| Summary: | <i>Aeromonas hydrophila</i> was isolated from an African black-footed penguin (<i>Spheniscus demersus</i>) that died while in zoo captivity. At necropsy, the virulence of <i>A. hydrophila</i> appeared to be enhanced by stress, so was assessed in the presence of in vitro and in vivo stressors and serial passaging in mice. Virulence genes from the isolate were amplified by PCR. In vitro assays were conducted to test the hemolytic activity, cytotoxicity, and effect of stress hormones on <i>A. hydrophila</i> virulence. In vivo assays were conducted to test the stress effect on mortality of <i>A. hydrophila</i>-infected mice and virulence in mice. Two virulence genes coding for hemolysin (<i>ahh1</i>) and aerolysin (<i>aerA</i>) were detected, and the cytotoxic potential of the isolate was demonstrated in baby hamster kidney and Vero cells. Some or all mice inoculated with <i>A. hydrophila</i> and exposed to stress hormones (epinephrine and norepinephrine) or low temperature died, while mice inoculated with <i>A. hydrophila</i> and exposed to fasting or agitation stressors or no stressors survived. We concluded that stress can be fatal in mice experimentally infected with <i>A. hydrophila</i> and that serial passaging in mice dramatically enhances the virulence of <i>A. hydrophila</i>.
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