Identification and pathogenicity of Plesiomonas shigelloides from Acipenser dabryanus in China

Acipenser dabryanus is designated as a critically endangered species on the IUCN Red List and known as a first-class protected animal in China. Recently, A. dabryanus fries (approximately 45 days old) in a fishery institute in south-western China suffered from a high rate of mortality. In this study, a strain of Plesiomonas shigelloides was isolated from the liver and heart of the infected fry, named 22S, and confirmed to be related to the mortality of A. dabryanus fries. Plesiomonas shigelloides was identified using biochemical characterization and molecular analysis. A 16S rDNA gene sequence alignment analysis found that the isolate shared 100% homology with P. shigelloides (NCBI GenBank Accession Number-NR 044827) in this gene. Antimicrobial susceptibility testing showed that 22S was resistant to penicillin, oxacillin, clindamycin, trimethoprim and lincomycin and sensitive to nitrofurantoin, levofloxacin tablets and norfloxacin. The LD50 value was 5.94*105 cfu/g. Clinical symptoms of challenged fish were the same as those of naturally infected fish. Histopathology of the infected fish showed that the viscera and gill were severely damaged. This is the first report suggesting that P. shigelloides infection is associated with mass mortality of A. dabryanus. The results of this study elucidate the pathogenesis and severe pathogenicity of P. shigelloides in cultured A. dabryanus and highlight the need for appropriate disease control and prevention measures.