Investigating the Impact of Varied Dietary Protein Levels on Litopenaeus vannamei: An Exploration of the Intestinal Microbiota and Transcriptome Responses

This study explored the effects of dietary protein levels on Litopenaeus vannamei with its intestinal microbiota and transcriptome responses. Previous studies on the effects of dietary protein levels on L. vannamei have focused on growth performance, antioxidant indices, and digestive enzyme activity, but few studies have been conducted at the microbiological and molecular levels. In this study, five isolipid experimental diets with protein levels of 32% (P32), 36% (P36), 40% (P40), 44% (P44), and 48% (P48) were used in an L. vannamei (0.63 ± 0.02 g) feeding trial for 56 days. At the end of the feeding trial, the growth performance, immunity, intestinal health, and transcriptional responses of L. vannamei were determined. This study demonstrated that higher protein levels (P44) led to superior weight gain and growth rates for L. vannamei, with lower feed conversion ratios (FCR) observed in the P48 and P44 groups compared to the P32 and P36 groups (p ≤ 0.05). The P44 and P48 groups also showed a notably higher protein efficiency ratio (PER) compared to others (p ≤ 0.05), and there was no significant difference between them. Upon Vibrio parahaemolyticus infection, the P48 group exhibited a significantly lower survival rate (SR) within 48 h, while during 72 h of white spot syndrome virus (WSSV) infection, the P44 group had a notably higher survival rate than the P32 group (p ≤ 0.05). Digestive enzyme activity and antioxidant levels in L. vannamei initially increased and then decreased as protein levels increased, usually peaking in the P40 or P44 groups. Lower dietary protein levels significantly reduced the relative abundance of beneficial bacteria and increased the relative abundance of pathogenic bacteria in the intestines of L. vannamei. Transcriptome sequencing analysis revealed that most differentially expressed genes (DEGs) were up-regulated and then down-regulated as dietary protein levels increased. Furthermore, KEGG pathway enrichment analysis indicated that several immune and metabolic pathways, including metabolic pathways, glutathione metabolism, cytochrome P450, and lysosome and pancreatic secretion, were significantly enriched. In summary, the optimal feed protein level for L. vannamei shrimp was 40–44%. Inappropriate feed protein levels reduced antioxidant levels and digestive enzyme activity and promoted pathogen settlement, deceasing factors in various metabolic pathways that respond to microorganisms through transcriptional regulation. This could lead to stunted growth in L. vannamei and compromise their immune function.