Objective    To study the virulence variation of Enterococcus faecalis before and after the deletion of Hyaluronidase gene (hyl) in animal model of Enterococcus faecalis.     Methods    A mouse peritonitis model and a rabbit endocarditis model were established by being infected with Enterococcus faecalis wild strain (OG1RF) and allelic gene defective mutant (hyl

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mutant) and reverse mutation of hyl

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mutant. Virulence variation of different strains was evaluated.     Results    The mouse peritonitis model and the rabbit endocarditis model

were successfully established with Enterococcus faecalis. Hyl

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mutant’s virulence was one-fifth as much as that of wild strain OG1RF

and reverse mutation of hyl

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mutant had similar virulence as OG1RF. The peripheral blood leukocyte counting and peritoneal fluid cytokine TNF-α concentration assay were made at 12 h after infection. The number of leukocytes and the level of TNF-α were significantly lower in mice injected of hyl

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mutant than those injected of OG1RF

but showed similarity in mice injected of hyl

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mutant and OG1RF. In rabbit endocarditis model

the pathogenic and pathologic changes of heart in mice injected of hyl

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mutant were less severe than those injected of OG1RF.     Conclusion    hyl

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is important in the pathogenesis of Enterococcus faecalis

and probably is a major gene to encode virulence factor of Enterococcus faecalis.