Isolation of Vibrio parahaemolyticus from abalone (Haliotis diversicolor supertexta L.) postlarvae associated with mass mortalities

Outbreaks of mass mortality among cultured postlarvae of abalone Haliotis diversicolor supertexta aged between 7 and 30 days occurred since 2002 on the south coast of China. Among 24 bacterial strains isolated from diseased abalone postlarvae on 2216E Marine and TCBS agar plates during an outbreak in July 2003, 23 were avirulent whilst a predominant strain (designated as strain 2) was highly virulent to postlarvae with an LD50 value under 1.0 × 103 colony forming units (CFU) ml− 1. All the moribund/dead postlarvae exhibited the same gross symptoms as that observed in natural outbreaks. The same bacteria could be re-isolated from postlarvae after bacterial challenge using 2216E Marine and TCBS agar plates. API analysis identified it to be Vibrio parahaemolyticus with 99% confidence. 16S and ITS rDNA sequencing analysis also revealed it to be highly homologous with Vibrio parahaemolyticus. Kanagawa reaction and PCR amplification of TDH gene on strain 2 proved to be negative. Antibiotic susceptibility tests showed that strain 2 exhibited 56.25% of susceptibility to chemotherapeutic agents tested, and was resistant to penicillin G, amikacin, trimethoprim-sulfamethoxazole, tetracycline, ciprofloxacin, novobiocin and neomycin. Results in this study reveal that V. parahaemolyticus strain 2 is an infectious agent to abalone postlarvae in South China.