Feeding red tilapia a diet with the probiotic Pediococcus acidilactici (2-3 g per kg of feed) boosted growth, improved blood health, and raised the fish's natural IGF‑1 and related gene levels, leading to better disease resistance against a common bacterial infection.

Despite the widespread practice of using probiotics in aquaculture for their potential benefits, the consistency of their impact on disease resistance in fish is still being scrutinized. This study aimed to address this gap by investigating the influence of the probiotic Pediococcus acidilactici (PCA) on hematological indices, body composition, serum metabolites, water quality, growth metrics, the transcriptomic profile of genes related to immune function, and the ability of red tilapia (Oreochromis niloticus × O. mossambicus) to resist challenge with the bacterial pathogen Aeromonas hydrophila. A total of 180 tilapia fingerlings (average weight 17.10 ± 0.46 g) were distributed into four groups and fed a basal diet supplemented with 0 (PCA0), 1 (PCA1), 2 (PCA2), and 3 (PCA3) g of PCA per kg of diet for 63 days. Water quality analysis indicated a significant decrease in both salinity and dissolved oxygen concentrations in all PCA-supplemented groups compared to a PCA-free diet (P < 0.05). Fish fed diets having 2 or 3 PCA had superior FBW, SGR, and ADG, while these levels significantly improved the FCR compared to the untreated diet (P < 0.001). In fish supplemented with PCA, body composition analysis showed a statistically significant increase in protein and ash content, accompanied by a statistically significant decrease in lipid content (P < 0.05). Furthermore, dietary PCA significantly enhanced phagocytic index, and serum IgG and IgM levels in a dose-dependent response. Notable improvements in hematological variables and blood protein fractions were observed in the PCA2 and PCA3 groups compared to other groups (P < 0.05). Dietary PCA administration sustained liver enzymes and kidney functions (P < 0.05). Dietary supplementation with PCA resulted in a significant upregulation (P < 0.05) of growth-related genes (GH, IGF-1, and IGF1-R) and immunity-associated genes (CXC chemokine, IL8, and IL1β) in a dose-dependent manner. Furthermore, dietary PCA improved the histological structure of the liver and enhanced intestinal integrity in Tilapia. Following pathogen challenge, the survival rate was significantly higher in all PCA-supplemented groups compared to the control group, with the highest survival observed in the PCA3 group. In summary, this research highlights the significant benefits of dietary Pediococcus acidilactici supplementation at 2 and 3 g/kg for improving growth, blood physiology, immunity, and disease resistance in red tilapia. These findings underscore the potential of Pediococcus acidilactici as a practical and effective strategy for enhancing productivity and sustainability in red tilapia aquaculture.