Comparative effects of viable and gamma-irradiated Bifidobacterium longum subsp. infantis on immune modulation and resistance to furunculosis in rainbow trout (Oncorhynchus mykiss)

BACKGROUND: This study investigated the effects of dietary supplementation with live (B-PRO) and gamma-irradiated (B-PAR) Bifidobacterium longum subsp. infantis on growth performance, innate immune-related parameters, antioxidant status, and resistance to Aeromonas salmonicida in rainbow trout (Oncorhynchus mykiss). RESULTS: After 60 days of feeding, both B-PRO and B-PAR were associated with improved growth performance, feed utilization, and body composition compared with the control group, with generally higher values in B-PRO. Hematological parameters, including erythrocyte and leukocyte counts, were elevated in treated groups. Innate immune indicators (lysozyme activity, complement C3, and bactericidal activity) and antioxidant status (SOD, GSH-Px, and total antioxidant capacity) were significantly improved, accompanied by reduced malondialdehyde levels. Gene expression analysis showed tissue-dependent modulation of TNF-α, IL-6, and IL-8, indicating a non-uniform immunoregulatory response. Following A. salmonicida challenge, survival was ~95% in both treated groups versus ~15% in controls. Histopathological evaluation indicated reduced tissue damage in supplemented fish. CONCLUSIONS: Dietary supplementation with both live and gamma-irradiated B. infantis is associated with improved growth, modulation of innate immune-related parameters, enhanced antioxidant capacity, and increased disease resistance in rainbow trout. The comparable effects of both preparations suggest that bacterial viability is not essential for several observed responses under the present conditions. However, mechanistic interpretations remain limited due to the absence of direct assessment of immune signaling pathways.