Gut microbiota adaptation to low and high carbohydrate-to-protein ratio diets in grass carp (Ctenopharyngodon idella)

This study attempted to explore the adaptability of gut microbiota in grass carp to diets with different carbohydrate-to-protein ratios in order to better understand the immunological and metabolic changes in the organism. Three different ratios of carbohydrate and protein diets (12.5%-carbohydrate 46.3%-protein (LCHP), 27.5%-carbohydrate 33.7%-protein (MCMP), and 42.5%-carbohydrate 16.9%-protein (HCLP)) were satiety-fed to grass carp. After feeding, the growth performance, gut microbial community composition and function were analyzed. Growth performance results showed that LCHP group exhibited the significantly lower specific growth ratio than MCMP group, and HCLP group displayed the significantly higher feed coefficient and lower survival rate. The community composition results showed that the abundance of the beneficial genus Cetobacterium was the highest in MCMP group, and the abundance of the harmful genera Ruminiclostridium_9 and Phreatobacter was the highest in the LCHP and HCLP groups, respectively. Furthermore, the ratio of Firmicutes to Bacteroidetes considered as an indicator of gut microbiota dysfunction, was significantly higher in the LCHP and HCLP groups than that in the MCMP group. The community functional analysis showed that the abundance levels of cofactors and vitamin metabolism were significantly lower and the abundance levels of neurodegenerative diseases were significantly higher in the LCHP and HCLP groups than those in the MCMP group. The abundance levels of energy metabolism and a range of biomolecules metabolism were significantly decreased in the HCLP group. These results suggested that high-carbohydrate diets have a greater negative impact on the gut microbiota than high-protein diets. The gut microbiota results echoed the growth performance parameters, reflecting that gut microbiota may play an important role in the influence of different carbohydrate-to-protein ratios diets on host. Bacterial genera such as Ruminiclostridium_9 and Phreatobacter may play key roles in the effects of highprotein and high-carbohydrate diets on host health and metabolism, respectively.