Objective This study aims to identify the pathogen of a widespread disease outbreak among Heliocidaris crassispina farms in Dongshan, Fujian, in May 2024, providing a basis for disease control and prevention.

Method Samples were collected from diseased H. crassispina in a farm in Dongshan County, and then inoculated onto thiosulfate citrate bile salts sucrose (TCBS) agar medium and 2216E medium. After 24-48 h of culture in an incubator, single colonies were picked from the culture medium and purified repeatedly to obtain a dominant strain with the number HD4. The pathogenic strain HD4 was identified and described through the observation of the morphological characteristics, physiological and biochemical characteristics, and molecular biology identification, artificial regression infection test, and then the whole-genome sequencing and drug susceptibility test were carried out.

Results Strain HD4 was isolated as the dominant bacterium from the surface lesions. Combined with morphological characteristics, Biolog identification, sequence analysis, and physiological/biochemical profiles, the strain was confirmed as Vibrio mediterranei. A dominant strain (HD-3) isolated from the infected H. crassispina was verified as identical to pathogenic strain HD4. The artificial regression infection test resulted in H. crassispina mortality, demonstrating the significant pathogenicity of V. mediterranei HD4 to H. crassispina. Genome sequencing revealed that pathogenic strain HD4 harbored 49 virulence genes, including luxS-SRH lyase, llpA lipoprotein, chemotaxis proteins cheW and cheY, gmhA/lpcA (GDP-mannose-6-phosphate isomerase), kdsA (lipopolysaccharide-related enzyme), flagellin fliM, and tapT (tyrosine kinase), etc., which are involved in promoting bacterial invasion and inducing tissue damage in H. crassispina. Drug susceptibility testing showed that the pathogenic strain HD4 was highly sensitive to six aquatic antibacterial agents, including enrofloxacin, thiamphenicol, flumequine, sulfamonomethoxine sodium, sulfamethoxazole+trimethoprim, and florfenicol.

Conclusion V. mediterraneus HD4 is identified as the pathogen of the disease outbreak, and the multiple pathogenic factors it carried played an important role in the pathogenesis. The study provides a scientific reference for the control and prevention of abalone diseases.