One of the largest challenges for the sustainable development of global aquaculture is the threat of infectious diseases. Preventative strategies that reduce antibiotic use are required to ensure fish health, minimise infectious diseases and subsequent pharmaceutical interventions. Recent strategies involve health-promoting feed supplements, such as astaxanthin and probiotic bacteria. Astaxanthin, a widely used carotenoid, offers colouration and antioxidant properties that can improve fish growth and fish survival when challenged with a pathogen. Probiotics can provide fish with a range of health benefits ranging from enhanced feed digestion, synthesis of vitamins, boost of innate immune response and active defence against potential pathogens.
In this study, we tested if novel probiotic blends (Bacillus subtilis and/or Bacillus indicus) can be used as alternative health and/or colouration supplements to astaxanthin in two cyprinid species, mirror carp (Cyprinus carpio) and Red Comet goldfish (Carassius auratus auratus). Using experimental feed trials and 16S rRNA microbial profiling, the impact of the probiotic on fish growth and microbial community within the distal gastrointestinal tract was assessed. In addition, in mirror carp, blood samples were tested for immunology and haematological parameters, while in goldfish colouration of the skin was analysed.
Mirror carp fed astaxanthin showed significantly increased growth whereas B. substilis /B.indicus supplementation had non-significant effects on growth performance. Our results provide the first insights into how the supplementation of astaxanthin changes the microbial composition in cyprinid species. In mirror carp, astaxanthin and the probiotic blend induce a significant shift in gut microbial communities. Mirror carp fed B. substilis/ B.indicus showed several indices of potential microbial and health benefits such as increased diversity, an abundance of potentially beneficial bacteria and enhancement of the phagocytic activity and creatinine blood levels. However, no effect on colouration, growth or the microbial community was found in goldfish, highlighting substantial species-specific differences in response to probiotics, in two closely related cyprinid species. Further research into the efficacy and site of colonization of supplemented bacteria in fish gastrointestinal tracts, and the mechanisms underlying the observed shifts in the host microbiota, is required to fully understand species-specific responses to probiotic supplementation.