Antimicrobial resistance in commensal Escherichia coli and Enterococcus spp. isolated from pigs subjected to different antimicrobial administration protocols.

The antimicrobial resistance (AMR) in human and animal pathogens is a global concern, and antimicrobial use (AMU) is considered the most important driver for its increase. The aim of this study was to assess AMR in Escherichia coli and Enterococcus spp. in faecal samples of pigs subjected to four different AMU protocols from birth to finishing: G1, no in-feed antimicrobials; G2: a total average dose 6018 mg antimicrobials/pig; G3: a total average dose 8127 mg antimicrobials/pig; and G4: a total average dose 15,678 mg antimicrobials/pig. Faecal samples were collected at six time points and AMR was assessed in both bacteria. The microbiota composition was assessed by 16S rRNA sequencing. Minor differences on the microbiota profile was observed among groups, but a lower Firmicutes:Bacteroidetes ratio was noted in G4. Escherichia coli and Enterococcus spp. strains isolated from all groups showed a high level of multi-drug resistance (MDR). The amount of antimicrobials used was significantly positively associated with the probability of MDR in both bacteria. Approximately 43% of the variation in MIC90 for colistin could be explained by AMU, and a one-day increase in administration of colistin increased MIC90 by 0.05 μg mL-1. In conclusion, the results suggest that the higher the use of antimicrobials in farms, the higher the MDR frequency and resistance to the highest priority critically important antimicrobials for humans in commensal gut bacteria of pigs.