Washed leaves ready to eat? Only in manufacturers’ declarations. The cut and pre-washed lettuce leaves sold in foil bags are a veritable breeding of salmonella and E. coli – these are the conclusions of a study announced by British scientists in the journal “Applied and Environmental Microbiology”. A few months ago we wrote about two people in the UK having died and more than 150 had been infected with E. coli after eating prepackaged lettuce, and we urged people to thoroughly wash the lettuce out of the packet. Today we advise you not to buy it at all.
The authors of the “salty” report are scientists from the University of Leicester. For the purposes of the study, scientists recreated the conditions in sealed foil bags in which ready-made lettuce mixtures are sold in stores. So it was humid and anaerobic, which favored the release of juices by the cut leaves. The authors pointed out, inter alia, on the rate of multiplication of the bacterial colony in such conditions and the mobility of individual bacteria.
It turned out that after five days of storing tightly packed lettuces in the refrigerator, the number of salmonella bacteria increased from 100 to 100.
In addition, the juices secreted by the cut lettuce favored the formation of biofilm on the leaf surface. As the authors explain, biofilm helps various bacteria to permanently attach to the surface on which they are located, e.g. to medical implants, stainless steel or tooth enamel. It was no different in this case: the biofilm produced by the salmonella sticks on the leaves of lettuce caused the bacteria to strongly bind to their surface. The bacteria were thus impossible to remove even by washing thoroughly under running water.
At the same time, it was noticed that the usual bacterial flora, which normally lives on lettuce, did not acquire the skills similar to salmonella: it did not bind permanently to the leaf surface and did not significantly increase in number in response to juices secreted by damaged leaves.
Of all the leafy vegetables, salmonella grows most preferably on spinach
“Lettuce is a vegetable that is usually grown in open fields, where there is a risk of infection with salmonella bacteria, for example through insects, manure or bird droppings. And while these types of salmonella outbreaks are not common, they pose a significant public health problem, ‘says Dr Primrose Freestone, co-author of the study.
“In such situations it does not help to irrigate the crops with clean water, rinse the leaves, etc. – adds the scientist. – In fact, from such procedures, ie rinsing with recycled water, you can even transfer the contamination to previously healthy crops.
“Interestingly, previous studies have shown that salmonella are so strongly attracted to the juices produced by the leaves and roots of lettuce that they can enter the vascular system of the germinating plant through them. Once inside, they cannot be removed, ”says Freestone.
The scientist also explains that of all leafy vegetables, salmonella grows most willingly on spinach. “Our pathogen is definitely spinach-loving” – jokes the researcher.
She adds that pre-washed, sliced and wrapped lettuces are more and more commonly available on store shelves, and customers are more and more willing to use this solution. Also in fast-food restaurants or in dishes served on board airplanes, pre-prepared salad mixes are used, which wait in refrigerators for at least a few days. Meanwhile, very little research has been done on how salmonella behaves when stored under these conditions.
“We wanted to investigate what is happening to a colony of salmonella in a foil bag containing torn lettuce leaves to better understand the potential risk to consumers and prepare the ground for future research to develop ways to reduce the binding strength of bacterial bacilli to leaf surfaces,” concludes Dr. Freestone.
In July 2016. We wrote about two people who died of poisoning after eating a prepackaged salad mix. We quoted the BBC, which stated that as many as 144 people got poisoned, more than 60 patients required hospitalization, and that the cause of the infections was most likely the pathological E. coli bacteria marked with the symbol 0157: «Public Health England claims that infections with this bacterium are rare happen compared to other foodborne infections. The organization suggests, however, that prepackaged lettuce, vegetables and fruit should be carefully washed to reduce the risk of food contamination. It may happen that pathogenic bacteria penetrate into these products.
The first symptoms of poisoning usually appear 3-4 days after infection, but sometimes they become manifest later, even after two weeks.
British journalist Joanna Blythman in her 2015 book Swallow This: Serving Up The Food Industry’s Darkest Secrets writes that even careful washing of lettuce and vegetables does not always guarantee avoiding contamination.
Blythman says this is due not only to how it is packaged and stored, but also to how the plants are grown. While growing, lettuce requires a lot of water. Therefore, if water contaminated with pathological bacteria, such as E. coli, Listeria or Salmonella, is used for irrigation, they can penetrate into the lettuce cells. In such a situation, careful washing will not remove them.
We are unable to sterilize all the vegetables and fruits we eat. There will always be some risk. However, by following the basic rules of hygiene, we can significantly reduce the likelihood of food poisoning. And when it comes to pre-packaged lettuces, it might be better to rule out the risk altogether and simply not to buy or order them in restaurants ».