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INTRODUCTION. 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.
MATERIAL AND METHODS. The objective of present study was to investigate the actual levels of resistance to glyphosate in diverse isolates of Escherichia coli and Salmonella spp., and its ability to induce resistance.
Minimal inhibitory concentrations (MIC) of glyphosate and the glyphosate containing herbicide product Roundup™ were determined using a broth microdilution method for 120 Salmonella spp. and 113 E. coli isolates. Two strains of E. coli (ESBL and non-ESBL) were further passaged daily in gradually increasing concentrations of glyphosate and Roundup.
RESULTS. The MIC values ranged within 40-80 mg/ml for glyphosate and Roundup in Salmonella spp. and 5-10 mg/ml for glyphosate and 20-40 mg/ml for Roundup in E. coli. Antibiotic resistance status, host species, geographic location, phylogroup or time of isolation relative to the introduction of glyphosate did not affect the levels of resistance. Resistance induction response for Roundup was similar in two E. coli strains, with early extinctions of bacterial populations at 2x MIC. However, there was an increased level of tolerance at 2x MIC to glyphosate in both strains.
DISCUSSION. Our results demonstrate differences in glyphosate sensitivity between E. coli and Salmonella, which were not affected by the antibiotic resistance status or isolate origin. Although glyphosate resistance does not occur easily, at least in our experimental setting, it is possible to select for increased tolerance. Further investigations are necessary to determine the ubiquity and mechanism of this phenomenon, as well as relevance to animal husbandry.