Abstract
We investigated whether the maximum residual levels of trimethoprim permitted in food (Acceptable Daily Intake-ADI) could select for de novo trimethoprim resistance in Escherichia coli in vivo. We designed chronic infection models of E. coli in Galleria mellonella and exposed them to sub-ADI doses of trimethoprim through a single-dosing regimen. The emergence of trimethoprim resistance was determined by isolating the target bacteria on selective agar plates, followed by species confirmation using MALDI-TOF mass spectrometry. The minimum inhibitory concentration (MIC) was assessed via the E-test to determine E. coli susceptibility to trimethoprim. Notably, exposure to as low as one-tenth of the ADI dose through a single-dosing regimen resulted in the selection of trimethoprim-resistant E. coli. Our findings indicate that trimethoprim doses ten-fold lower than the established ADI threshold could induce resistance to trimethoprim in E. coli. These results highlight the importance of considering antimicrobial resistance induction as a key factor when determining ADI levels in food.