Phytobiotic Essential Oils as Antibiotic Alternatives in Aquaculture: Antimicrobial and Antioxidant Properties of Garlic, Thyme, Thyme Conehead, Rosemary, and Eucalyptus

ABSTRACT The increasing threat of antimicrobial resistance in aquaculture demands sustainable alternatives to conventional antibiotics. Plant‐derived essential oils are attractive candidates owing to their antimicrobial, antioxidant, and immunomodulatory activities, yet their volatility and chemical instability hinder incorporation into aquafeeds. This study integrated chemical profiling, bioactivity assessment, and spray‐drying microencapsulation to evaluate five Eos, garlic, thyme, conehead thyme, rosemary, and eucalyptus, as functional feed additives (illustrated in the Graphical abstract below). Gas chromatography–mass spectrometry (GC–MS) revealed distinct chemotypes: garlic oil was rich in propyl propane thiosulfonate (30.3%); thyme oil in thymol (35.1%) and p‐cymene (31.6%); conehead thyme oil in carvacrol (19.9%); rosemary oil in eucalyptol (43.0%); and eucalyptus oil in eucalyptol (68.9%). Antioxidant capacity varied markedly, with conehead thyme (34.7 μmol/mL) and garlic (17.8 μmol/mL) outperforming thyme (12.7 μmol/mL), while rosemary (0.79 μmol/mL) and eucalyptus (0.32 μmol/mL) were weak. Antimicrobial testing showed potent activity for garlic and both thyme oils against Escherichia coli , Pseudomonas aeruginosa , Staphylococcus aureus , and others, with several minimum inhibitory concentrations comparable to or lower than gentamicin. All EOs exhibited antifungal activity, in some cases approaching or exceeding fluconazole. Spray‐drying with maltodextrin or starch produced spherical microcapsules (mean 19–28 μm) with encapsulation efficiencies of ~67%–71% and process yields of 53%–79%. Release in PBS (pH 7.4) was rapid, consistent with porous particle morphology. Overall, spray‐dried EO microcapsules preserved bioactivity and handling stability, supporting their feasibility as natural antibiotic alternatives in aquaculture feeds and motivating in vivo validation.