Dietary intake affects the structure and function of microbes in host intestine. However, the succession of gut microbiota in response to changes in macronutrient levels during a long period of time remains insufficiently studied. Here, we determined the succession and metabolic products of intestinal microbiota in grass carp (Ctenopharyngodon idellus) undergoing an abrupt and extreme diet change, from fish meal to Sudan grass (Sorghum sudanense). Grass carp hindgut microbiota responded rapidly to the diet shift, reaching a new equilibrium approximately within 11 days. In comparison to animal-diet samples, Bacteroides, Lachnospiraceae and Erysipelotrichaceae increased significantly while Cetobacterium decreased significantly in plant-diet samples. Cetobacterium was negatively correlated with Bacteroides, Lachnospiraceae and Erysipelotrichaceae, while Bacteroides was positively correlated with Lachnospiraceae. Predicted glycoside hydrolase and polysaccharide lyase genes in Bacteroides and Lachnospiraceae from the Carbohydrate-Active enZymes (CAZy) database might be involved in degradation of the plant cell wall polysaccharides. However, none of these enzymes was detected in the grass carp genome searched against dbCAN database. Additionally, a significant decrease of short chain fatty acids levels in plant-based samples was observed. Generally, our results suggest a rapid adaption of grass carp intestinal microbiota to dietary shift, and that microbiota are likely to play an indispensable role in nutrient turnover and fermentation.

A Escherichia coli strain producing phytase named DH5α was chosen from four E. coli stains, the enzyme activity was 17.88 U/mL after fermented at 37°C for 2 days. The enzymatic properties of crude enzyme solution were studied. The optimal temperature was 55°C; optimal pH was 2.5 and 7.0, enzyme had certain resistance to heat and different pH conditions, 75% activity could be maintained when treated at 65°C for 30min, and 60% activity still remained when treated in the pH range of 2 to 9 for 30 min and 60 min. Resistance to KH 2 PO 4 of phytase was poor, low concentrations of KH 2 PO 4 (15mg/L) could inhibit the enzyme activity to below 40%; all kinds of metal ions had no significant activation or inhibition effect on enzyme, the enzyme had resistance to pepsin but no resistance to trypsin, after treated by trypsin for 30min, only 66.69% of the activity retained, while treated by pepsin , the activity could still maintain more than 90%.