Quorum quenching (QQ), the enzymatic degradation of N-acyl homoserine lactones (AHLs), has been suggested as a promising strategy to control bacterial diseases. In this study, 10 AHL-degrading bacteria isolated from the intestine of barramundi were identified by 16S rDNA sequencing. They were able to degrade both short and long-chain AHLs associated with several pathogenic Vibrio species (spp.) in fish, including N-[(RS)-3-Hydroxybutyryl]-l-homoserine lactone (3-oh-C₄-HSL), N-Hexanoyl-l-homoserine lactone (C₆-HSL), N-(β-Ketocaproyl)-l-homoserine lactone (3-oxo-C₆-HSL), N-(3-Oxodecanoyl)-l-homoserine lactone (3-oxo-C₁₀-HSL), N-(3-Oxotetradecanoyl)-l-homoserine lactone (3-oxo-C₁₄-HSL). Five QQ isolates (QQIs) belonging to the Bacillus and Shewanella genera, showed high capacity to degrade both synthetic AHLs as well as natural AHLs produced by Vibrio harveyi and Vibrio alginolyticus using the well-diffusion method and thin-layer chromatography (TLC). The genes responsible for QQ activity, including aiiA, ytnP, and aaC were also detected. Analysis of the amino acid sequences from the predicted lactonases revealed the presence of the conserved motif HxHxDH. The selected isolates were further characterized in terms of their probiotic potentials in vitro. Based on our scoring system, Bacillus thuringiensis QQ1 and Bacillus cereus QQ2 exhibited suitable probiotic characteristics, including the production of spore and exoenzymes, resistance to bile salts and pH, high potential to adhere on mucus, appropriate growth abilities, safety to barramundi, and sensitivity to antibiotics. These isolates, therefore, constitute new QQ probiotics that could be used to control vibriosis in Lates calcalifer.

A 60‐day study was carried out to determine the effect of dietary selenium nanoparticles (SeNP) on growth, digestive enzymes, and health status of Asian seabass (Lates calcarifer, 46.5 ± 0.2 g) juveniles reared in high saline water (48 ppt). Five levels of SeNP were added to a basal diet (45% protein, 15% lipid), including control (0), 0.5 (SeNP0.5), 1.0 (SeNP1), 2 (SeNP2), and 4 (SeNP4) mg SeNP kg⁻¹ diet. Fish were stocked into fifteen 2,000 L tanks (50 fish tank⁻¹) filled with 1,800 L sand‐filtered seawater (26.5 ± 1.5°C, 48.0 ± 0.2 ppt) in a flow‐through system. Each dietary treatment was performed in three replicates. The growth rate positively increased in both linear and quadratic trends with increasing dietary SeNP level (P < 0.05). The liver Se concentration increased with increasing SeNP in diet (P < 0.05). Gut total protease, trypsin, chymotrypsin, alkaline phosphatase (ALP), lipase, and α‐amylase activities were significantly enhanced in the SeNP4 group compared to the other treatments (P < 0.05). Antioxidant capacity improved in fish‐fed SeNP2 and SeNP4 diets regarding catalase and superoxide dismutase activities and the liver glutathione content. Serum lysozyme and hemolytic activities and white blood cells’ respiratory burst activity in the control were lower than in fish‐fed SeNP‐supplemented diets. Serum total protein, globulin, and globulin/albumin ratio in fish‐fed SeNP1, SeNP2, and SeNP4 diets were higher than the other groups (P < 0.05). The interleukin-10 and granulocyte-macrophage colony-stimulating factor genes’ relative transcription levels in the gut of fish‐fed SeNP4 were higher than the other groups. Serum cholesterol, triglycerides, ALP, aspartate aminotransferase, alanine aminotransferase, and lactate dehydrogenase significantly decreased in fish with increasing SeNP content in the diet. In conclusion, supplementing the diet with 4 mg kg⁻¹, SeNP was recommended to improve growth and health indices in L. calcarifer juveniles reared in high saline water (48 ppt).