Probiotic feeding improves the immunity of pacus, Piaractus mesopotamicus, during Aeromonas hydrophila infection

The utilization of probiotic bacteria have been widely tested and applied in intestinal microflora modulation, through competitive exclusion of the pathogenic bacteria. Aeromonas hydrophila is a gram-negative bacterium, responsible for fish outbreaks in farms around the world, and is one of the major loss causes for neotropical fish farmers. 660 pacus (67 ± 7 g) were distributed in 20 tanks (n = 33), constituting five groups (four tanks for each treatment): four groups were fed with different levels (2, 4, 8 and 16 g kg−1) of Bacillus cereus and Bacillus subtilis (1:1, 108 CFU g−1), and the fifth group was fed with a control diet (without probiotic). Pacus fed with probiotic showed increment in the ROS production associated to elevated neutrophil and monocyte counts and increased phagocytic activity without affecting the growth parameters. Probiotic fed fish presented higher survival rates, subjected to an i.p. challenge with 108 CFU mL−1 of A. hydrophila. The results demonstrated a dose response effect and the ideal level of the probiotic (Bacillus cereus and Bacillus subtilis, 1:1, 108 CFU g−1) in P. mesopotamicus diets was around 8 g kg−1, as the highest survival rates and immunological responses were found in groups of fish fed with this diet. In addition, too much probiotic should be avoided, as pacus fed diet with 16 g kg−1 showed partial suppression of these responses.