Effect of iron on expression of superoxide dismutase by Aeromonas salmonicida and associated resistance to superoxide anion

Superoxide dismutase activity was detected in Aeromonas salmonicida under iron‐replete and iron‐limited culture conditions. Under iron‐replete conditions an iron superoxide dismutase, molecular mass 50,400 Da, was identified based on inhibition by hydrogen peroxide but not by millimolar concentrations of cyanide. When the available iron in the culture medium was limited by addition of the non‐assimilable iron chelator 2,2‐dipyridyl, a manganese superoxide dismutase, molecular mass 45,600 Da, was identified, which was resistant to inhibition by either hydrogen peroxide or cyanide. The change in enzyme production would appear to be iron dependent, as addition of FeCl 3 in excess to iron‐limited broths resulted in only the iron superoxide dismutase being synthesised. Examination of the location of the superoxide dismutase enzymes revealed that the manganese superoxide dismutase expressed under iron limitation is located in the periplasm, while the iron superoxide dismutase has a cytoplasmic location. The periplasmic manganese superoxide dismutase was able to protect A. salmonicida against extracellular riboflavin‐generated superoxide, with A. salmonicida grown under iron‐limited conditions exhibiting a 32‐fold increase in minimum bactericidal concentration of riboflavin compared to cells cultured under iron‐replete conditions. Furthermore, in a time‐course study of bactericidal activity of exogenously generated superoxide against A. salmonicida , bacteria grown under iron‐replete conditions and expressing cytoplasmic iron superoxide dismutase were rapidly killed, whilst those grown under iron limitation expressing periplasmic manganese superoxide dismutase survived for the duration of the experiment.