Objective This study aimed to isolate and identify the pathogenic bacteria from diseased pearl gentian grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatu ♂), characterizing the isolate's species, physiological and biochemical properties, drug sensitivity profiles, and pathogenicity, thereby providing a theoretical foundation for disease prevention and control.

Method Diseased groupers were collected from a farm in Zhanjiang City. Under aseptic conditions, a dominant bacterial strain (OR539201) was isolated from the spleen of grouper. The morphological observation, 16S rRNA gene sequence analysis, physiological and biochemical identification of this isolate were performed. Further analyses including drug susceptibility testing and regression infection experiments, with bacterial loads quantified in the spleen and liver, and virulence genes were also carried out.

Result The results of morphological observation, 16S rRNA sequence analysis, and physiological and biochemical identification indicated that the strain OR539201 was belonged to Photobacterium damselae subsp. damselae. The strain formed circular colonies with rounded, full, and smooth morphology when grow upon the LB agar plate. Phylogenetic analysis of the 16S rRNA gene revealed a close relationship between this strain and P. damselae, which are both belonging to the genus Vibrio. Biochemical tests showed positive reactions for oxidase activity, urea, glycerol, and lysine, while negative reaction for salicin, aesculin, and mannitol. The drug sensitivity test revealed a varying degrees of resistance to different antibiotics, with moderate sensitivity to 6 antibiotics such as polymyxin, amikacin, and ceftriaxone, and sensitivity to 13 antibiotics including ofloxacin, ciprofloxacin, and cefalexin. Regression infection experiments demonstrated that this strain possesses significant pathogenic effects on grouper, inducing surface ulcers, red proliferative nodules, liver enlargement, intestinal erosion, and brain redness that were consistent with natural disease manifestations. Bacterial load in the spleen and liver of groupers in the experimental group were significantly higher than those in the control group, confirming the presence of P. damselae. The virulence gene detection results show that the strain possesses the gene (plpV) related to phospholipase activity.

Conclusion This study isolated and identified a potential pathogen from diseased groupers successfully, which was identified as P. damselae subsp. damselae. The strain exhibited significant pathogenicity that could induced severe lesions and fish deaths.