Bacterial single cell protein (BSCP) obtained from methanotrophic bacteria was tested as an alternative protein source to fishmeal (FM) in compound diets for rainbow trout (Oncorhynchus mykiss) juveniles (initial body weight, BW = 11.3 +/- 0.2 g, mean +/- standard deviation). Different levels of FM replacement (0, 25, 50, 75 and 100%) by a BSCP (0, 3.75, 7.5, 11.25 and 15% of Uniprotein (R) Aqua) were tested in isoproteic (crude protein: 42%) and isolipidic (crude fat: 21%) extruded diets. The trial was divided in two different parts, a nutritional trial that lasted 83 days during which the impact of experimental diets was evaluated in terms of key performance indicators (KPIs) associated to fish growth performance and feed utilization, which was followed by a challenge with a pathogenic bacterium (Aeromonas salmonicida subsp. salmonicida) to evaluate the immunological competence of fish fed the experimental diets (15 days). At the end of the nutritional trial, somatic growth in terms of body weight (BW) responded quadratically (P < 0.05) to the dietary inclusion of BCSP with the lowest BW values (183.4 +/- 4.5 g) recorded in fish fed 100% FM replacement with BSCP, whereas the rest of dietary groups showed similar BW values (191.3-201.2 g). Feed intake was not affected by the level of FM replacement by BSCP, whereas feed conversion ratio (FCR) slightly increased with increasing levels of BSCP (P < 0.05), even though the lowest FCR values were found in fish fed 50% FM replacement by BSCP (0.79 +/- 0.01) in comparison to the control group (0.81 +/- 0.01). When using quadratic linear regression, the optimal dietary FM replacement by BSCP in terms of BW values was estimated as 41.7%. The nutritional quality of the fillet was not altered with experimental diets; all dietary groups showed similar amino acid levels as well as their content in HUFA n-3 and n-6 that remained unaltered (P > 0.05) and lipid nutritional indexes (P > 0.05). No negative effects were observed in the histological organization of the liver nor the proximate intestine, neither in the lipid peroxidation levels nor activity of antioxidative stress enzymes in the liver from fish fed increased inclusion levels of BSCP (P < 0.05). The replacement of FM by the tested BSCP obtained from methanotrophic bacteria did not cause an imbalance or dysbiosis in the intestinal microbiota in rainbow trout regardless of the level of FM replaced. The replacement of FM by BSCP at 50% improved disease resistance in rainbow trout exposed to furunculosis when compared to the control diet (cumulative mortality = 83.3 +/- 7.2% vs. 49.2 +/- 15.6%; P < 0.05), which suggested that this ingredient may also have some immunomodulatory properties that would protect fish from infections, although their mode of action deserves further attention. Current results indicated that the tested BSCP from methanotrophic bacteria is a safe, functional, and sustainable alternative protein source for aquafeeds.
