Introduction
Infectious diseases represent a major challenge for aquaculture, leading to substantial economic losses and compromising fish welfare. Consequently, identifying viable and environmentally sustainable solutions is a key priority for the sector. Numerous efforts have been made and are continually being undertaken to develop efficient preventive and curative strategies to mitigate the impact of infectious diseases affecting fish while also seeking to avoid, or at least minimize, the use of antibiotics.
An effective approach for the implementation of selective breeding strategies or the development of novel preventive or control measures against infectious diseases involves deciphering the molecular differences between fish full-sibling families that exhibit differential degrees of resistance or susceptibility to a particular pathogen. In this regard, omics technologies, such as genomics, transcriptomics, and proteomics, have proven to be powerful tools to identify a wide range of molecular markers associated with disease resistance. However, the use of metabolomics for this purpose is not so widespread and, to the best of our knowledge, it has not previously been applied to compare the profiles of full-sibling fish families exhibiting different levels of susceptibility to a specific pathogen. This study aimed to identify metabolic markers associated with resistance to the bacterium Aeromonas salmonicida subsp. salmonicida in turbot (Scophthalmus maximus), with the ultimate goal of incorporating resistance-associated metabolites into functional diet formulations .
Materials and methods
Untargeted metabolomic analyses of the head kidney and liver from two full-sibling turbot families with differing susceptibility to the pathogen A. salmonicida subsp. salmonicida were conducted. Metabolites found in higher abundance in the resistant family were preliminarily screened in zebrafish (Danio rerio ) larvae using an A. hydrophila infection model. Based on the promising results, two of these metabolites were incorporated into the diet at a concentration of 1.5% to further evaluate not only their protective effect in turbot against A. salmonicida, but also their palatability, impact on growth, and potential histopathological effects. In order to better understand the mechanisms underlying the protection induced by the resistance-associated metabolites, the modulation of the intestinal microbiota (via 16S rRNA amplicon sequencing) and transcriptomic changes (via RNA-Seq analysis) were assessed in response to the functional diet that demonstrated a significant protective effect.
Results
A comparative metabolomic analysis was conducted on two full-sibling turbot families with differing susceptibility to the pathogen, revealing a subset of metabolites with differential abundance in the head kidney and liver. Preliminary screening in zebrafish larvae identified 5-L-glutamyl-taurine as a promising protective compound. However, due to the high cost of this metabolite, its precursors—taurine and L-glutamic acid—were tested. Both showed protective effects in zebrafish larvae infected with A. hydrophila , leading to their selection for dietary trials in turbot. While dietary supplementation with L-glutamic acid did not significantly affect survival, the taurine-enriched diet significantly improved survival in turbot challenged with A. salmonicida, resulting in a 34% higher survival rate compared to controls, despite increased bacterial loads observed at 24 hours post-infection (hpi). Moreover, taurine supplementation had no negative effects on the evaluated parameters.
Taurine also modulated the intestinal microbiota, reducing alpha diversity through the expansion of Vibrionaceae and Marinobacteraceae . RNA-Seq analysis of head kidney and intestinal tissues suggested that taurine’s protective effect may involve attenuation of the early inflammatory response, potentially minimizing tissue damage. These changes emphasize the complex interplay between diet, immune response, and microbial communities. Overall, this study highlights taurine’s potential as a functional dietary additive to enhance disease resistance in aquaculture, while underscoring the value of immunometabolism research in developing strategies to combat infectious diseases.