Retrospective Study on Antimicrobial Susceptibility of Staphylococcus hyicus Strains Isolated in Northern Italy...

Sudden outbreaks of exudative epidermitis (EE) are managed with antibiotics. This retrospective observational study evaluated the antimicrobial susceptibility of Staphylococcus (S.) hyicus strains isolated in Northern Italy between January 2022 and April 2025. Isolates were recovered from clinical cases submitted to IZSLER diagnostic laboratories. To ensure data independence, only one isolate per pig farm or strains from the same farm isolated at least 6 months apart were included in the final analysis. Strains resulted from bacteriological examinations on skin swabs (65) and synovial fluid (1) of pigs with EE. Antimicrobial susceptibility was assessed by determining the MIC of 16 antimicrobials in broth microdilution. MIC₅₀ and MIC₉₀ values were determined, and strains were classified as wild-type (WT) or non-WT according to EUCAST ECOFFs. Each antimicrobial showed different sub-populations with reduced susceptibility. MIC₅₀ and MIC₉₀ values were identical or differed by one dilution for most antimicrobials. The highest ratios of non-WT strains were penicillin (97%), tetracycline (91%), and clindamycin (90%). Conversely, non-WT strains ranged between 3 and 6% for oxacillin, cefazolin, ceftiofur, and rifampin.