Iron utilization and siderophore production by Streptococcus phocae isolated from diseased Atlantic salmon (Salmo salar)

We present here the first evidence of the presence of iron uptake mechanisms in Streptococcus phocae, a beta-hemolytic bacterium frequently involved in disease outbreaks in seals causing pneumonia or respiratory infection as well as warm water streptococcosis in Atlantic salmon (Salmo salar). Fifteen Atlantic salmon isolates and type strain were able to grow in the presence of the chelating agent 2,2-dypiridyl and produced siderophores using the universal Chrome Azurol S assay in solid and liquid media. Based on the biochemical homogeneity of the bacterium and the results of the siderophore production assays, two representatives of Atlantic salmon isolates and the seal type strain were examined. All of them were able to utilize apo-transferrin, transferrin hemin, hemoglobin, ferric ammonic citrate and ferric chloride as iron sources when added to iron-deficient media; although S. phocae possesses a stronger capacity for iron acquisition from hemoglobin than hemin. Whole cells of all S. phocae strains, grown under iron-supplemented or iron-restricted conditions were able to bind hemin, indicating the existence of constitutive binding components located at the S. phocae cell surface. In all strains, at least two iron-regulated membrane proteins increased when S. phocae was cultured in iron-restricted medium. Iron did not play a regulatory role in the synthesis of some proteolytic enzymes and hydrophobic properties, since these activities were similar when the strains were cultured in iron-rich or iron-restricted conditions.