Early findings from researchers in Spain has found organic produce can be contaminated with a range of bacteria.

The study suggests that amoebas that live on organic leafy vegetables can shelter human pathogens like Pseudomonas, Salmonella, and Helicobacter.

Details are based on a poster presentation at the European Congress of Clinical Microbiology and Infectious Diseases (ECCMID) in Lisbon, Portugal, from April 23 to 26. All abstracts were peer reviewed by a congress committee. There is no paper yet, but the research has been submitted to a medical journal for publication.

However, other scientists warned that while conclusions are interesting, the fact that data has not yet been peer reviewed means they cannot be put into context.

Presence of DNA detected
The work found vegetables can become contaminated with certain single-celled organisms such as free-living amoebae (FLA), which feed on bacteria and can act as hosts to pathogenic bacteria which resist FLA digestion.

“Food and food-related environments create an ideal meeting place for free-living amoebae and pathogenic bacteria. However, comparatively little is known about the occurrence and diversity of free-living amoebae on organic vegetables and their role in transmitting human pathogens,” said Yolanda Moreno from Universitat Politècnica de València in Spain.

There is rising demand for organically grown fruit and vegetables as people want to eat healthy diets and because of concerns about potential contamination from pesticides, chemical fertilizers and herbicides. However, during growth, harvest, transportation and further processing and handling, fresh produce can be contaminated with pathogens from human or animal sources, through contact with soil, irrigation water, air, rain, insects, and during industrial produce-washing.

Researchers collected 17 samples of lettuce and spinach from supermarkets in Valencia between November 2020 and May 2021. They used a metagenomics technique that identifies DNA in all bacteria present inside FLA. Results determined what kinds of microbes were in each sample but it is not known if they were alive or dead.

A third of samples contained 52 potentially disease-causing types of bacteria including Legionella, Salmonella and Arcobacter. Presence of such bacteria inside the FLA suggests they are vehicles that can transmit pathogens capable of reaching humans.

“Contamination can arise as a consequence of treating soil with organic fertilizers such as manure and sewage sludge and from irrigation water. Leafy greens are particularly susceptible to fecal 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,” said Moreno.

Hazard found but risk unclear
Despite the findings, researchers said larger studies were needed from different countries to understand more about the microbiological quality and safety of organic vegetables.

John Fawell, of Cranfield University, said presence did not necessarily mean significant risk.

“We need better and more systematic data to show how big a problem this is and to take steps to minimize any risks. A hazard has been demonstrated, the next stage is to assess how big a risk this poses to consumers and what steps they should take to mitigate that risk,” he said.

Willem van Schaik, from the University of Birmingham, said: “As vegetables are grown on soil, it is almost unavoidable that organisms from soil, or water that is used for irrigation, are present on leafy greens and this includes amoeba discussed in the abstract of this study. These organisms are very widespread in the environment and are extremely rare causes of disease in humans. 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 foodborne infections.”

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