The pirarucu (Arapaima gigas) from Amazon basin is currently the largest farmed fish species and its production is increasing rapidly in Brazil. However, there is a concern about bacterial disease outbreaks and resulting mortalities in pirarucu farms. The use of probiotic bacteria as prophylactic method is recognized as beneficial practice to enhance fish production. The aim of this thesis was to isolate autochthonous probiotic strains from the pirarucu intestine, characterize their potential probiotic characteristics in vitro, and perform in vivo colonization and growth experiments. To this end, the intestinal microbial community of A. gigas was assessed at two different growth stages using high-throughput sequencing (HTS) analysis. Proteobacteria, Fusobacteria and Firmicutes were the most abundant phyla. At genus level Bradyrhizobium and Cetobacterium were the most abundant in adult and juvenile fish, respectively. In a further trial two isolated lactic acid bacteria (LAB): Lactococcus lactis subsp. lactis and Enterococcus faecium, showed in vitro suitability as probiotics. Two potential pathogens: Citrobacter freundii and Pseudomonas sp., were also isolated and their pathogenicity assessed. Subsequently, an in vivo experiment was performed to assess the potencial of LAB strains to colonise and modulate the gut microbiome of pirarucu after 21 days of feeding. The results showed a high abundance of Cetobacterium in all treatments. Additionally, both probiotic treatments decreased the levels of Clostridiales in pirarucu intestine and showed adherence to the fish mucosal tissue. Finally, a growth experiment was performed to assess the ability of to the candidate probiotics to improve growth parameters after 42 days of feeding. HTS confirmed that Cetobacterium was the most abundant genus in all treatments. Fish fed with L. lactis subsp. lactis presented higher percentage of increase (%I) of weight, specific growth rate (SGR), and monocytes in blood. The strain E. faecium interacted with the microbial gut community and was able to populate the mucosal tissue. In conclusion, both LAB strains presented probiotic characteristics and should be considered as probiotics in A. gigas farming. These probiotics could contribute to a reduction in antibiotics use in pirarucu farms, thus, adding value to the species as a sustainable aquaculture product.

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