Microbiota como bioindicador de salud en especies marinas de interés acuícola

Résumé

The microbiota of teleost fish plays a crucial role in various physiological functions. Knowing and understanding the microbiota of domesticated fish is very useful both for improving the productivity of the industry and for the progress of the sustainability of the sector. The interest of this work is to contribute knowledge to the emerging field of study of the microbiota of some of the marine fish farmed in the Mediterranean area. To study the microbiota associated with different body regions of these animals (gastrointestinal tract and skin mucus), several molecular biology techniques have been used, both in the fields of microbiology and immunology. In Chapter 1, changes in the systemic and local immune system and in the diversity of the skin mucus microbiota during an episode of chronic stress in gilthead sea bream were analyzed. In addition, stress and growth parameters were analyzed. In this experiment, no differences were found in stress indicators in serum or skin mucus. Cellular and systemic immunity was not affected. In contrast, significant differences were obtained in local immune parameters (increase in the level of total immunoglobulins and peroxidase activity, decrease in total levels of immunoglobulins M) in the skin mucus of chronically stressed fish with respect to unstressed fish. Regarding microbial richness and diversity, a significant increase in both the richness and diversity of microbial communities was observed in chronically stressed fish. The beta diversity analysis showed that there were significant differences between both bacterial communities, especially at 14 days. The taxonomy results indicated an increase in the abundance of bacterial species with the potential ability to control the proliferation of pathogenic species and a decrease in species considered opportunistic. The results obtained show the high sensitivity of the skin mucosa and the tendency to protect the tissue from both the immune system and the local microbiota against the alterations caused by a stressful agent. In Chapter 2, variations in the skin mucus microbiota and alterations in the immune system (systemic and local) following bacterial infection caused by Vibrio harveyi were examined in sea bass. A significant decrease in protease activity in both systemic and local immunity was detected in infected fish compared to not infected fish. Additionally, a significant decrease in lysozyme activity and a significant increase in peroxidase activity were observed in local immunity in infected fish. Regarding bacterial richness and diversity, the results showed a significant increase in bacterial diversity in infected fish. On the other hand, beta diversity indicated that the bacterial communities of infected and uninfected fish were significantly different. Finally, the linear discriminant analysis, effect-size (LEfSe) to detect biomarkers determined that the biomarkers associated with the infected fish were mainly Proteobacteria (V. harveyi) for the infected group while Verrucomicrobia (Rubritalea) was especially associated with the not infected group. In conclusion, the results of this experiment suggest the early role played by both protease and lysozyme in skin mucus in vibriosis infection. The analysis of the composition of the microbiota suggests a possible dysbiosis in the infected fish, since there is an alteration both in composition and function. Furthermore, the LEfSe assay detected V. harveyi as a biomarker of the infected group, demonstrating its potential as a tool for early infection detection. Finally, in Chapter 3 the possibility of using fecal content, instead of posterior intestinal tissue, as a non-invasive method, to determine the microbial composition of the gastrointestinal tract of gilthead sea bream, was studied. The results obtained showed that there are no significant differences between the bacterial composition of the intestinal tissue compared to that of the faeces, both in diversity and richness as well as in taxonomy, highlighting the possibility of using this matrix and thus avoiding animal sacrifice.