Juveniles of Pseudoplatystoma reticulatum (36.25 g and 19.32 cm) were randomly distributed in 16 tanks with 80 L of water. The fish were fed with diets containing probiotic (2%), prebiotic (0.5%), synbiotic (2% probiotic +0.5% prebiotic) and diet without additives (control). After 60 days of feeding, the fish were challenged with inoculation of the Aeromonas hydrophila (0.9 × 106 CFU/mL). Growth performance, physiological, immunological and haematological analyses were evaluated. Fish fed with probiotic, inulin and synbiotic diet showed greater (p = 0.018) weight gain when compared to control fish, and the condition factor was better (p < 0.001) in fish fed with probiotic. Cortisol concentrations increased in all treatments after challenge (p = 0.27), however, after A. hydrophila injection of 6 h, the cortisol levels increased significantly in fish that received control diet. The haematocrit (p < 0.001) and lymphocytes percentages (p < 0.001) decreased after A. hydrophila injection of 3 and 6 h, and neutrophils percentages increased (p < 0.001), without difference among treatments. The not supplemented and supplemented with prebiotic and synbiotic fish groups had decreased haemoglobin concentration (p = 0.01) at after 3 and 6 h. The juvenile catfish fed with probiotic (2% Bacillus subtilis), prebiotic (0.05% inulin) and the synbiotic combination for 60 days had better productive performance, ensuring greater weight gain and growth. The supplemented diets inhibited the cortisol increase when fish are challenged with A. hydrophila, but it does not inhibited the effects of the stress imposed by the bacterial infection.