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A new strain of Paenibacillus polymyxa S3 with antagonistic effects on 11 major fish pathogens (especially Aeromonas hydrophila), but had no toxicity to grass carp, was screened from the sediment of fishponds. In vivo colonization studies showed that strain S3 could be colonized and distributed in the gill and abdomen of the grass carp. Bioassay results showed that the weight growth rate of grass carp in the strain S3 oral group (16.01%) and strain S3 immersion group (16.44%) was significantly higher than those of the control group (8.61%). At the same time, the activities of ACP, AKP, CAT and GSH‐Px in the serum of grass carp in oral and immersion groups were significantly higher than those of the control group. In addition, the treatment with strain S3 could significantly upregulate the expression of the antioxidant‐related genes and immune‐related genes Keap1, Nrf2, C3, LZM, IgM, TLR‐4 and MyD‐88 in grass carp tissues. The challenge test showed that strain S3 treatment significantly increased the survival rate of grass carp infected with Aeromonas hydrophila. Whole genome sequencing analysis showed that strain S3 had 16 active metabolite gene clusters, indicating that it had abundant gene resources, which provided important support for its development for fish microecological preparations. In summary, a new strain of Paenibacillus polymyxa S3 with antibacterial activity against a variety of fish pathogens was screened in this study and its probiotic function was evaluated, proving its potential value in fisheries. [ABSTRACT FROM AUTHOR]

This study evaluated the inhibition and interaction of Bacillus velezensis BvL03 as a probiotic agent against Aeromonas hydrophila. Strain BvL03 isolated from sediment samples of fish ponds had excellent antimicrobial activity against several fish pathogenic bacteria, especially Aeromonas, including A. hydrophila, A. veronii, A. caviae, and A. sobria. The successful amplification of lipopeptide antimicrobial chemical biosynthetic genes, including iturin family (ituA, ituB, and ituD), bacillomycin family (bacA, bacD, and bacAB), surfactin family (srfAB, srfC, and srfAA), and subtilosin family (albF and sunT) from the genome of BvL03 strain, confirmed its predominant antimicrobial activity. The challenge test suggested that BvL03 significantly decreased fish mortality when challenged with A. hydrophila, which had a cumulative mortality of 12.5% in the treatment group. Toxicity and hemolytic activity of A. hydrophila after co-cultured with BvL03 were relieved as confirmed by the cell experiments, when the initial inoculated concentration of BvL03 was 109 cfu/mL or higher. Moreover, the BvL03 strain labeled with GFP protein (BvL03-GFP) and AhX040 strain labeled with mCherry protein (AhX040-mCherry) were injected into grass carps. The fluorescence levels were monitored by using In Vivo Imaging System (IVIS), in which the green color was steadily increasing, whereas the red color was gradually weakening. Whole genome sequencing revealed that strain BvL03 possesses 15 gene clusters related to antibacterial compounds, including 5 NRPS gene clusters and 3 PKS gene clusters. These results suggested that B. velezensis BvL03 has the potential to be developed as a probiotic candidate against A. hydrophila infection in aquaculture. [ABSTRACT FROM AUTHOR]