Emergence of multidrug-resistant Staphylococcus epidermidis in Nile tilapia (Oreochromis spp.): virulence, antimicrobial resistance, and nanoparticle-based control.

BACKGROUND: Staphylococcus epidermidis represents an emerging zoonotic threat impacting aquatic ecosystems, livestock, and human health. This study investigated the causative agent behind summer mortality episodes affecting cultured Oreochromis spp. (180 ± 20 g) in Egyptian fish farms, where diseased specimens exhibiting characteristic ulcerative dermatopathy were collected from Ismailia governorate. RESULTS: Comprehensive diagnostics excluded TiLV and NNV, while bacteriological analysis identified Gram-positive cocci producing distinctive white hemolysis-negative colonies on blood agar and red colonies on mannitol salt agar. Polyphasic characterization confirmed the isolates as S. epidermidis (16 S rRNA GenBank MN153038), marking the first genomic record of this pathogen in tilapia. Antimicrobial profiling revealed alarming multidrug resistance (54.5% of tested agents), including resistance to β-lactams (oxacillin, ampicillin, and cefoxitin) that suggests possible methicillin-resistant (MRSE) phenotypes, despite retained vancomycin susceptibility (MIC = 4 µg/mL). Controlled challenge trials demonstrated dose- and route-dependent virulence, with scale removal during immersion exposure precipitated 95% mortality in fingerlings (2.3 ± 0.75 g) versus 55% in intact fish. In contrast, intraperitoneal injection caused 40–50% mortality in adults/juveniles. Notably, silver nanoparticles (AgNPs) exhibited size-dependent antimicrobial activity: 10-nm AgNPs showed superior efficacy (MIC = 1.25 µg/mL; MBC = 2.5 µg/mL) compared to 100-nm AgNPs (MIC = 10 µg/mL) and zinc oxide nanoparticles (MIC = 125 µg/mL). The enhanced activity of smaller AgNPs is attributed to their greater surface area and improved biofilm penetration. CONCLUSION: These results highlight S. epidermidis as an emerging threat in tilapia aquaculture, particularly given its multidrug resistance. The demonstrated efficacy of AgNPs, especially at smaller particle sizes, offers a promising alternative for controlling such resistant infections in aquaculture settings.