Identification and analysis of potential virulence-associated genes in Haemophilus parasuis based on genomic subtraction.

Haemophilus parasuis is a commensal bacterium in the swine upper respiratory tract, but virulent strains can invade host and cause Glässer's disease under certain conditions. In order to explore the virulence difference in H. parasuis strains, a virulent isolate, SH0165 (serotype 5), and an avirulent isolate, 7140 (serotype 4), were selected to investigate genetic differences by genomic subtraction. A total of 560 clones were screened by dot-blot and PCR, and 33 fragments present in SH0165 but absent from 7140 were identified. They encode eight gene categories, the sclB family, restriction modification system, phage products, transport system, outer membrane proteins, metabolism, and DNA synthesis and insertion sequences. We investigated the distribution of 21 fragments in 15 reference strains. Six fragments were more prevalent in virulent strains than avirulent strains based on statistical analysis; three fragments were SH0165-specific, and the remaining 12 were diversely distributed. Having analyzed these significantly differential fragments, including the sclB family, ABC-type transporter, fhuA and nhaC, we presume that these genes are associated with virulence in H. parasuis.