P.‐C. LIU, K.‐K. LEE AND S.‐N. CHEN. 1996. The pathogenicity of six Vibrio harveyi strains in tiger prawn, Penaeus monodon, was studied, using both live bacteria and extracellular products (ECP). The organisms originally isolated from diseased penaeids were more virulent using both live bacteria and ECP (LD50, 4.87–8.65 times 104colony‐forming units (cfu) and 1.20–1.51 μg protein g‐1body weight) than the two reference strains originally isolated from either sea water (ATCC 25919; LD50, 3.18 times 106cfu and 2.70 μg protein g‐1body weight) or diseased Talorchestia sp. (ATCC 14126, 0.418 times 106cfu and 2.34 μg protein g‐1body weight). Each strain was reisolated from the haemolymph and the hepatopancreas of moribund prawns following each bacterial challenge. Both the live bacteria and the ECPs of the penaeid isolates exhibited stronger proteolytic (caseinase), phospholipase and haemolytic activities than those of the reference strains. These results indicate that there are differences between penaeid and non‐penaeid isolates of V. harveyi in pathogenicity and reveal that proteases, phospholipases, haemolysins or exotoxins might play leading roles in the pathogenicity of V. harveyi in the tiger prawn, Penaeus monodon.

Outbreaks of serious mortality among cultured kuruma prawns (Penaeus japonicus) with white spotted syndrome in the carapace occurred in the summer of 1993 in I-Lan, Taiwan. A swarming bacterium, strain Swy, was isolated from the hepatopancreas of the moribund prawns using tryptic soy agar supplemented with 1% NaCl and/or thiosulphate citrate bile salt sucrose agar. This strain was characterized and identified as Vibrio alginolyticus on the basis of a number of biochemical tests. The Swy strain was virulent to both kuruma prawns (P. japonicus) and tiger prawns (P. monodon) with LD50 values of 4.43 x 10(4) and 1.57 x 10(5) cfu g body weight-1, respectively.

T h e surface hydrophobicity of strains of Vibrio alginolyticus, V. carchariae, i ? damsela, V. harveyi and V. vulnzjicus, isolated from either diseased cultured grouper (Epinephelus malabaricus) or penaeids (Penaeus monodon and P. japonicus) was determined using three different methods : the salt aggregation test (SAT), bacterial adhesion to hydrocarbons test (BATH), and adherence to nitrocellulose filters test (NCF). T h e results obtained indicate that all the strains tested showed some degree of hydrophobicity with the type strain of V. harveyi (ATCC 25919) showing strong hydrophobic properties in all the methods. T h e SAT method used in the present study was modified to a microtitre tray test, an easier test to read than the conventional in glass slide methodology. All the 13 test strains were positive in the BATH test when n-octane was used as the solvent, but only one strain was positive when n-hexadecane was used as the solvent. It is suggested that this method using n-octane as solvent is suitable for assaying the hydrophobicity of pathogenic vibrios isolated from diseased aquatic animals.

Protease and virulence of the extracellular products (ECP) of Vibrio carchariae strain EmI82KL, a causative agent of gastroenteritis in E pinephelus coioides, cultured on different media were studied. The bacteria grown on peptone agar, nutrient agar or brain heart infu­sion agar produced higher protease activities than that grown on tryptic soy agar (TSA) in terms of protein content. The addition of ethylenediamine di(o-hydroxyphenylacetic acid) or horse serum in TSA enhanced the ECP protease production while the addition of grouper serum apparently reduced the enzyme activity indicating the presence of protease inhibitor(s) in the fish serum. Furthermore, the use of grouper meat or peptone as a single nutrient source remarkably enhanced the production of ECP protease. Adding proteinaceous materials from animal sources (horse serum, grouper meat or peptone) on agar manifested higher ECP protease activity than that from plant source (TSA), indicating the intestine of carnivorous groupers might favour the existence, survival or infection of the bacterium. The protease was a metal-chelator-sensitive serine protease since it was inhibited by 3,4-dichloroisocoumarin and phenylmethanesulfonyl fluoride while about 80% of its activity inhibited by chelating agents (ethylene-diaminetetraacetic acid and ethylene glycol-bis(y3-amino-ethylether) N,N,N′,N′-tetraacetic acid). The ECP obtained from each medium was not lethal to the groupers suggesting that the bacterium is low virulent. As grouper is carnivorous, therefore, the role of the protease played in the fish intestine probably is competing for nutrients and/ or associated with the cause of edema leading to gastroenteritis.