Isolation, Identification, and Pathogenicity of Retrieved from European Seabass () Farmed in Türkiye.

In this study, strain C24 was isolated from cases of winter mortalities of hatchery-reared European seabass () broodstock in Türkiye. The first mortalities were reported in September 2016 and occurred annually in early autumn/late winter until the end of February 2019, when 15% of accumulated mortality was recorded. Diseased moribund fish exhibited general septicemic signs, including dermal ulcerations with hemorrhagic margins, distended abdomens, and hemorrhages below the pectorals, pelvic fins, and at the operculum. Postmortem findings showed congestion in several internal organs, hemorrhagic ascitic fluid, and congested prolapsed anal openings. The representative bacterial isolate strain C24 was characterized as Gram-negative, motile, nitrite-producing, and as vibrio static agent O/129-sensitive. The full-length 16S rRNA sequence (Accession No. ON778781) and gene sequence (Accession No. ON792326) of the C24 strain showed high similarity to strains. Moreover, the whole-genome average nucleotide identity (ANI) values (ANI > 97.7%) against four strains above the species demarcation limit unambiguously identified the C24 isolate as a member of this species. A preliminary virulence-gene analysis showed that the isolate C24 encoded at least three exotoxins, including two aerolysins and a thermolabile hemolysin. The experimental infection showed that the C24 isolate exhibited low to moderate virulence in experimentally infected European seabass juveniles. Interestingly, antimicrobial susceptibility testing revealed that the C24 isolate was susceptible to nalidixic acid, ciprofloxacin, and several other antibiotics but resistant to tilmicosin, kanamycin, streptomycin, and ampicillin. To our knowledge, this study is the first to report that could be considered an emerging bacterial pathogen in Türkiye, and it may threaten the international European seabass production.