Bacillus Velezensis Fia2 as an Oxydifficidin Producing Strain and its Effects on the Growth Performance, Innate-Immunity, Intestinal Microbiota, and Resistance to Aermonas Salmonicidia Infection in Crucian Carp (Carassius Carassius)

In aquaculture, biological control of bacterial infections is a promising strategy that, aside from preventing infection, also benefits the host in several ways. In this study, a bacterial strain FiA2 isolated from the gut of crucian carp exhibited broad spectrums of activity against multiple aquatic bacterial pathogens. Crucian carp, when supplemented with FiA2 in diet, resists infection, with a survival rate of 45%. Moreover, the increase in total weight gain (15.87%) and up-regulated (P<0.05) mRNA levels of IGF-1 and IGF-2 in the muscular tissue of crucian carp demonstrate the positive role of FiA2 in their growth. Similarly, the innate immune-related genes in the liver, the spleen, and the head-kidney of the fish in the FiA2 supplemented group were significantly up-regulated (P<0.05). In addition, FiA2 modulated intestinal microbiota by; at the phylum levels, Actinobacteriota and Firmicutes increased, whereas Bacteriodota and Proteobacteria decreased. Similarly, the control group at species level was dominated by Aeromonas veronii and Plesiomonas shigelloides. Furthermore, the bioactive compound was successfully separated and identified as oxydifficidin, with a 559.47 m/z value. The oxydifficidin retained its activity even after being exposed to high temperatures, ultraviolet light, proteolytic enzymes, chemical reagents, and different levels of pH. Overall, we conclude B. velezensis FiA2, capable of producing broad-spectrum oxydifficidin, is a novel probiotic strain with positive impacts on the growth performance, innate immunity, intestinal microbiota, and disease resistance of crucian carp