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

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. 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. 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.