Outbreaks of serious mortality among cultured abalone postlarvae have occurred across Southern China since July 2002. Five motile bacterial strains were isolated from diseased abalone postlarvae on tryptic soy agar supplemented with 1% NaCl (TSA1) and/ or thiosulphate citrate bile salt (TCBS) sucrose agar plates during an outbreak in August 2003 in Shanwei, Guangdong province. All isolates were characterized and identi¢ed asVibrio alginolyticus on the basis of biochemical characteristics and comparisonswiththose of the reference strainV. alginolyticus ATCC 17749. Strain 19 (a representative of ¢ve similar isolates) was virulent to abalone postlarvae with an LD50 value of 1.00 104 colony-forming unitsmL 1. All abalone postlarvae exhibited the same signs as in natural outbreaks.The same bacterium could be re-isolated from abalone postlarvae after bacterial challenge usingTSA1andTCBS plates. The results reveal thatV. alginolyticus is an infectious agent of abalone postlarvae.

Mass mortality among the post-larvae of cultured abalone Haliotis diversicolor supertexta has occurred on the south coast of China since 2002. The diseased abalone are generally 10 to 30 days old, and typical signs of the disease include them turning white in colour and falling off the diatom films on which they were cultured. Among sixteen different motile bacteria isolated from the diseased post-larvae, four were identified as Vibrio parahaemolyticus on the basis of biochemical characteristics when compared with those of a V. parahaemolyticus type strain ATCC 17802T. Isolate 25, a representative isolate of V. parahaemolyticus recovered from diseased abalone, was virulent for the post-larvae with an LD50 value of 3.5 × 105 CFU (colony forming units)/ml. All moribund post-larvae artificially infected with the bacterium turned white and fell off the diatom films on which they were cultured as seen to occur during natural outbreaks of the disease, and it was possible to recover the bacterium from artificially infected post-larvae. The results of the study indicate that V. parahaemolyticus is a pathogenic bacterium to abalone post-larvae.

An epidemic of mass mortality of abalone (Haliotis diversicolor supertexta) postlarvae aged 40 days or less has existed across south coast of China since the second half of 2002. Among 20 bacterial strains isolated from diseased abalone postlarvae on 2216E marine agar plates during an outbreak of postlarval disease in August 2005, a predominant strain (designated strain 20) was demonstrated to be virulent to postlarvae with an LD50 value of 1.0-105 colony forming units (CFU ml 1) on day 4, while the other 19 strains were either avirulent (16 strains) or weakly virulent (3 strains). The same bacterium could be re-isolated from postlarvae after bacterial challenge using 2216E marine agar plates. Preliminary toxicity tests of ECPs of strain 20 revealed that at 2.77 mg protein ml 1, crude ECPs completely liquefied postlarvae within 24 h, leaving only shells. API 20E analysis identified strain 20 as Klebsiella oxytoca. 16S and ITS rDNA sequencing and phylogenetic analyses further confirmed this identification. Antibiotic susceptibility tests showed that strain 20 exhibited 94% of susceptibility to 16 various antibiotics tested and only showed resistance to streptomycin. Results of this work demonstrated that K. oxytoca is also linked to this epidemic in Fujian, China. This is considered to be the first report regarding K. oxytoca involved in the mass mortality of postlarval abalone in south China and the world.