Response of Staphylococcus aureus isolates from bovine mastitis to exogenous iron sources

Staphylococcus aureus can survive in conditions of extremely low iron concentration. The ability of S. aureus to use two exogenous hydroxamate types of siderophores (desferrioxamine and ferrichrome) and four iron-containing proteins found in cattle (hemin, hemoglobin, ferritin, and lactoferrin) were tested on 16 reference and clinical isolates. For all strains tested, ferrichrome and desferrioxamine showed strong growth-promoting activities in a disk diffusion assay and in liquid medium. The heme proteins hemin and hemoglobin were also found to support growth in culture media lacking other iron sources, while lactoferrin failed to do so. On media containing the iron chelator dipyridyl, ferritin induced a growth inhibition effect that was further enhanced in the presence of lactoferrin in seven of the 13 tested strains. Staphylococcus aureus was able to bind hemin and the level of binding activity was not increased after growth in iron-rich or -poor media. Dot-blot competition tests showed that biotin-labeled lactoferrin binds to S. aureus, and this binding can be inhibited by unlabeled lactoferrin. Expression of lactoferrin-binding activity was independent of the level of iron in the medium and the iron saturation status of lactoferrin. For each strain tested, ligand blots showed lactoferrin-binding proteins of molecular weights ranging from 32 to 92 kDa. Possible functions of these lactoferrin-binding proteins could not be related to iron acquisition mechanism in S. aureus.