RESUMEN
A 50-day feeding trial was conducted to evaluate the effects of dietary supplementation of a novel marine psychrotrophic bacterium, Psychrobacter maritimus S, on growth performance and immune responses of Nile tilapia (Oreochromis niloticus) fingerlings. Triplicate groups of Nile tilapia (10 fish each) were fed for 50 days with three different diets: T0 (without probiotic, negative control), T0.5 (supplemented with 3.3 × 108 CFU g-1P. maritimus S), and T1 (supplemented with 6.6 × 108 CFU g-1P. maritimus S). The test diets were fed to fish at a daily rate of 3% of their live weights, three times per day. On the 50th day, fish were weighed and blood samples were collected to determine the main nonspecific humoral and cellular immune responses and digestive enzymes (protease, amylase, and lipase). Fish performance (weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio, and protein productive value) was also calculated. Growth rates and digestive enzyme activity were significantly increased (P < 0.05) with increasing dietary P. maritimus S up to 0.5%, and leveled off with further increase in bacterial concentrations. Phagocytic activity, lysozyme activity, alternative complement hemolysis, and hematological parameters were also significantly increased (P < 0.05) with increasing P. maritimus S concentration to 0.5%, followed by a slight decrease (P > 0.05) at 1% level. The expression of interleukin-4 and interleukin-12 genes was significantly upregulated, while heat shock protein gene was downregulated, with dietary supplementation of P. maritimus S up to 0.5% level. These results suggest that 0.5% of dietary Psychrobacter maritimus S supplementation could be considered as a novel probiont for optimum growth performance and immune response of Nile tilapia fingerlings.