Sessions

Antimicrobial resistance (AMR) represents one of the most serious challenges to global public health, with the use of antibiotics in animal husbandry playing a significant role in its emergence. Escherichia coli is a key indicator bacterium in AMR monitoring, as it can act both as a pathogen and as a carrier and disseminator of multiple resistance genes. The aim of the present study was to characterize the resistance profiles of multidrug-resistant E. coli strains isolated from large-scale pig farms in Hungary, with particular emphasis on extended-spectrum β-lactamase (ESBL) production.

A total of 203 strains were tested phenotypically, of which 110 were β-lactamase producers and 127 were confirmed as ESBL producers. Next-generation sequencing (NGS) was successfully performed on 116 isolates, leading to the identification of 82 distinct resistance genes with a total of 5,427 occurrences. Among the ESBL-encoding genes, members of the CTX-M and TEM families were detected, while in phenotypically positive but genotypically ESBL-negative strains, β-lactamase production was likely explained by overexpression of ampC and ampH genes.

Other major resistance mechanisms included aminoglycoside-modifying enzymes (aadA, APH), tetracycline resistance genes (tet(A), tet(B)), fluoroquinolone target protection (qnrB5) and efflux pumps, as well as chromosomal (pmrF, eptA, ugd) and plasmid-mediated (mcr-1) resistance to polymyxins. Resistance to trimethoprim-sulfamethoxazole was primarily associated with dfrA1, dfrA5, sul1, and sul2 genes, whereas macrolide and phenicol resistance was mainly linked to mobile genes such as mphA, mphB, floR, and catI. The widespread chromosomal presence of efflux pump and regulatory genes highlights the integrated role of intrinsic resistance mechanisms and environmental stress responses.

Overall, our findings demonstrate that multidrug-resistant E. coli populations present in Hungarian pig herds carry a complex genetic network in which mobile resistance genes and chromosomal adaptive mechanisms act synergistically to ensure bacterial survival. This combination poses not only a challenge for veterinary medicine but also a potential zoonotic risk, underscoring the importance of continuous monitoring and the prudent reduction of antibiotic use.