Vegetables are good for you but raw vegetables — and especially packaged salads — may be at risk of contamination by antibiotic-resistant bacteria that may be especially dangerous to consumers with compromised immune systems, German researchers who recently studied the problem say.
For their study, scientists purchased mixed salads, arugula and cilantro in German supermarkets. The samples were then analyzed to determine whether they contained antimicrobial resistance genes. They focused on E-coli, a mostly harmless bacterium found in sewage and runoff from farms but one that can become resistant to the antibiotics that are often used in large livestock operations
“The results of the comprehensive tests clearly show that a wide variety of transferable plasmids — gene carriers in bacteria that occur outside the chromosomes — have been found with resistance genes in the E. coli from fresh produce. Each of these plasmids carries resistance to multiple classes of antibiotics. E. coli bacteria with these properties have been found on all three analysed foods,” said Dr. Kornelia Smalla from the Julius Kühn Institute.
If these bacteria — though harmless themselves — occur on vegetables, they can enter the human intestine through the consumption of raw vegetables. Once ingested, the bacteria can pass on their plasmids to any other bacteria that may be present in the intestine, a process known as horizontal gene transfer. If a patient is treated with antibiotics, they may not be effective against bacteria that have incorporated these kinds of transferable resistance genes into their genome.
It is not known how frequently resistance genes are transferred in the human intestine and there is also little knowledge as to whether and to what extent diseases are caused by such resistant bacteria.
“We have to get to the bottom of these findings”, said Dr. Georg Backhaus, President of the Julius Kühn Institute.
Chlorine’s role in doubt
Another recent study found that chlorine, commonly used in agriculture to decontaminate fresh produce, can make food-borne pathogens undetectable. The study may help explain outbreaks of Salmonella enterica and Listeria monocytogenes among produce in recent years.
“This important work is a major breakthrough, after 100 years of relying on chlorine to sanitize foods and drinking water, and may explain the many unrecognized or untraceable disease outbreaks relying on the gold standard of culture recovery,” said lead study author Bill Keevil, Ph.D., chair in environmental healthcare within Biological Sciences and head of the Microbiology Group at the University of Southampton, United Kingdom.
It’s not just food that is potentially affected by the findings.
“This has big implications for many household and industrial cleaning agents, paints, cosmetics etc. sold with an antimicrobial claim,” said Keevil. “The efficacy of many validated products with a biocidal claim may need to be re-examined when they rely solely on culture recovery techniques [that involve chlorine].
“The problem with fresh produce of course is that many people eat it fresh, so it misses the cooking step which would kill most pathogens, one reason why companies have relied on chlorine washing before sale,” Keevil said.
What to do
Consumers should always wash raw vegetables, leaf salad and fresh herbs thoroughly with drinking water before eating them minimize the risk of ingestion of pathogens or antimicrobial-resistant bacteria.
Pregnant women and people with compromised immune systems as a result of advanced age, pre-existing conditions or medications should additionally refrain from eating pre-cut and packaged salads as a precaution against foodborne infections and should instead prepare salads themselves using fresh and thoroughly washed ingredients shortly before consumption.
However, washing alone is not sufficient to reliably remove the disease pathogens or antimicrobial-resistant bacteria that may be present on vegetables, the researchers cautioned. Therefore, in rare cases it is necessary that especially immuno-compromised persons heat vegetables and fresh herbs sufficiently (at least two minutes to 70°C inside the food) before consumption according to the instructions of their attending physicians.
The study of the Julius Kühn Institute, the University of Giessen and the University of Idaho on antimicrobial-resistant bacteria on plants was published in the October issue of the special-interest journal mBio.