Antibiotics are increasingly losing their medical effect. This is down to an increase in antibiotic-resistant bacteria occurring not only in humans and animals, but also on foodstuffs. Agroscope is investigating how antibiotic resistance can be transferred to plant-based foods.
Vegetable foodstuffs such as fresh salads are subject to numerous risks of contamination with undesirable microbes during production. Bacteria can, for instance, be transferred to the plants via organic fertilisers, the soil, animals, or irrigation water. Consumers ingest numerous bacteria by eating fresh raw products. Most of these bacteria are harmless, or even useful, for humans. Others, however, are undesirable, such as disease-causing salmonellae, enterohaemorrhagic E. coli (EHEC) and listeria, as well as antibiotic-resistant bacteria.
Antibiotic-resistant bacteria are of course widespread in the environment. In recent decades, however, their numbers have multiplied owing to the increased clinical use of antibiotics. Increasingly, so-called multiresistant bacteria, which are impervious to three or more classes of antibiotics, can also be found in the environment.
Focus on Irrigation
As part of the Agroscope Research Programme REDYMO, we are therefore investigating how antibiotic-resistant bacteria come to be present on salad plants, devoting particular attention to the antibiotic-resistant gut bacteria E. coli and enterococci, both of which serve as faecal indicators. Special focus is placed on irrigation water. After sampling plants and irrigation water, resistant bacteria are cultured in the laboratory on nutrient media with various antibiotic additives. Afterwards, the resistance of the isolated bacteria to up to 32 clinically important antibiotics is determined. Several multiresistant E. coli strains have so far been identified in this way.
Confirming Suspicions
To determine whether irrigation water contributed to the appearance of resistant gut bacteria on plants, bacterial isolates from water and plant samples with the same antibiotic resistance pattern underwent genetic analysis in the form of multi-locus sequence typing (MLST). This method is used to determine the origin of the bacteria, e.g in order to identify the responsible foodstuff in the case of a foodborne disease outbreak. These analyses have demonstrated that the irrigation water represents a source of contamination with antibiotic-resistant E. coli on plants.
Avoidance Measures
Agroscope experts conducted this study in partnership with the industry, specialist agencies and commercial farms. The occurrence of E. coli and enterococci in the irrigation water is investigated in various Swiss growing regions on salad plants, from cultivation through to the harvest product and processed cut salads. Based on this, measures for avoiding contamination of fresh and cut salads were summarised for vegetable growers in an Agroscope factsheet, coming soon in 2017.
Research Continues
Further studies are to be launched as part of the National Strategy for Antibiotic Resistance (StAR) and the Swiss National Science Foundation (SNSF)-financed National Research Programme ‘Antimicrobial Resistance’ (NRP72). The aim: to investigate additional contamination sources, such as the soil, or organic fertilisers used in agriculture.
From early 2017, Agroscope experts have been researching what resistances and bacteria from the soil, water or fertilisers are transferred to salad plants up to harvest, to what extent this occurs, and which resistances survive and reach humans. This should allow recommendations to be developed for agricultural practitioners, for monitoring programmes and for official guidelines. In this way, Agroscope hopes to contribute to a reduction in antibiotic resistance in the environment and agriculture, as well as to reduced transfer to the food chain.
