Quantitative Analysis Of Superoxide Dismutase In Aeromonas salmonicida Subsp. salmonicida: Re-Evaluation Of Its Role In Virulence     Andrew Dacanay1, R. Bjornsdottir 2, L. L. Brown1, S. C. Johnston1     1National Research Council-Institute for Marine Biosciences, 1411 Oxford Street, Halifax NS, Canada; 2 Icelandic Fish Health Laboratories, Akureyri, Iceland     Superoxide dismutase (SOD) catalyses the dismutation of the superoxide anion (O2-) into hydrogen peroxide (H2O2), which is, in turn, transformed into H2O and O2 by catalase.  This enzyme cascade is homeostatically important as it detoxifies free radicals produced by oxidative phosphorylation.  Free radicals are also produced by macrophages as part of phagocytosis as an antimicrobial strategy.  Superoxide dismutase has been implicated in pathogen survival by other bacteria such as Mycobacterium tuberculosis.  Despite this, SOD has not been considered a virulence factor in the pathogenic fish bacteria Aeromonas salmonicida subsp. salmonicida, as both virulent and avirulent strains of bacteria are able to produce the enzyme.  We present evidence for quantitative differences in SOD levels between virulent and avirulent strains of A. salmonicida.  These data suggest that SOD may be a virulence factor for A. salmonicida.  Virulence in this species may be more a matter of regulation of gene expression rather than gene presence or absence.