Characterization of two pathogenic Photobacterium strains isolated from Exopalaemon carinicauda causing mortality of shrimp

Bacterial and viral infectious diseases are serious problems in shrimp farming. During the culture of Exopalaemon carinicauda, we found that some individuals showed typical symptoms with pale color for their hepatopancreas, and whitish color for their body, which caused the final mortality of shrimp. In order to know the agent which caused the mortality of shrimp, we isolated the dominant bacteria and analyzed their characteristics. Histopathological observation showed that the tubule lumens of hepatopancreas were enlarged, and the cells located at the hepatopancreatic tubes were damaged severely. Two dominant strains named WH01 and WH02 were isolated from the hepatopancreas of diseased shrimp by thiosulfate citrate bile salt sucrose (TCBS) agar. Analysis on their partial 16S rRNA sequences showed that WH01 and WH02 had the highest similarity with Photobacterium damselae subsp. damselae. Furthermore, a specific molecular methodology (using toxR gene) and Biolog system phenotypic analysis confirmed that WH01 and WH02 were two strains of Photobacterium damselae subsp. damselae. Hemolysis test showed that both WH01 and WH02 strains could cause beta hemolysis on sheep blood agar plates. Artificial infection test revealed that both WH01 and WH02 could infect the shrimp E. carinicauda and lead to 100% mortality. The median lethal dose (LD50) for WH01 was 2.30 × 105 CFU/g (body weight) and the LD50 for WH02 was 1.09 × 105 CFU/g (body weight). In addition, these two strains could also infect the shrimp Litopenaeus vannamei to cause its mortality by intramuscular injection. These data indicated that Photobacterium might be a common pathogen of shrimps. The pathogenicity of Photobacterium to shrimp must be paid more attention for shrimp farming.