Introduction
The continued expansion of aquaculture requires sustainable protein sources beyond fishmeal, as pressure on wild fisheries and rising costs limit its long-term viability (FAO, 2018). Bacterial meal (BM), a single-cell protein produced from bacteria grown on non-food substrates, emerges as a promising alternative due to its high protein content, favourable amino acid profile, and low land and water requirements (Pesante et al., 2022), yielding good growth and health in farmed fish, including salmon (Aas et al., 2006). BM also serves as a source of postbiotics, nonliving microbial components, whereas Gram-negative bacterial meals have shown dose-dependent improvements in growth performance, microbiota diversity, and inflammatory regulation in fish (Wang et al., 2023). In the current study, graded dietary inclusion of a Gram-negative bacterial biomass produced from agricultural sidestreams was evaluated for its effects on Atlantic salmon smolt performance, smoltification success, and key health and welfare indicators.
Materials and Methods
The feeding trial was conducted at the Nofima Research Station for Sustainable Aquaculture (Sunndals��ra, Norway) using four diets containing 0, 2.5, 5, or 10% marine BM, each fed to triplicate groups of Atlantic salmon parr (~80 g initial body weight). Fish were reared for 9.5 weeks in 12 tanks under controlled freshwater/seawater conditions and continuously fed using automatic feeders, with feed intake and mortalities recorded daily. Growth performance, feed utilisation, smoltification capacity, and biometry were assessed. Moreover, blood biochemistry, hindgut histology, and enzyme activities were analysed. Head kidney transcriptomics was performed using Nofima's 44k Atlantic salmon microarray (Salgeno - 2) using the STARS bioinformatics platform (Krasnov et al., 2011).
Results and Discussion
Feed intake (% BW/day) increased stepwise with dietary BM inclusion, with statistically significant differences observed between the control and 10% BM after seawater transfer, and overall, between 5% BM and the control. Feed intake correlated with growth, and fish in all BM treatments were numerically larger at the end of the trial, although the growth differences were not statistically significant. Relative liver weight (HSI) was significantly higher in the BM-fed fish, which may be linked to immune stimulation by bacterial cell wall components rather than to high-fat-driven liver enlargement. No differences in mortality were observed across treatments. In terms of population structure, the BM groups showed significant differences in size distribution (higher kurtosis at 5–10% BM), consistent with a stronger clustering around one body size rather than evenly spread "small-to-large" categories, an observation that is relevant given that dominance hierarchies can influence growth dispersion in salmon populations (Soares et al., 2019). This aligns with the broader theory that bacterial meals/postbiotics can affect behaviour and stress responsiveness via microbiota-born signals (e.g., Knobloch et al., 2022). Blood biochemistry showed the lowest cholesterol at 10% BM and the highest in the control group, whereas triglycerides tended to show the opposite trend (higher at 5–10% BM). Hematocrit and the remaining blood parameters were unchanged. Histology of the distal intestine (AI-based Aiforia�� analysis) revealed no signs of enteritis in any group, and the distal intestine layers (including lamina propria and submucosa) showed no inflammatory thickening. Nevertheless, distal enterocyte height (mucosa width) was significantly lower in fish fed 2.5% BM compared with the control. Vacuolisation was medium-to-high in all groups (numerically higher in BM fish), interpreted as an "active" distal intestine consistent with lipid digestion/absorption. Mucous cell numbers decreased slightly in BM groups, with a statistically significant reduction in large mixed-mucin (blue) mucous cells at 2.5% BM. Intraepithelial lymphocytes were significantly reduced at 10% BM (and their localisation shifted toward the mucosal surface), suggesting immunomodulation without an enteritis phenotype. The eosinophilic granular cells varied, being highest at 2.5% BM. These outcomes are compatible with the mixed performance evidence for bacterial protein meals in salmonids, with some studies reporting improved growth and nutrient utilisation even at high inclusion levels (e.g., up to 36% bacterial protein meal; Aas et al., 2006), while other functional feed strategies have shown a shift in microbiota composition without growth gains (Baumg��rtner et al., 2022). Overall, the trial supports the inclusion of BM (2.5-10%) as gut-safe under these conditions, with functional effects on feed intake and immune-related gut cell profiles, while highlighting that the "optimal" inclusion window likely depends on balancing immune stimulation against potential metabolic trade-offs.
Acknowledgment
This research was funded by MicroHarvest GmbH and Nofima.
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