Differential display RT-PCR was used to investigate differential gene expression of H. parasuis in response to conditions that begin to mimic in vivo conditions encountered during disease. Pathogens encounter low iron availability in vivo, therefore, gene expression in response to iron restriction was investigated. H. parasuis commonly causes meningitis, therefore, gene expression in response to porcine cerebrospinal fluid was also investigated. Ten and nine genes were found to be up-regulated in H. parasuis in response to each condition, respectively. A gene fragment predicted to encode a phosphodiesterase was up-regulated in response to both conditions and was PCR amplified as either a 300 bp or a 1400 bp fragment in the reference strains. A correlation of the 300 bp amplicon with virulent strains and a 1400 bp amplicon with avirulent strains was initially suggested, however, further investigation indicated this correlation isn't clear. A gene fragment encoding a predicted hydrolase was present in the genomes of all references strains, however, is only expressed in response to iron restriction in strains associated with a high virulence potential.
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