Surface-Expressed Enolase Contributes to the Pathogenesis of Clinical Isolate SSU ofAeromonas hydrophila

Abstract

In this study, we demonstrated that the surface-expressed enolase from diarrheal isolate SSU ofAeromonas hydrophilabound to human plasminogen and facilitated the latter's tissue-type plasminogen activator-mediated activation to plasmin. The bacterial surface-bound plasmin was more resistant to the action of its specific physiological inhibitor, the antiprotease α₂-antiplasmin. We found that immunization of mice with purified recombinant enolase significantly protected the animals against a lethal challenge dose of wild-type (WT)A. hydrophila. Minimal histological changes were noted in organs from mice immunized with enolase and then challenged with WT bacteria compared to severe pathological changes found in the infected and nonimmunized group of animals. This correlated with the smaller bacterial load of WT bacteria in the livers and spleens of enolase-immunized mice than that found in the nonimmunized controls. We also showed that the enolase gene could potentially be important for the viability ofA. hydrophilaSSU as we could delete the chromosomal copy of the enolase gene only when another copy of the targeted gene was supplied intrans. By site-directed mutagenesis, we altered five lysine residues located at positions 343, 394, 420, 427, and 430 of enolase inA. hydrophilaSSU; the mutated forms of enolase were hyperexpressed inEscherichia coli, and the proteins were purified. Our results indicated that lysine residues at positions 420 and 427 of enolase were crucial in plasminogen-binding activity. We also identified a stretch of amino acid residues (₂₅₂FYDAEKKEY₂₆₀) in theA. hydrophilaSSU enolase involved in plasminogen binding. To our knowledge, this is the first report of the direct involvement of surface-expressed enolase in the pathogenesis ofA. hydrophilaSSU infections and of any gram-negative bacteria in general.