The isolation of a novel Streptomyces termitum and identification its active substance against fish pathogens

The emerging outbreak of bacterial diseases poses a major threat to the aquaculture industry, and the abuse of antibiotics leads to the emergence of drug-resistant pathogens; hence, the development of antibacterial agents from natural resources to control and manage fish bacterial diseases has become a promising strategy. In this study, seven potential Streptomyces spp. with antimicrobial activity against fish pathogenic bacteria were isolated from the soils of different regions in China. Streptomyces termitum N-15, which showed the strongest and most stable antibacterial activity, was selected for the fish disease control experiment. Then, we investigated the effect of a basal diet and N-15 supplementation (108 ​CFU/mg diet) on growth performance, blood immune parameters (ACP and AKP), and the expression of immune genes. The results demonstrated that the ACP and AKP activities of serum were significantly increased in the N-15-supplemented group. Moreover, N-15 upregulated the expression level of Toll-like receptor 4 (TLR-4) in the kidney, liver, spleen and head kidney. The challenge test showed that supplementation with N-15 could significantly improve the survival rates of grass carp infected with A. veronii. Chalcomycin A was purified from Streptomyces termitum N-15 and found to have antimicrobial activity against fish pathogenic bacteria for the first time. Furthermore, the complete genome of S. termitum N-15 was found to consist of an 8,887,222 bp circular chromosome with 72.89% GC content and a plasmid genome, including 7744 protein-coding sequences (CDSs). The genome of S. termitum N-15 contained gene clusters for the synthesis of multiple secondary metabolites, including melingomycin, sulforaphane, oromycin, chlortetracycline, isomycin, and primycin, by antiSMASH online software. Gene cluster 1.5 revealed a chalcomycin A biosynthesis gene cluster (chmGⅠ, chmGⅡ, chmGⅢ, chmGⅣ, chmGⅤ). In summary, S. termitum N-15 and its secondary metabolites are expected to be developed into inoculants for promoting fish health in aquacultural systems.