Antibacterial Activity and Safety of Oregano Oil–Lauric Acid Cationic Nanostructured Lipid Carriers in Nile Tilapia (Oreochromis niloticus)

Aquaculture faces increasing challenges from bacterial diseases and the widespread use of antibiotics, which contribute to antimicrobial resistance and environmental concerns. Oregano oil–lauric acid cationic nanostructured lipid carriers (OE-L+NLCs) were developed in this study and their antibacterial activity and safety were evaluated in Nile tilapia (Oreochromis niloticus). The nanoparticles were prepared using a hot homogenization–ultrasonication method and characterized based on their particle size, surface charge, morphology, and release profile. Antibacterial activity was assessed against six isolates of Streptococcus agalactiae using minimum inhibitory and bactericidal concentration assays, while their stability was evaluated under different storage conditions and simulated gastrointestinal environments. In vivo safety was determined through median lethal concentration analysis, survival assessment, and histopathology. The developed OE-L+NLCs showed a mean particle size of 175.90 ± 4.94 nm, positive zeta potential of +44.02 ± 0.54 mV, and low polydispersity index (0.15 ± 0.00). The formulation exhibited sustained carvacrol release (70.46 ± 0.96% over 24 h) and strong antibacterial activity against all six isolates, with MIC and MBC values of 0.25 and 0.25–0.49 mg/mL, respectively. The nanoformulation maintained antibacterial activity during 90 days of storage and under simulated gastric conditions. In vivo toxicity assessment showed that OE-L+NLCs exhibited a higher LC50 value (11.11 mg/mL) than free oregano oil (5.89 mg/mL), together with improved survival and reduced histopathological alterations in Nile tilapia. Overall, the formulation provides an effective and stable delivery platform for natural antimicrobials, offering a promising alternative to antibiotics for sustainable disease management in aquaculture.