Identification of virulence genes in bacterial fish pathogens: a genomic and proteomic approach

The increase demand for food and ornamental fish necessitates intensive fish farming. This practice is extremely important in land-scarce Singapore where the ornamental fish industry contributes significantly to the nation's economy. However. intensive fish farming often leads to disease problems resulting in serious economic losses. Bacterial pathogens are one of the major causative agents of fish diseases and they include Aeromonas hydrophila, Vibrio species and Edwardsiella tarda. There is thus an urgent need to understand the pathogenesis of these bacterial pathogens. Out laboratory adopts the genomic and proteomic approach to examine the virulence factors at the molecular level. In the genomic approach, genomic subtraction and TnphoA transposon mutagenesis allow the identification of virulence genes such as hemolysin (HlyA), protease (oligopeptidase), and histone-like protein (HU-2) in A. hydrophila and arylsulfate transferase (astA), phosphate-specific transport genes (pstS. Cq A and B) and catalase in E. tarda. In addition to the identification of virulence genes, it is also essential to study the dynamic proteome. Since most virulence factors are secreted or on the cell surface, we made an attempt to study the extracellular proteins using the proteomic approach (two-dimensional gel electophoresis and masE spectrometry). A combination of these two approaches will undoubtedly enhance the understanding of the pathogenic mechanisms of these bacteria and this will in turr. pave the way for the development of diagnostic kits, vaccines and new therapeutics.