Copper and Antimicrobial Residues in the Liver and Kidney—Antimicrobial Resistance and Cu Tolerance Unrelated in Escherichia coli from Piglets’ Faeces

Antimicrobials, widely used in livestock, have induced the emergence of antimicrobial-resistant bacteria, prompting farmers to explore alternatives like copper. This study aims to determine antimicrobial residues and Cu concentrations in the liver and kidney of piglets and to investigate the correlation between Cu and antimicrobial use and the resistance to Cu and antimicrobials of <i>Escherichia coli</i> isolated from piglets' faeces. Antimicrobial residues were quantified by UHPLC-ToF-MS; Cu was quantified using FAAS; microbiological methods were used for <i>E. coli</i> isolation, CuSO₄ minimal inhibitory concentration (MIC), and antimicrobial susceptibility; and to detect genes, Real-Time PCR was used. Cu concentrations and antimicrobial residues in piglet livers and kidneys revealed no significant differences. Antimicrobial residues were detected in a significant number of livers and kidneys. While Cu concentrations in the liver were within adequate ranges, those in the kidney exceeded the recommended levels. <i>E. coli</i> isolates from piglet faeces exhibited high antimicrobial drug resistance (AMR), with no clear link to Cu exposure. The genes <i>cop</i>A, <i>pco</i>A, and <i>pco</i>D, associated with Cu tolerance, were predominantly found in isolates with a CuSO₄ MIC of 8 mM. Cu was not used excessively, suggesting that Cu did not replace antimicrobials. <i>E. coli</i> was mostly resistant to antimicrobials and it was not possible to demonstrate that Cu was the trigger for this resistance. There was no relationship between Cu tolerance and AMR in <i>E. coli</i> isolates. This study highlights the need for further research on the complex interplay between metals, antimicrobials, and bacterial resistance in livestock, impacting 'One Health'.