Completed project: Resistance on lettuce plants

Antibiotic resistance can spread from soil, water and slurry to cultivated vegetables. A study shows the risks at every stage from planting to harvesting.

Antibiotic-resistant bacteria occur in the environment, in places such as soil, bodies of surface water and organic fertilisers used in agriculture. They are presumed to pass from these “reservoirs” to humans via plant-based food. To break these transmission routes, it is first necessary to understand the role played by environmental reservoirs. Using lettuce as a representative system for plant products, Jörg Hummerjohann and other scientists at the Agroscope research institute have investigated these processes. Their goal was to obtain an insight into the interface between the environment and the food chain and to observe how antibiotic-resistant bacterial load or individual resistance genes develop during the growth phase through to harvest.

Slurry use has definite impact

The researchers conducted experiments with both outdoor and greenhouse lettuce. They grew the lettuce in the conventional away, but in different soils – with and without slurry, with river water and with sterilised water. Regardless of the soil or water type during the early growth phase, the outdoor lettuce plants had similarly low levels of antibiotic-resistant bacteria and resistance genes. Furthermore, these levels declined as the plants grew. Using slurry made a difference since it resulted in higher resistance gene loads. However, these also declined over time, falling to similar values to those in plants that had not been manured.

Pre-harvest waiting times reduce risk

With the greenhouse plants, the researchers also investigated the effect of slurry and water that had first been contaminated with multidrug-resistant (ESBL-producing) E. coli bacteria. In both cases, the resistant bacteria were transferred to the seedlings. While they were only detectable for a few days, their clinically most significant resistance genes were still on the plants up to four weeks after transmission. The researchers therefore recommend observing strict waiting times if non-potable water or slurry is applied to plants, since both may contain multidrug-resistant E. coli bacteria of the type investigated in the project. This preventive measure is important for growers of crops such as lettuce which are eaten raw, because – unlike products that are cooked – any antibiotic-resistant bacteria that might be present are not killed off during preparation.

May 2022