Phenotypic and genetic characterization of bacteria isolated from diseased cultured sea cucumber Apostichopus japonicus in northeastern China

During the winter-spring from 2004 to 2006 in northeastern China cultured Japanese sea cucumber Apostichopus japonicus suffered from a serious disease. Clinical signs included swollen mouth, skin ulceration and massive mortality. Clinical samples taken during this period were studied. Thirty-one bacterial samples were isolated from diseased sea cucumbers and identified through biochemical tests, 16S rRNA gene sequence analysis and PCR amplification, followed by pathogenicity determination. The results showed that the 31 isolates belonged to the genera Vibrio (64.5%), Shewanella (12.9%), Serratia (12.9%), Pseudoalteromonas (6.4%) and Flavobacterium (3.2%). The 3 prominent strains were Vibrio splendidus (41.9%), Shewanella (12.9%) and Serratia odorifera biogroup I (12.9%). Pathogenicity tests demonstrated that 13 out of 31 isolates were pathogenic, including 8 strains of V splendidus, 3 strains of Shewanella sp. and 2 strains of Pseudoalteromonas tetraodonis. The pathogenic V. splendidus showed the highest frequency of appearance. Median lethal dose (LD50) values (14 d) of V. splendidus, Shewanella sp. and P. tetraodonis were 1.74 x 10(7), 7.76 x 10(6), 7.24 x 10(7) CFU g(-1) body weight of sea cucumber, respectively. The virulences differed by species: Shewanella sp. > V splendidus > P. tetraodonis. This is the first report of Shewanella sp. virulence in sea cucumber.