Lethal attribute of serine protease secreted by Vibrio alginolyticus strains in kuruma prawn Penaeus japonicus

Toxicity of the extracellular products (ECP) and the lethal attribute of se rine protease secreted by five pathogenic Vibrio alginolyticus strains from various sources in kuruma prawn Penaeus japonicus were studied. The ECPs o f organisms originally isolated from diseased kuruma prawn or small abalone Haliotis diversicolor supertexta were more lethal (LD50 value of 0.48 or 0 .41 mu g protein/g prawn) than those from diseased tiger prawn P. monodon, yellow-fin porgy Acanthopagrus latus or horse mackerel (LD50 value of 0.98- 1.17 mu g protein/g prawn). All the ECPs manifested strong, weak and no act ivities against gelatin, sheep erythrocytes and chitin, respectively. In im munodiffusion tests using rabbit antiserum to a purified 33 kDa serine prot ease of strain Swy against ECP of each tested strain produced one single pr ecipitation band in each treatment. Furthermore, the serine protease was su ggested to be the dominant protease secreted by V. alginolyticus strains te sted since the majority of enzymatic activity of the respective ECP was inh ibited by phenylmethanesulfonyl fluoride (PMSF). A higher inhibition of ser ine protease activity by PMSF resulted in lower mortality rate of the ECPs injected into the prawns suggesting that the protease is one of the major l ethal factor(s) secreted by V. alginolyticus.