Infectious diseases are one of the biggest burdens in the growing aquaculture industry. Novel alternative strategies to improve fish health and increase disease resistance are urgently needed. The carotenoid astaxanthin is a widely used feed supplement with well-known health benefits for the host and is used as a/or colourant for fish flesh/skin. However, due to high productions cost, its use is restricted to high-value species. In the following experimental study, we tested a novel two-species probiotic - Bacillus indicus and B. subtilis - as a potential alternative health supplement to astaxanthin in mirror carp (Cyprinus carpio). The main goal of the study was to use experimental feed trials and 16 S rRNA microbiome profiling to compare the effects of B. indicus and traditional astaxanthin supplementation on fish performance and health, based on the overall microbial community of the gastrointestinal tract.
Over a 7-week carp trial, fish fed the probiotic showed a greater specific growth rate (SGR) compared to the control, however not significant. Fish fed astaxanthin demonstrated significantly increased SGR. The haematological and immunological analysis showed significant results in fish fed the probiotic (e.g., increased phagocytic activity). In carp, we found that both treatment groups significantly increased microbial diversity in the distal intestine (Figure 1). The probiotic increased the abundance of potentially beneficial bacteria including Bacillus spp. Overall, the probiotic blend induced a wider community shift compared to astaxanthin. In addition, a greater number of bacteria metabolic pathways were altered in carp fed the probiotic compared to astaxanthin. However, during the microbial analysis, the probiotic bacteria species could not be detected in the distal intestine.
The probiotic supplement B.Indicus/B.subtilis significantly increased gut microbial diversity in carp, similar to astaxanthin supplementation. However, the probiotic induced greater changes in abundance across the microbial community. Increased microbiome diversity is likely linked to healthier fish, for example, our bloodwork analyses demonstrated significantly increased phagocytic activity in fish fed the probiotic, indicating an increased immune response. Overall, we conclude that the B.inidicus/subtilis probiotic has great potential to be used as a replacement for astaxanthin.