IMMUNOMODULATION IN NILE TILAPIA Oreochromis niloticus FOLLOWING A DIETARY SUPPLEMENTATION OF DIFFERENT Bacillus subtilis STRAINS

Tilapia is an economically important aquaculture species cultured around the world, with production only second to carp species. Despite their relative high tolerance to a broad range of stressors, compared to other fish species, major losses still occur as a result of bacterial diseases. Because of the controversy around antibiotic application, the scope has shifted to the use of more environmentally friendly agents, such as probiotics, to increase both immune responses and fish health.  Although the number of probiotic studies in tilapia has increased in recent years, much work remains to be done as the mechanisms of action of many probiotic candidates remain to be deciphered in tilapia. This study assessed the effects of two probiotic candidates of Bacillus subtilis at the systemic and organ level functionalities of the innate immune response in Nile tilapia.

For this work, sixty non-sexed fish (161±3.9 grams) were split into three groups and were maintained in one recirculating aquaculture system. On day 0, twelve fish per group were tagged intramuscularly near the dorsal fin to follow the individual fish immune responses over time. Each group was fed a diet supplemented with spores of either of two B. subtilis strains or a control diet for 51 days. On days 14, 28 and 51, blood was collected from caudal vessels of the fish, and lysozyme content and alternative complement activities were measured in the plasma. On day 51, spleen and anterior kidney samples were collected to measure phagocytic activity, respiratory burst, and immune gene expression of pro-inflammatory cytokines.

Throughout the trial, both plasma lysozyme concentration and plasma alternative complement of tilapia were significantly increased (p<0.05) by varying degrees after dietary supplementation with either probiotic strain, relative to the non-treatment control.

On day 51, there was a trend towards increased phagocytic and respiratory burst activities observed in immune organ derived leukocytes. Consistent with this data, there were elevated levels of pro-inflammatory cytokine mRNA levels in these immune organs. Moreover, the occurrence of these results could be more tightly associated with supplementation of the probiotic strains, given that Bacillus bacteria were observed to populate the intestines of the treatment groups.

These results suggest the potential roles of these probiotic candidates to stimulate immune responses of Nile tilapia over time, without impairing the growth of the host.