Characteristics of Staphylococcus epidermidis Isolated from Humans and Animals

Staphylococcus epidermidis is an important opportunistic pathogen and a potential reservoir of antimicrobial resistance determinants, yet comparatively little is known about isolates originating from both humans and companion animals. This study characterized 320 S. epidermidis isolates obtained from humans, cats, and dogs within a One Health framework using phenotypic, molecular, proteomic, and genomic approaches. Antimicrobial susceptibility testing revealed high frequencies of resistance to oxacillin (67.19%), erythromycin (66.56%), tetracycline (55.63%), and clindamycin (53.75%) by disk diffusion. PCR-based analysis identified blaZ as the most prevalent resistance determinant (50.63%), whereas whole-genome sequencing of a selected subset revealed a broader and heterogeneous repertoire of resistance-associated genes. Among 67 sequenced isolates, 26 known sequence types were identified, including several STs occurring in isolates from more than one host species. Core-genome phylogenetic analysis demonstrated substantial genomic diversity without complete segregation according to host species or clinical status. Genotype–phenotype concordance was generally limited and gene dependent, and discrepancies were also observed between PCR- and WGS-based detection for selected resistance determinants. Biofilm formation was significantly influenced by incubation temperature, host species, and clinical status, while growth kinetics showed pronounced strain-specific variability and no consistent relationship with biofilm-forming capacity. MALDI-TOF MS profiling and partial rpoB phylogeny likewise showed considerable overlap among isolates from different host groups. In the Galleria mellonella infection model, mortality was inoculum dependent and varied substantially among strains. Overall, the findings demonstrate marked phenotypic and genomic heterogeneity among S. epidermidis isolated from humans and companion animals and indicate that some clonal backgrounds and resistance determinants occur across host species, highlighting the relevance of this species within a One Health context.