Substituting fish meal with a bacteria protein (Methylococcus capsulatus, Bath) grown on natural gas: Effects on growth non-specific immunity and gut health of spotted seabass (Lateolabrax maculatus)

This study aimed to evaluate effects of substituting fish meal (FM) by methanotroph (Methylococcus capsulatus, Bath) bacteria meal (FeedKind®, FK) on growth performance, non-specific immunity and gut health of Lateolabrax maculatus. Six iso-nitrogenous diets were formulated with 0, 3.5%, 7%, 10.5%, 14% and 17.5% FK to substitute 0 (control group, FM content: 35%), 10%, 20%, 30%, 40% and 50% fish meal, referred to as FM, FK3.5, FK7, FK10.5, FK14, and FK17.5 respectively. Fish (initial body weight: 11.00 ± 0.02 g) were fed experimental diets for 56 days. Results showed that growth of fish fed the FK3.5 diets was significantly increased (P < 0.05) but that there was no significant difference (P > 0.05) in growth of the other groups (FK7-FK17.5) compared with control group. Although survival of fish in the 50% substitution group was significantly decreased (P < 0.05) compared with the other groups, there was no overall trend in the survival data. The activities of serum acid phosphatase (ACP), alkaline phosphatase (AKP), gut lipase and protease and the contents of serum total protein and albumin in the FK3.5 group were significantly higher than those of control group (P < 0.05). In contrast, the activities/contents of serum ACP, AKP, lysozyme and complement 4 in the FK10.5, FK14 and FK17.5 groups were significantly decreased compared with control group. There was no significant difference in gut mucosal structure among all groups. The analysis of gut flora showed that the relative abundance of Lactobacillus in the FK7 and FK10.5 groups was significantly higher than other groups. In conclusion, dietary FK inclusion level up to 14% of total diet to replace 40% FM is possible without negatively affecting the growth performance of spotted seabass. Furthermore, the replacement of FM with 3.5% FK enhances the growth and immune, and substituting 20–30% of FM by 7–10.5% FK increased the abundance of beneficial bacteria in intestine of fish.