Sessions

Erysipelothrix rhusiopathiae is a clinically relevant zoonotic pathogen that may cause septicemia and substantial production losses, particularly in waterfowl. Integrated phenotypic and genomic information on antimicrobial resistance (AMR) in poultry-derived isolates remains limited.

In this study, we characterized 38 E. rhusiopathiae isolates recovered from clinical poultry cases in Hungary in 2024 (32 geese, 2 ducks, and 4 turkeys). Minimum inhibitory concentrations for 18 antimicrobial agents were determined by broth microdilution.

Nineteen isolates showing reduced susceptibility were subjected to Illumina whole-genome sequencing followed by resistome, mobile genetic element, genomic island, and phylogenomic analyses. For penicillin, amoxicillin, ceftriaxone, cefotaxime and ceftiofur, no isolate met the applied resistance criterion. The highest resistance frequency was observed for oxytetracycline (10.5%), whereas resistance to doxycycline and florfenicol was 5.3% and to tylosin and tiamulin 7.9%. Among the sequenced isolates, tetM was detected in nine genomes, while tetT and erm(47) occurred in two genomes each. Tetracycline and macrolide phenotypes showed strong genotype-phenotype concordance. tetM was consistently located in a chromosomal genomic context close to Tn6009-associated mobile genetic elements, while erm(47) was located within 10 kb of ISErh6. In isolate 99790, a 64,833-bp genomic island carried ermC. The 19 genomes formed four descriptive phylogenetic clusters, and tetM was enriched in the largest cluster; however, this pattern should not be interpreted as direct evidence of clonal spread without recombination-aware and epidemiologically linked analyses.

Overall, β-lactam activity remained preserved, but mobile-element-associated AMR determinants were already present in poultry-derived E. rhusiopathiae in Hungary. These findings support standardized phenotypic surveillance combined with genomic monitoring to identify emerging resistance before widespread dissemination.