Atypical Aeromonas salmonicida infection in the black rockfish, Sebastes schlegeli Hilgendorf, in Korea

Cultured black rockfish, Sebastes schlegeli, suffered mass mortalities during winter 2008 and spring 2009 in Korea, showing clinical signs of ulcer lesions and haemorrhages over their body surface. The aetiological agent was identified as Aeromonas salmonicida (strains RFAS-1, -2 and -3), which is a non-pigmented, slow-growing bacterium. Phenotypes of RFAS strains showed variation, while 16S rRNA, gyrB, rpoD, dnaJ and recA gene sequences of all the strains were affiliated to A. salmonicida. In particular, vapA gene sequences of the strains were most closely related to one of the five subspecies of A. salmonicida subsp. masoucida (=KCCM 40239T). LD50 values of RFAS-1 for intraperitoneal and intramuscular injection were 1.5 x 105.25 and 1.5 x 106.4 cfu/rockfish, respectively. However, A. salmonicida strains KCCM 40239T and SAS-1, which originate from masou and chum salmon, respectively, were not pathogenic to black rockfish. RFAS strains, possessing A-layer protein on their surface, exhibited beta-haemolytic activity against rockfish erythrocytes and capability to survive in rockfish serum, which seem to be associated with virulence.