Effect of Amorphous Halomonas-PHB on Growth, Body Composition, Immune-Related Gene Expression and Vibrio anguillarum Resistance of Hybrid Grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatu ♂) Juveniles

Poly-β-hydroxybutyrate (PHB) is a bacterial metabolite produced by bacteria such as <i>Halomonas</i> sp. that serves as a carbon and energy storage compound for bacteria under nutrient-limited conditions. Two experiments were conducted to investigate the effects of dietary supplementation with <i>Halomonas</i>-PHB on hybrid grouper (<i>Epinephelus fuscoguttatus</i> ♀ × <i>E</i>. <i>lanceolatu</i> ♂). In experiment I, juvenile groupers were fed basal diets supplemented with 3% <i>Halomonas</i>-PHB (3% HM-PHB) containing 1.4% PHB and 3% <i>Halomonas</i> (3% HM) without PHB, as well as a control diet, for seven weeks. The results showed no significant difference in survival rate, weight gain, and crude fat content between the 3% HM-PHB group and the control group; however, the crude protein of the 3% HM-PHB group was significantly lower than that of the control group. Furthermore, supplementation with 3% HM-PHB increased the fatty acids content in fish muscles, including long-chain unsaturated fatty acids C18:1n9, EPA, and DHA. In experiment II, groupers were fed a basal diet supplemented with 6.5% <i>Halomonas</i>-PHB (6.5% HM-PHB) containing 3% PHB and 6.5% <i>Halomonas</i> (6.5% HM) containing no PHB, as well as a basal diet (Control). After seven weeks of rearing, the fish were challenged with <i>Vibrio anguillarum</i> for 48 h. Although no significant difference in survival rate and growth was observed among different groups, the dietary supplement of 6.5% <i>Halomonas</i>-PHB improved the survival rate of <i>V. anguillarum</i> challenged grouper and significantly increased the gene expressions of catalase (<i>CAT</i>) and superoxide dismutase (<i>SOD</i>) in blood, interleukin 1 (<i>IL1</i>) and interleukin 10 (<i>IL10</i>) in the liver, spleen, head kidney, and blood (<i>p</i> < 0.05). In conclusion, dietary supplementation of <i>Halomonas</i>-PHB had no significantly positive effect on fish growth performance but increased the content of fatty acids, including long-chain unsaturated fatty acids C18:1n9, EPA, and DHA in fish muscle; it also improved the <i>V. anguillarum</i> resistance, possibly through increasing immune-related gene expression in different tissues and organs. Our findings offer compelling evidence that <i>Halomonas</i>-PHB can be utilized as a feed additive in intensive grouper farming to enhance the groupers' resistance to <i>Vibrio</i>.