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A recent study examining genetic variations among the 3 completely sequenced white spot syndrome virus (WSSV) genomes isolated from China, Thailand and Taiwan revealed five major differences among them. Of these differences, a deletion region between ORF 23/24 and a variable region of ORF14/15 prone to recombination were of particular evolutionary significance. Focusing on these regions, 81 WSSV isolates from India were characterized by sequencing polymerase chain reaction (PCR) amplicons. The Indian strains carried a 10,970 bp deletion in the ORF 23/24 region relative to WSSV-TW and WSSV-TH-96-II. Analysis of the ORF 14/15 regions revealed two novel strains of WSSV with unique sequences which could have evolved by recombination. None of the WSSV isolates had a transposase sequence or VP35 gene as reported for Taiwan isolates. The Indian strains were closely related to Thailand strains suggesting movement of a putative ancestor from Thailand to other parts of the world including India.

Aims: The study was aimed at investigating the presence of typical and atypical virulence genes in isolates belonging to the Harveyi clade (Vibrio harveyi and Vibrio campbellii). Methods and Results: Forty-eight vibrio isolates belonging to the Harveyi clade were screened for the presence of virulence genes that are typical for these bacteria and those found in human pathogenic vibrios such as Vibrio cholerae, Vibrio parahaemolyticus and Vibrio vulnificus and aquatic pathogenic Vibrio anguillarum. The virulence genes were amplified by PCR with specific primers, and the presence further confirmed by dot blot hybridization. The virulence genes vhh, chiA, vhpA, toxR(Vh), luxR and serine protease, typical of Harveyi clade were detected in all the isolates. The haemolysin gene hlyA and the virulence regulator gene toxR(Vc) specific to V. cholerae and the V. anguillarum-specific flagellum gene (flaC) were present in some of the isolates. Challenge tests with gnotobiotic Artemia nauplii did not show any correlation between the presence of the virulence genes and virulence of the isolates. Conclusion: From our results, there appears a remote possibility that vibrios belonging to the Harveyi clade might acquire virulence genes from other vibrios in the aquatic environment through horizontal gene transfer. Significance and Impact of the Study: Vibrios belonging to the Harveyi clade may be an important reservoir of virulence genes of other (human pathogenic) Vibrio species in the aquatic environment. The acquisition of virulence genes by horizontal transfer might increase the ability of Harveyi clade vibrios to infect aquatic organisms by increasing their virulence to a specific host by broadening their host range. The detection of such genes may forewarn the hatchery operators about a potentially virulent pathogen and thus help to develop management measures to handle the problem of vibriosis

Outer membrane protein A (OmpA) is a component of the outer membrane of Edwardsiella tarda and is wildly distributed in Enterobacteriaceae family. The gene encoding the OmpA protein was cloned from E. tarda and expressed in Escherichia coli M15 cells. The recombinant OmpA protein containing His(6) residues was estimated to have a molecular weight of ~38kDa. In Western blot the native protein showed expression at ~36kDa molecular weight which was within the range of major outer membrane proteins (36-44kDa) observed in this study. All E. tarda isolates tested harbored the ompA gene and the antibody raised to this protein was seen to cross react with other Gram negative bacteria. The OmpA protein characterized in this study was observed to be highly immunogenic in both rabbit and fish. In Enzyme linked immunosorbent assay, rabbit antisera showed an antibody titer of 1: 128,000. Common carp vaccinated with recombinant OmpA protein elicited high antibody production and immunized fish showed a relative percentage survival of 54.3 on challenge.

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