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The bacterial communities associated to the gastrointestinal tract in fish, are involved in physiology, development, and immune responses against pathogenic bacteria. However, for Piaractus brachypomus (White Cachama) a tropical fresh water fish from the Amazonas and Orinoco basins and the second most produced fish in Colombia, these communities are still largely unknown. The study of bacterial communities and how they are affected have increased in fish with economic value. In this study, reared individuals of P. brachypomus were chosen to investigate the structure of intestinal microbiota by culture-dependent methods, Denaturing Gradient Gel Electrophoresis (DGGE) and 16S rRNA gene survey, taking into consideration the anterior and posterior gastrointestinal tract of juvenile and adult fish. Results showed that intestinal microbiota of P. brachypomus was dominated by Fusobacteria, Spirochaetes, Firmicutes and Proteobacteria across life stages and gastrointestinal sites. We observed statistically significant differences in beta-diversity between life stages. Some genera significantly higher in juvenile individuals e.g. Brevinema , had a very low abundance in adult ones, suggesting that developmental stage may select some bacterial groups, and Cetobacterium , a B12 vitamin producing bacteria, showed significant dominance across gut sites and life stages, suggesting an important role in P. brachypomus digestion. Culture dependent techniques allowed the isolation of bacteria of interest in aquaculture, previously reported fish probiotic and pathogenic bacteria were obtained. The combination of different approaches allowed the first assessment of the gastrointestinal associated bacterial communities of P. brachypomus , showing that developmental has an influence on its composition and structure. • First high-throughput screening of P. brachypomus gastrointestinal bacterial microbiota, second most farmed fish in Colombia. • Strains of concern in aquaculture such as: Plesiomonas, Edwardsiella, Aeromonas, Lactococcus and Enterococcus, were isolated. • Relevant information of composition and variables that affect the gut bacterial communities of (P. brachypomus) was obtained. [ABSTRACT FROM AUTHOR]