Characterization of Actinobacillus pleuropneumoniae isolates from pigs in Piedmont, Italy, by whole-genome sequencing: insights into antimicrobial resistance.

BACKGROUND: Actinobacillus pleuropneumoniae (APP) remains a major respiratory pathogen in swine production worldwide. The limited cross-protection of available vaccines and the increasing occurrence of antimicrobial resistance (AMR) highlight the need for integrated phenotypic and genomic surveillance to support effective local control strategies. METHODS: Twelve APP isolates previously collected in Piedmont (Italy) were investigated. Antimicrobial susceptibility was assessed by minimum inhibitory concentration (MIC) testing against a panel of antimicrobials commonly used in swine medicine. Whole-genome sequencing was performed using Illumina technology, followed by resistome profiling and comparative genomic analyses. Virulence-associated genes, including apx toxin genes and capsular polysaccharide loci, were also characterized. RESULTS: Phenotypic testing revealed resistance to tetracycline, macrolides, β-lactams, tiamulin, and sulfamethoxazole/trimethoprim in a subset of strains. Genomic analysis identified tetracycline- and phenicol-resistance genes (tet, flor) in one isolate, while genes associated with macrolide and elfamycin resistance were widespread across the dataset. All isolates carried major apx toxin genes, with variable distribution of apxII and apxIII operons. A serovar 9/11 isolate showed a partial deletion within the capsular polysaccharide biosynthesis locus, confirmed by PCR and sequencing. Comparative genomics also suggested distinct genomic lineages among serovar 6 isolates. CONCLUSION: The combined phenotypic and genomic approach provides valuable insights for evidence-based antimicrobial use and provides additional genomic information on clinically relevant APP field isolates and supports future studies on AMR and genomic diversity.