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The genotypic characterization of some bacterial pathogens that were incriminated in disease outbreaks among Haffara seabream "Rhabdosargus haffara" and marbled spinefoot "Siganus rivulatus" was investigated in full through the current study. A total of 250 fish samples (125 of each species) were collected along the Red Sea coasts of Hurghada City, Egypt. Fish samples were inspected for clinical signs, post-mortem changes, and bacteriological examination. The investigated fishes displayed septicemic signs and external skin lesions characteristic for streptococcosis and tenacibaculosis. Enterococcus spp., Streptococcus spp., and Flavobacterium spp. were isolated from moribund fishes using selective media. These isolates were phenotypically and genetically identified and characterized. The identities of bacterial isolates were confirmed as Enterococcus fecalis, Streptococcus iniae, Enterobacter cloacae, and Tenacibaculum maritimum based on sequencing the 16S rRNA gene. Enterococcus fecalis was the most common pathogen isolated from S. rivulatus and R. haffara and accounted for 46.8% and 47.8% of the total isolates, respectively. Accordingly, the present study proved that Enterocoocus spp., Streptococcus spp., and T. maritimum are important pathogenic bacteria incriminated in wild fish outbreaks in the Red Sea in Egypt. These findings proved the importance of regular and permanent bacteriological examination of wild fish to overcome fish mortalities, which lead to economic losses. Finally, sequencing and phylogenetic relationship techniques proved their usefulness as an ideal assay to develop a reliable, accurate, and rapid detection method for bacterial pathogens in the aquatic environment. [ABSTRACT FROM AUTHOR]

In farmed saltwater fish, vaccination is considered the first barrier against pathogenic agents. In Egypt, the vaccine application can diminish the dilemma of summer mass kills of farmed marine fish. The efficacy of booster vaccination with a bivalent vaccine against vibriosis was studied in European seabass. Fish were vaccinated by direct immersion (DI) in water containing bivalent Vibrio alginolyticus and Vibrio parahemolyticus whole cells killed vaccine. Immersion vaccination is more applicable under semi‐intensive fish culture, but less potent than injection. Accordingly, to increase antigen uptake and conferred protection, vaccinated fish randomly received booster vaccination via direct immersion (DI imm.) or intraperitoneal injection (i.p.) at 21 days post‐prime vaccination (21 dpv). Vaccinated and unvaccinated fish were monitored for 3 months and challenged with V. alginolyticus and/or V. parahemolyticus at 42 dpv. Specific and non‐specific immune responses were evaluated in blood samples. At 52 dpv (10 days post‐challenge), the relative immunoglobulin T (IgT) expression was assessed in the gills, hindgut, head kidney and spleen. Notably, imm. + imm. and imm. + i.p. vaccination yielded significantly higher specific antibody titre at 70 dpv, whereas the former was associated with significantly higher IgT expression in the gills and hindgut (p <.05) than the latter. High IgT levels in mucosal organs (gills and hindgut) might be the first indicator of the involvement of IgT in mucosal immune responses. Relative per cent survival among booster‐vaccinated fish ranged from 70% to 90%. The results support the use of this vaccination protocol as a highly effective tool to protect seabass from vibriosis and to improve fish health and environment. Booster doses are strongly recommended under farming environments. [ABSTRACT FROM AUTHOR]