What’s lurking in your lettuce?

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When people are trying to eat more nutritiously, one of the first things they go to is salad. The variety of ready-to-eat salads has greatly expanded in the past few years to accommodate consumer demand. Unfortunately, fresh produce such as salad, may be contaminated with bacteria. According to Dr. Kornelia Smalla, a professor from the Julius Kuhn Institute (JKI), contaminated salads may be exposed to bacteria resistant to antibiotics. Professor Dr. Georg Backhause, president of Julius Kuhn Institute notes this type of bacteria is often found in manure, sewage sludge, soil and bodies of water and can pose a health risk to consumers. 1

In order to evaluate the total quantity of transferable antimicrobial resistant genes (termed the “transferable resistome” by scientists), researchers bought samples of mixed greens, arugula and cilantro in German grocery stores to evaluate Escherichia coli, intestinal bacteria considered mostly harmless, on these foods. Researchers honed in on E-coli bacteria that are resistant to the antibiotic tetracycline as this antibiotic is used in livestock farming, and can promote the production and growth of resistant bacteria in organs like the intestine. E-coli as well as portions of the antibiotics are passed by animals then migrate to fields through organic fertilizers like manure. According to Smalla, "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 analyzed foods." The president of the German Federal Institute for Risk Assessment (BIR), Dr. Andreas Hensel notes the urgency in discovering the health risk for consumers. 1

While the bacteria are harmless on vegetables, once consumed, the bacteria can distribute their plasmids to other pathogenic bacteria living in the intestine known as horizontal gene transfer. This type of gene transfer allows bacteria to quickly adapt to changing environmental settings. In patients treated with antibiotics, this type of bacteria that has been incorporated into transferrable resistant genes can grow in favor of less viral bacteria. While salad is limited in E-coli contamination, it is unknown how often resistance genes are moved around in the human intestine. Little is also known of the extent to which diseases are due to resistant bacteria. 1

To be safe, raw vegetables, leafy salads and fresh herbs should always be washed thoroughly with clean water prior to eating to limit the risk of consuming pathogens or antimicrobial-resistant bacteria.

Women that are pregnant, the elderly and individuals with compromised immune systems should avoid eating pre-cut, ready-to-eat salads as a precaution to prevent food-borne infections. Salads should instead be prepared fresh using thoroughly washed vegetables.

Unfortunately, disease pathogens and antimicrobial resistant bacteria in vegetables is not always reliably washed away. In some rare cases, immunocompromised individuals may want to heat vegetables and fresh herbs to and internal temperature of ~160 degrees Fahrenheit. 1

To reduce risk of eating contaminated vegetables, consumers can:

  • Pay attention to CDC (Center for Disease Control) recalls through the CDC web site https://www.cdc.gov
  • Limit use of pre-packaged salads
  • Wash and spin produce well prior to consumption
  • Steam or grill fresh vegetables to destroy harmful bacteria
  • Use lightly steamed, crisp veggies in a salad and toss with seasonings, oil, and vinegar for a delightful and safe way to enjoy veggies


  1. Khald Blau, Antje Bettermann, Sven Jechalke, Eva Fornefeld, Yann Vanrobaeys, Thibault Stalder, Eva M. Top, Kornelia Smalla. The Transferable Resistome of ProducemBio, 2018; 9 (6) DOI: 1128/mBio.01300-18

Submitted by Lisa Andrews, MED, RD, LD





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