Immunostimulatory feed supplements have an increasingly interest in aquaculture management. Generally, an individual supplement was used in fish diets but it is expected that the use of multi-supplements may show synergistic enhancements in fish performance, health, and immunity. Therefore, the present investigation was carried out to evaluate the use of dietary probiotic Lactobacillus plantarum and whey protein concentrate (WPC) in practical diets for Nile tilapia, Oreochromis niloticus. Hence, probiotic L. plantarum, WPC and their mixture were incorporated into a basal fish diet (300 g/kg crude protein) as follows: T1 = a basal control diet, T2 = a basal diet containing L. plantarum, T3 = a basal diet containing 1.0 g WCP/kg diet and T4, T5 or T6 = basal diets containing probiotic L. plantarum + 1.0, 2.0 or 3.0 g WCP/kg diet, respectively. Fish (15.2 ± 0.6 g) were fed on one of the tested diets up to apparent satiation twice a day for 60 days. After that, fish were intraperitoneally injected with pathogenic bacteria Aeromonas sobria and fish mortality was observed for 10 days postchallenge. Fish growth and feed intake were significantly improved by dietary probiotic L. plantarum (T2) and/or WPC (T3) over the control group (T1), and highest fish performance was observed in T5-T6 fish groups. Similarly, highest values of haematocrit, glucose, total proteins, albumin, and globulin were significantly observed in T5-T6 fish groups. Likewise, fish fed dietary probiotic L. plantarum (T2), WPC (T3), and their mixture (T4-T6) showed antioxidants and immune-stimulating activities better than the control group. Fish fed the control diet were more susceptible to A. sobria infection showing highest fish mortality (75.0%). Meanwhile, dietary probiotic L. plantarum (T2), WPC (T3), and their mixture (T4-T6) enhanced significantly the fish resistance to A. sobria infection resulting in maximum values of relative percent of fish survival (73.3%-80.0%) in T5-T6 groups. The present investigation recommended the use of probiotic L. plantarum with 2.0 g WPC/kg diet to improve the growth, antioxidant, immunity responses and tolerance of Nile tilapia to A. sobria infection. Fresh whey was collected from the nearby village of the Central Laboratory for Aquaculture Research (CLAR), Abbassa, Abo-Hammad, Egypt, and transferred immediately to the Microbiology Lab, CLAR. All samples were analysed on the same day of sampling and kept at refrigerated (2-8°C) condition until doing the microbiological analysis. A 10 ml of fresh liquid whey was homogenized with 90 ml of 0.85% (w/v) sterile sodium chloride solution to make serial dilutions up to 10 −6 for each sample. A 1.0 ml sample from each of the corresponding dilutions (10 −5 and 10 −6 ) were inoculated into De Man, Rogosa and Sharpe agar (MRS agar; Merck, Germany) for lactobacilli isolation (De Man et al., 1960) and incubated at 30°C for 24-48 hr. After incubation, plates with 30-300 colonies were enumerated and the calculated results were expressed as colonyforming unit (CFU) per ml by mul...
