Salads are popular foods for people who strive to eat a balanced and healthy diet. These consumption goods are often being sold pre-cut and packaged in plastic containers. Fresh produce like this is known to sometimes be contaminated with hygiene related germs. A research team led by Prof. Dr. med. Kornelia Smalla from the Julius Kühn Institute (JKI) has now proven that there are also germs on these articles that are resistant to antibiotics.
"We need to get to the bottom of this finding," says Dr. Georg Backhaus, President of the Julius Kühn Institute. It is known that antibiotic-resistant bacteria can occur in manure, sewage sludge, soil and bodies of water. "This alarming evidence for these bacteria on the plants is concurrent with similar findings in other food stuffs," adds Prof. Dr. med. Dr. Andreas Hensel, President of the Federal Institute for Risk Assessment (BfR). "What this means with regard to consumer health risks is now a priority question."
For the investigations, Professor Smalla's research team acquired mix salads, rocket and spice plant coriander from various German supermarkets. The samples were then examined to determine the totality of the transmissible antibiotic resistance genes (the researchers speak of the 'transferable resistoma') in Escherichia coli, a mostly harmless intestinal bacterium on these foods. The experts focused on the part of the Escherichia coli bacteria that is resistant to tetracycline. This is because tetracycline antibiotics are used in animal husbandry, where they can promote the development and propagation of resistant germs, for example in the intestines of farm animals.
Not only these germs, but also traces of these antibiotics are excreted, eventually ending up on the fields through organic fertilizers such as manure. Smalla's conclusion: "The results of the extensive investigations clearly show that a considerable variety of transferable plasmids, that is, non-chromosomal genetic carriers in bacteria, with resistance genes in the E. coli, can be found on fresh produce. These are resistant to several classes of antibiotics. E. coli bacteria with these properties were found on all three food classes that were tested."
If such naturally harmless bacteria occur on vegetable food stuffs, they can enter the human intestine when these foods are consumed raw. Once ingested, the bacteria can pass their plasmids in the gut to potentially pathogenic bacteria. This is called horizontal gene transfer.
In nature, horizontal gene transfer enables bacteria to adapt quickly to changing environmental conditions. When a patient is treated with antibiotics, bacteria that have incorporated such transmissible resistance genes into their DNA have an advantage and proliferate more quickly than their competitors. The frequency of transmission of resistance in the human intestine is unknown due to the low occurrence of E. coli on lettuce. There is also a lot to be learned to what extent and how often these resistant bacteria cause diseases.
