Phenotypic and genomic features of virulent mecA-carrying Staphylococcus hyicus isolated from a pig farm with fatal exudative epidermitis

Staphylococcus hyicus is recognized as one of causative agents of porcine exudative epidermitis in piglets. However, research on clinical pathogenic S. hyicus remains limited. In this study, multidrug-resistant S. hyicus was isolated from a large-scale pig farm with cases of fatal exudative epidermitis in piglets. By characterizing the phenotypes and genotypes of S. hyicus isolates, we provide insights for clinical management of exudative epidermitis. Two 21-day-old piglets with clinical signs underwent necropsy and histopathological examination. 48 samples from lactating sows and suckling piglets were collected for pathogen identification. Antimicrobial and disinfectant susceptibility of the isolates was determined using broth microdilution, as well as whole-genome sequencing (WGS) was used to identify antimicrobial resistance genes. Moreover, comparative genomic analysis with public genomes in the Genbank database was performed. The virulence of mecA -carrying S. hyicus was evaluated using the Galleria mellonella infection model. Exudative epidermitis was first observed in 3-day-old piglets. Through necropsy, pathogen isolation, and other diagnostic approaches, S. hyicus was preliminarily identified as the primary causative agent of this episode, and 23 S. hyicus were isolated from samples. Antimicrobial susceptibility testing indicated most of isolated S. hyicus were susceptible to doxycycline, vancomycin and linezolid, while resistant to florfenicol, erythromycin, spectinomycin, amoxicillin, ceftiofur, enrofloxacin that are commonly used for pigs. Twelve resistance genes were identified by WGS, including aadD , ant(6)-Ia , aph(2'')-Ia , blaZ , erm(B) , erm(C) , fexA , lnu(B) , lsa(E), mecA , tet(L) , and tet(M). Compared with 39 GenBank genomes, mecA -carrying S. hyicus in our study carried more resistance genes and exhC islands closely related to previous Chinese strains. Based on our findings, we implemented a targeted treatment protocol that brought the outbreak under control and reduced piglet mortality to below 5%. This study successfully isolated pathogenic mecA -carrying S. hyicus responsible for high mortality in piglets, characterized its drug resistance phenotypes and genotypes, offering valuable insights for the prevention and control of this pathogen in piglets.