Enteric septicemia of catfish, columnaris disease, and streptococcosis, caused by Edwardsiella ictaluri, Flavobacterium columnare, and Streptococcus iniae, respectively, are the most common bacterial diseases of economic significance for the pond-raised channel catfish Ictalurus punctatus industry. Several management practices are used by catfish farmers to prevent large financial losses from these diseases such as the use of commercial antibiotics and other chemicals. In order to discover environmentally benign alternatives, using a rapid bioassay, we evaluated a crude extract from the roots of muscadine Vitis rotundifolia against these fish pathogenic bacteria and determined that the extract was most active against F. columnare. Subsequently, several isolated compounds from the root extract were further evaluated against F. columnare. Among these isolated compounds, (+)-hopeaphenol (2) and (+)-vitisin A (3) were found to be the most active against F. columnare, with 24-h 50% inhibition concentrations of 4.0±0.7 and 7.7±0.6 mg/L, respectively, and minimum inhibitory concentrations of 9.1±0 mg/L for each compound. Efficacy testing of 2 and 3 is necessary to further evaluate the potential for these compounds to be used as antibacterial agents for managing columnaris disease.

Endemic freshwater sponges (Demosponges, Lubomirskiidae) dominate in Lake Baikal and are multicellular filter-feeding animals represent a complex consortium of many species of eukaryotes and prokaryotes. In recent years, mass disease and death of the L. baicalensis have been an urgent problem of Lake Baikal. The etiology and ecology of these events remain unknown. Bacteria in microbiomes of diseased sponges of the families Flavobacteriaceae and Oxalobacteraceae were dominant. Both species are opportunistic pathogens common for freshwater ecosystems. The aim of our study is to analyze the genomes of strains Janthinobacterium sp. SLB01 and Flavobacterium sp. SLB02, isolated from diseased sponges to identify the reasons for their joint dominance. The first one attacks the other cells using type VI secretion system, suppress gram-positive bacteria with violacein pigment and regulate its own activity via quorum sensing. It makes the floc and strong biofilm by exopolysaccharide biosynthesis and PEP‐CTERM proteins expression. The second one utilizes the fragments of cell walls produced of polysaccharides. Named two strains have noticeable difference in carbohydrates acquisition. We described the possible way of joint occupation of ecological niche into freshwater sponge microbial community. This study expands understanding about symbiotic relationship of microorganisms with freshwater Baikal sponges.

Using qRT-PCR analysis, the LcIL-8 expression level indicated broad expression in most detected tissues, with the most predominant expression in whole blood at 6 to 24 h after infection with S. iniae at concentrations of 105 colony-forming unit (CFU)/fish (p < 0.05). After immersion with F. covae, the LcIL-8 transcript was upregulated in the gills, liver and intestine, with the highest expression level found in the gills. However, LcIL-8 was downregulated in all tested tissues at 48 and 96 h after infection with the two pathogenic strains, indicating that Lc-IL8 has a short half-life during the early immune responses to pathogens. MIC analysis of rLcIL-8 protein against S. iniae was 10.42 ± 3.61 µg/mL. Furthermore, the functional analyses clearly demonstrated that 10 and 100 µg of rLcIL-8 protein efficiently enhanced the phagocytic activity of Asian seabass phagocytes in vitro (p < 0.05). Additionally, in vivo injection of S. iniae following the rLcIL-8 protein indicated that 50 and 100 µg of rLc-IL-8 were highly effective in protecting fish from the tested pathogen (p < 0.001). These results indicate that rLcIL-8 plays a crucial role in the defense mechanisms against bacterial infections in the target fish.