Antimicrobial resistance in Salmonella enterica serovar Choleraesuis isolated from pigs in northern Italy.

Salmonella enterica subspecies enterica serovar Choleraesuis (S. Choleraesuis) is a pathogen known to cause severe systemic infections in pigs and occasionally in humans. Despite the relevance of S. Choleraesuis due to its re-emergence in pig herds in recent years in the EU, little is known about its antibiotic resistance profile. The aim of this study was to evaluate the antimicrobial susceptibility of 188 S. Choleraesuis strains isolated from clinically diseased pigs in Northern Italy between 2022 and 2024. Antimicrobial susceptibility testing to a panel of antimicrobials was evaluated using the Minimal Inhibitory Concentration (MIC) method according to CLSI procedures. Phenotypic resistances to sulfisoxazole (90.2 %), ampicillin (85.0 %), enrofloxacin (83.5 %), florfenicol (73.9 %), tetracycline (71.3 %), trimethoprim-sulfamethoxazole (59.1 %) and flumequine (44.2 %) were the most frequently observed in the S. Choleraesuis population. Among the 188 strains, 59 isolates of S. Choleraesuis displaying multidrug-resistant (MDR) profiles were subject to Whole Genome Sequencing in order to detect possible genetic determinants responsible for the observed MDR profiles and to evaluate the correspondence between the observed phenotypic resistance to antimicrobials and the in-silico prediction. The most frequently detected genetic determinants in the MDR isolates were blaTEM-1B, tet(A), floR, sul2 and sul1. A strong agreement between phenotypic and genotypic resistance for ampicillin, florfenicol, tetracycline and sulfamethoxazole was observed, while low agreement was observed for colistin and trimethoprim-sulfamethoxazole. While similar investigations have been extensively conducted for other Salmonella serotypes, data on antimicrobial resistance in S. Choleraesuis are still limited. Therefore, further studies are needed to better understand antimicrobial resistance in this serotype.