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Aeromonas is the main pathogen causing bacterial diseases in fish. The disadvantages of chemical drugs to control fish diseases have been highlighted, and it is urgent to find an eco-friendly control method. In this study, an actinomycete strain with antibacterial activity against fish pathogenic bacteria was screened from soil samples. Combined with morphological characteristics, physiological and biochemical characteristics, and gyrB gene and whole genome comparison analysis, it was identified as a new strain of Streptomyces enissocaesilis, named Streptomyces enissocaesilis L-82. The strain has broad-spectrum antibacterial activity against fish pathogens. A substance with a mass-to-charge ratio of 227.20 [M + H] + was isolated and purified by high-performance liquid chromatography and mass spectrometry. It was presumed to be a derivative of 5-dimethylallylindole-3-acetonitrile. The strain is safe and non-toxic to crucian carp, and can stably colonize crucian carp and inhibit the proliferation of A. hydrophila. After feeding the feed containing 1 × 108 CFU/mL strain concentration, the weight growth rate and specific growth rate of crucian carp increased, the activity of ACP and SOD in serum increased, and the survival rate of crucian carp increased after challenge. Genome-wide analysis showed that the strain had strong ability to metabolize and tolerate extreme environments. And has a strong potential for disease resistance. Therefore, the strain is expected to be developed as a feed additive for fish farming. Key points: • The new Streptomyces enissocaesilis L-82 has a broad spectrum and stable antibacterial activity and meets the safety standards of feed additives. • Strain L-82 can colonize crucian carp, improve the growth, antioxidant, and immune performance of the host, and improve the survival rate after being infected with A. hydrophila. • Genome-wide analysis suggests that the strain has great disease resistance potential and is expected to be developed as a feed additive for fish culture. [ABSTRACT FROM AUTHOR]

The Streptomyces lateritius Z1‐26 was isolated from soil samples which showed broad‐spectrum antibacterial activity against a broad range of fish pathogens. The In Vivo Imaging System (IVIS) monitored that strain Z1‐26 could survive and colonize in the gills and abdomen of crucian carp. The effects of dietary supplementation with strain Z1‐26 were evaluated with respect to the growth performance, antioxidant capacity, and immune response of crucian carp. The results showed that the Z1‐26‐fed fish had a significantly higher growth rate than the fish fed the control diet. The immune and antioxidant parameters revealed that the non‐specific immune indicators (AKP, SOD, and LZM) of the serum, the expression of immune‐related genes (IgM, C3, and LZM), and antioxidant‐related genes (Nrf2 and Keap1) of the immune organs were significantly increased, whereas the expression of pro‐inflammatory factors (IL‐1β, IL‐8, and TNF‐α) of the immune organs was significantly down‐regulated in crucian carp fed strain Z1‐26 compared with fish fed a control diet. Moreover, fish fed with Z1‐26 supplemented diets showed a significantly improved survival rate after Aeromonas hydrophila infection. In addition, the whole genome analysis showed that strain Z1‐26 possesses 28 gene clusters, including 6 polyketide synthetase (PKS), 4 non‐ribosomal peptide‐synthetase (NRPS), 1 bacteriocin, and 1 lantipeptide. In summary, these results indicated that strain Z1‐26 could improve the growth performance and disease resistance in crucian carp, and has the potential to be developed as a candidate probiotics for the control of bacterial diseases in aquaculture. [ABSTRACT FROM AUTHOR]