First report on bacterial isolation from marbled flounder ( <i>Pseudopleuronectes yokohamae</i> ) associated with skin ulcers

Skin ulceration syndrome (SUS) is a significant disease affecting the marbled flounder ( Pseudopleuronectes yokohamae ) in aquaculture. This study aimed to identify the pathogen responsible for SUS outbreaks. Three bacterial species were isolated from infected flounder, with isolate Isolated strain 1 confirmed as pathogenic through artificial infection experiments. Isolated strain 1 is highly pathogenic, significantly reducing the survival rate of P. yokohamae . The strains were identified using morphological, physiological, biochemical analyses, 16S rRNA, and gyrB gene sequencing. Isolated strain 1 was found to be a Gram-negative bacillus, closely related to Vibrio splendidus . Phylogenetic analysis showed that the 16S rRNA and gyrB gene sequences of Isolated strain 1 clustered with V. splendidus , with high bootstrap confidence levels (99.09% and 100%, respectively). Consequently, V. splendidus was identified as the causative agent of SUS in P. yokohamae . Antibiotic susceptibility tests revealed that Isolated strain 1 was sensitive to norfloxacin, ciprofloxacin, enrofloxacin, ceftazidime, doxycycline and florfenicol, but resistant to tetracycline, gentamicin, erythromycin and amoxicillin. These findings provide a foundation for developing disease prevention and treatment strategies in marbled flounder aquaculture.