Effects of dietary heat-killed Pseudomonas aeruginosa strain VSG2 on immune functions, antioxidant efficacy, and disease resistance in Cyprinus carpio

A probiotic strain of Pseudomonas aeruginosa VSG2 was previously isolated from the gut of Indian major carp Labeo rohita. But, its efficacy as a heat-inactivated probiotic on the immune response of fish remains unexplored. Thus, present study investigated the effect of heat-killed P. aeruginosa VSG2 (HK-PA) supplementation on the immune function, digestive enzymatic activities, and antioxidant status of Cyprinus carpio. Moreover, the effect of HK-PA supplementation on the susceptibility of C. carpio to Aeromonas hydrophila challenge was also evaluated. To do so, diets containing five different concentrations of HK-PA (mg kg(-1)) (0 [basal diet], 10 [PA10], 20 [PA20], 30 [PA30], and 40 mg kg(-1) [PA40]) were fed to fish for 8 weeks. The results revealed that immunerelated factors, such as lysozyme, protein level, alkaline phosphatase, and alternative complement pathway levels increased in fish fed with HK-PA, but differences were significant in PA20-PA40 compared to the control. The immunoglobulin level was higher (p < .05) in PA30, compared to the control. Superoxide dismutase (SOD), glutathione, glutathione peroxidase (GPx), and myeloperoxidase levels significantly increased in serum or skin mucus of fish fed with HK-PA. Accordingly, malondialdehyde levels decreased in fish fed with HK-PA. Among intestinal enzymatic activities, protease activity was higher in the HK-PA fed groups compared to the control. Moreover, mRNA expression of antioxidant genes (SOD, GPx, and catalase) significantly increased in the liver of PA20-PA40. Fish in the PA30 exhibited the highest relative post-challenge survival (71.54 +/- 5.37%) against A. hydrophila infection. Overall, dietary administration of HK-PA at 30 mg kg(-1) (PA30) for 8 weeks improved (p < .05) the skin mucosal and serum immune parameters, strengthened antioxidant capacity, enhanced intestinal enzymatic activities, and increased the resistance against a pathogen challenge. The heat-killed P. aeruginosa VSG2 represents a promising dietary supplement for carps in aquaculture.