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 (LD 50 ) values (14 d) of V. splendidus, Shewanella sp. and P. tetraodonis were 1.74 × 10 7 , 7.76 × 10 6 , 7.24 × 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. KEY WORDS: Sea cucumber · Apostichopus japonicus · Bacterial pathogen · Vibrio splendidus · Shewanella sp. · Pseudoalteromonas tetraodonis Resale or republication not permitted without written consent of the publisherDis Aquat Org 91: [223][224][225][226][227][228][229][230][231][232][233][234][235] 2010 gradient gel electrophoresis (DGGE) (Becker et al. 2004). Six isolates obtained from the lesions of diseased sea cucumbers were identified as Vibrio sp. but the pathogenicity assays of isolates were not successful (Becker et al. 2004). It is commonly believed that this disease of sea cucumbers is due to bacterial infections; however, the agent that initiates the skin ulcerations has not been identified (Becker et al. 2004). Most of the studies mainly focused on isolation and identification of pathogens.No information is available for the distribution, diversity and appearance of pathogens in cultured sea cucumbers in China. The objective of this study was to carry out a precise characterization of bacterial pathogens responsible for outbreaks that affected cultured sea cucumbers during the winter-spring period from 2004 to 2006 in northeastern China. MATERIALS AND METHODSSampling and processing. Seventy-three sea cucumbers were collected from 11 farms located in the suburbs of Dalian in northeastern China during winterspring from 2004 to 2006 whenever a disease case was reported (Table 1). Three moribund or newly dead sea cucumbers showing disease signs were collected for bacteriologic assays at each sampling time from these areas. All samples were transported to our lab at 4°C within 24 h. The body weights of the sea cucumbers were ...

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