New research suggests that disease-causing amoebas that live on organic leafy vegetables can shelter human pathogens like Pseudomonas, Salmonella, and Helicobacter and are a potential risk to public health.
There is a growing demand for organically grown fruit and vegetables as people strive to eat healthy diets and amid concerns over 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.
Vegetables can become contaminated with certain protozoa (single-celled organisms) such as free-living amoebae (FLA), that feed on bacteria and can act as hosts to pathogenic bacteria (the so-called "Trojan horses") which resist FLA digestion and could be a threat to public health.
"Food and food-related environments create an ideal meeting place for free-living amoebae and pathogenic bacteria", explains Dr. Moreno. "However, comparatively little is known about the occurrence and diversity of free-living amoebae on organic vegetables and their role in transmitting human pathogens."
To conduct a preliminary study of the FLA microbiome isolated from organic vegetables, researchers collected 17 samples of lettuce and spinach from local supermarkets in Valencia between November 2020 and May 2021.
To investigate the degree of contamination, researchers used a metagenomic technique that identifies DNA in all bacteria present inside FLA. The results were assessed to determine what kinds of microbes (Microbiome) were in each sample.
The main bacterial types identified were Flavobacterium (found in 10% of vegetable samples) and Pseudomonas (10%), many of which do not cause disease in humans. However, a third of samples (34%) contained 52 potentially disease-causing types of bacteria including Legionella, Salmonella, and Arcobacter. The resulting infections can cause illnesses—including pneumonia and gastrointestinal illness.
Moreover, the FLA species Vermamoeba vermiformis that commonly causes severe infections in humans was found in a fifth (19%) of vegetable samples; and Acanthamoeba castellanii, that can cause blindness and encephalitis were identified in almost two thirds (63%) of samples.
"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", says Dr. Moreno. "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."
Leafy greens are particularly susceptible to fecal contamination due to their proximity to the ground and the likelihood of humans consuming them without cooking.