Antimicrobial Resistance to WHO-Categorised Critically Important Antimicrobials in Piglet-Derived Escherichia coli and Enterococci: A One Health Approach

Antimicrobial resistance (AMR) is a critical health issue affected by human, animal, and environmental factors within a One Health framework. Livestock production, particularly pig farming, contributes to the emergence and spread of resistant bacteria through antimicrobial use and selective pressure. This study characterized the antimicrobial resistance profiles of Escherichia coli and enterococci isolated from piglet faeces in Portugal, with emphasis on antimicrobials classified by the World Health Organization (WHO) according to their importance for human medicine. A total of 340 E. coli and 130 Enterococcus isolates were tested using disk diffusion, and the resistance frequencies were calculated at isolate level. E. coli showed high resistance to commonly used antimicrobials, including amoxicillin–clavulanic acid (67.1%) and ciprofloxacin (43.2%), a highest priority critically important antimicrobial (HPCIA). Resistance was also found to human-only drugs such as piperacillin (67.4%) and, at a lower frequency, imipenem (2.6%), a last-resort carbapenem. Multidrug resistance was detected in 61.2% of isolates. Among enterococci, E. faecium exhibited high resistance to imipenem (64.4%), while E. faecalis and other Enterococcus species showed notable resistance to ciprofloxacin. Overall, resistance crossed all WHO antimicrobial categories. This finding highlights the potential role of pig production in the dissemination of AMR across ecosystems and suggests the need for strengthened One Health surveillance and antimicrobial stewardship efforts.