Cell wall fractions from Methylococcus capsulatus prevent soybean meal-induced enteritis in Atlantic salmon (Salmo salar)

• We evaluated the enteritis-preventing effects of bacterial meal (BM) products.
• There were no effects of PER, while RET and the other fractions prevented enteritis.
• Large molecules and water insoluble components in BM prevented enteritis.

Plant based feed ingredients pose a risk for developing intestinal inflammatory diseases in fish. The salmon is in particular sensitive to solvent extracted soybean meal (SBM). Recent studies have shown that dietary inclusion of a bacterial meal (BM) grown on natural gas containing mainly Methylococcus capsulatus can prevent SBM-induced inflammation. The present study aimed to identify BM fractions and products that have this beneficial effect. A fish meal (FM) based diet, a diet with 200 g kg− 1 SBM, and six diets with 200 g kg− 1 SBM in combination with basic BM, autolyzed BM (AUT), permeate (PER) or retentate (RET) from filtration of AUT, nucleic acid reduced M. capsulatus (MCap), and M. capsulatus grown on methanol (MeOH) were made for this study. The inclusion of the BM products were equivalent to 150 g kg− 1 basic BM. The diets were fed for 4 weeks to triplicate tanks of juvenile Atlantic salmon (Salmo salar) reared in salt water. No significant (P < 0.05) differences were found for feed intake and growth among fish fed the experimental diets. Morphometric measurements of the length of proliferating cell nuclear antigen (PCNA) stained cells in distal intestinal tissue revealed that all BM products except the PER provided significant protection against SBM-induced enteritis. The effective component in the BM products was present in the retentate that mainly consisted of large molecules and water insoluble components. Nucleic acids and small water soluble molecules did not seem to provide any protective effect.