The ability of strains from serotype groups O1 and O2 to reduce Fe in the form of different chelates was investigated. All strains, grown in M9 minimal medium supplemented with 0.2% Casamino acids, reduced Fe complexed by citrate, nitrilotriacetic acid and EDTA. In whole cells, the degree of reduction was dependent on the Fe ligand and on the strain, with the greatest values corresponding to ferric dicitrate and serotype group O1 strains, respectively. The ferric-reductase activity increased, over the basal levels, when the cells were grown with iron added as ferric dicitrate, haemin or haemoglobin. All strains also reduced ferricyanide, a compound that is not transported into the bacterial cells. Ferricyanide reduction was also increased when the cells were grown in the presence of an iron source. All of the cell fractions (periplasm, membranes and cytoplasm) showed Fe-reducing activity, with the highest values observed in the presence of Mg, NADH and FAD in the assay buffer. Cytoplasmic ferric-reductase could be visualized using native polyacrylamide or starch gel electrophoresis, whereas the periplasmic and membrane reductase(s) could only be detected on starch gels. The results indicate the presence of different ferric-reductase activities in which could be involved in the different iron-acquisition systems present in this micro-organism, i.e. siderophore-mediated systems and siderophore-independent mechanisms.


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