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Background and objectives: Residues of glyphosate, the most used herbicide in the world, are commonly found in the environment and food supply chain. Recently, its effects on microorganisms and antibiotic resistance have been recognised, raising concerns about the effects of glyphosate in animal feed on microbiome. The objective of this study was to investigate the ability of glyphosate to induce resistance in vitro in Escherichia coli isolated from farm animals.
Materials and methods: We used two strains of E. coli (with and without ESBL resistance markers). After initial determination of the minimal inhibitory concentration (MIC), we passaged them daily in gradually increasing concentrations of glyphosate alone and as a part of formulation (Roundup LB plus). To assess the stability of resistance, we determined the final MIC after the stability passage (in the absence of glyphosate).
Results: Resistance induction response for Roundup was similar for ESBL and non-ESBL E. coli strains, with early extinctions of bacterial populations at 2x MIC. The ESBL strain was also unable to grow at concentrations >MIC, whereas the non-ESBL strain readily adapted to growth at 2-4x MIC of glyphosate.
Conclusion: Our results demonstrate that there are differences between glyphosate alone and as a part of herbicide formulation and individual bacterial strains in the ability to induce resistance. Overall, although it is not easy to induce resistance to glyphosate, it is nonetheless possible.