Synergistic of Lactobacillus plantarum L20 and Sargassum polycystum hydrolysate enhances growth, immunity, and disease resistance against necrotizing hepatopancreatitis-like diseases-causing Aeromonas veronii in giant freshwater prawn (Macrobrachium rosenbergii)

Aeromonas spp. cause significant mortality in freshwater giant prawns (Macrobrachium rosenbergii), posing a major challenge to freshwater aquaculture. With growing restrictions on antibiotic use, there is an urgent need for sustainable alternatives to prevent aeromoniasis outbreaks. In this study, pathogenic A. veronii was isolated and identified from diseased prawns. The lethal concentration of A. veronii UG3 in M. rosenbergii was determined through immersion challenges at varying concentrations (1 × 10⁷ CFU/mL, 2 × 10⁷, CFU/mL, 4 × 10⁷ CFU/mL, 6 × 10⁷ CFU/mL, 8 × 10⁷ CFU/mL and 1 × 10⁸ CFU/mL). Postlarvae (PL15) of M. rosenbergii were fed for 49 days with diets supplemented with either a prebiotic premixture (hydrolysate of Sargassum polycystum) or a synbiotic premixture (a combination of Lactobacillus plantarum and S. polycystum hydrolysate), with each treatment administered in triplicate. Following the feeding trial, prawns were challenged via immersion with A. veronii UG3 at a concentration of 2 × 10⁷ CFU/mL. Tissue samples from the cephalothorax and abdomen were collected for gene expression and histopathological analyses. Necrotizing hepatopancreatitis-like disease was observed in the control group upon infection, with A. veronii UG3 harboring virulence genes including those encoding flagella, hemolysin, cytotonic heat-labile toxin, lipase, elastase, and chitinase. Among all treatment groups, the synbiotic-fed prawns exhibited the highest weight gain, specific growth rate, and survival rate. This group also showed enhanced immune responses, with upregulation of anti-lipopolysaccharide factor (ALF) gene expression following the feeding trial, and increased expression of immune-related genes such as IMD, prophenoloxidase, and heat shock protein 70 (HSP70) after bacterial challenge. Furthermore, histological examination revealed that the synbiotic diet preserved the structural integrity of the gastrointestinal tract during infection. These findings demonstrate the potential of synbiotic supplementation to improve growth performance, enhance immune function, and increase disease resistance in M. rosenbergii. The use of such functional diets presents a promising and sustainable approach to mitigating production losses in freshwater prawn aquaculture while reducing dependence on antibiotics.