our results also stress the need to educate the public on safe and proper handling of fresh organic vegetables before eating them fresh or slightly cooked.’ 

Professor Willem van Schaik, Director of the Institute of Microbiology and Infection and Professor of Microbiology and Infection at the University of Birmingham, who was not involved in the study, pointed out that it’s not just organic vegetables at risk. 

‘The observations reported here are somewhat unsurprising, but are not unique to organic vegetables as is suggested here,’ he said.

‘It is good to read that the researchers have highlighted the advice that all leafy greens should be washed before use, which will greatly reduce the risk of food-borne infections.’