Abstract
Russian sturgeon (Acipenser gueldenstaedtii) is an endangered fish species and also an important resource for the sturgeon aquaculture industry in Turkiye. Recently, a fatal and persistent bacterial disease occurred in the reared sturgeon kept in a trout farm in Turkiye. The disease outbreak has been with notable external signs including petechial hemorrhages and systemic anemia. This outbreak lasted for six weeks, and cumulative mortality reached around 35.00 - 40.00%. In this study, no parasitic and viral agents were observed in the sturgeons. Citrobacter gillenii was isolated from the diseased fish and identified by biochemical and molecular methods including API 20E and 20NE and 16S rRNA gene region sequencing, respectively. As a result, C. gillenii was identified for the first time in Russian sturgeon in Turkiye. The sequence was also deposited under the Genbank with MW057770 accession number. According to the result of disc diffusion method, bacteria were sensitive to enrofloxacin, streptomycin, amoxicillin and oxytetracycline and resistant to penicillin, trimethoprim/sulfamethoxazole, florfenicol and erythromycin. Also, ampC, sul1 and floR resistance genes were detected in the isolated bacteria. The results of this study provide important information for the diagnosis and treatment of this newly emerged disease of Russian sturgeon.