Aeromonas hydrophila is an important fish pathogen causing hemorrhagic septicemia. Proteolytic enzymes may play a critical role in invasive infections of this bacterium. In this paper, a 426-aa protease (designated ahMP) with zinc-binding motif was identified. SDS-PAGE analysis indicated that the recombinant ahMP was 64 kDa in size. The ahMP protease was classified as a zinc-dependent metalloprotease as its activity could be inhibited by zinc specific metal and metal ion chelators. The cells inoculated with ahMP underwent progressively cytopathic changes. To further confirm the role of this protease in A. hydrophila, we perform challenge experiments in fish and mice. In the ahMP-injected fish, death occurred within 48 h and the death rate was up to 16.7% (5/30), while most of ahMP-injected mice (8/10) showed clinical signs although there were no deaths. The organs from ahMP-injected mice and fish showed similar histopathological changes, including extensive vascular alterations, exudative lesions and cell necrosis. Together, these results suggest that a novel A. hydrophila metalloprotease, ahMP, is identified and is likely an important contributor to pathogenicity of this bacterium in fish.

Key words: Aeromonashydrophila, Fish, Histopathology, Metalloprotease, Mice