At the recent Japanese Society of Fisheries Science Symposium in Tokyo, Ka Yin Leung and colleagues from the National University of Singapore reported on their use of several techniques to identify and characterize virulence genes that cause fish pathogens.

The team first used suppression subtractive hybridization to examine genetic differences among virulent and avirulent strains of Aeromonas hydrophilia. More than 300 genomic regions of differences and 30 common virulent genes were found.

Transposon tagging, which involves inserting transposable elements out of sequence to mark a gene of interest, was their second approach. TnphoA transposon mutagenesis was used to construct a library of 490 PhoA+ fusion mutants from Edwardsiella tarda, another common fish pathogen. They injected each mutant into fish at a dose 10 times the wild type. Most of the fish, as expected, died. However, several mutants were determined to be attenuated in fish and may have applications as vaccines, Leung hopes.

The chair of the session, J. H. Crosa, from Oregon Health and Science University (Portland), warned of a possible surprise component to Leung’s approach: “He may be finding a lot of things that are not what he’s looking for.” However, Leung defended his team’s findings, stating that the virulent genes found were not housekeeping genes. He wishes to pull this work together into a database and optimistically thinks a DNA chip will be made within five years.

Currently, the team is beginning to use proteomics techniques in this effort. Unique virulence proteins may have use for diagnostic kits as well as for the design of new therapies to combat fish diseases.