Methanotroph (Methylococcus capsulatus, Bath) bacteria meal alleviates soybean meal-induced enteritis in spotted seabass (Lateolabrax maculatus) by modulating immune responses and the intestinal flora

The effect of methanotroph (Methylococcus capsulatus, Bath) bacteria meal (FeedKind & REG;, FK) supplementation in a high-soybean meal diet on growth, non-specific immunity, and gut health in spotted seabass (Lateolabrax maculatus) was investigated. Six experimental diets were formulated: control (containing 38% fish meal, FM, and 16.5% soybean meal, SBM), a high-soybean meal diet (containing 28% FM and 35% SBM), and four experimental diets each containing 28% FM and 35% SBM with the addition of FK at 2% (FK2), 4% (FK4), 6% (FK6) or 8% (FK8) in the diet to replace wheat gluten. All diets also contained 0.1% yttrium oxide to determine apparent digestibility coefficients (ADCs) of dry matter, crude protein and crude lipid. Following a 14-day acclimation period, fish were fed the diets for 56 days. Results showed that the FK2 and FK4 groups showed significantly higher weight gain (WG) compared to the SBM group (P < 0.05), whilst the FK2, FK6, and FK8 groups showed higher WG compared to the FM group (P < 0.05). The FK2, FK4, and FK8 had lower feed conversion rate (FCR) compared to the SBM group (P < 0.05). No differences were noted in overall survival. Intestinal lipase activity was significantly increased in the FK2 and FK4 groups and trypsin activity was significantly increased in the FK2 group compared to the SBM group (P < 0.05). The FK2 group showed significantly higher apparent digestibility coefficients of nutrients than the SBM group (P < 0.05). The expressions of anti-inflammatory genes (tgf & beta;, il-4, and il-10) in the intestine of the FK-supplemented group showed a significant increment (P < 0.05), whilst the expressions of pro-inflammatory genes (tnf & alpha;, il-1 & beta;, and il-8) significantly decreased (P < 0.05). The results of 16S rRNA gene sequencing showed that indexes of alpha diversity in the SBM group were decreased (P < 0.05) compared with all other diets. In addition, the FK2 group showed significantly higher serum acid phosphatase (ACP), alkaline phosphatase (AKP), complement 4 (C4), and total protein (TP), levels than the SBM group (P < 0.05). Inclusion of FeedKind in high SBM diets alleviated SBM-induced intestinal inflammation by modulating the expression of intestinal inflammatory factors and regulating intestinal flora. Overall, supplementation of SBM diets with 2%-4% FK improved growth, non-specific immunity, and ADC's in spotted seabass.