Characterization of a novel Spirillum-like bacterium that degrades ferrioxamine-type siderophores

Abstract

A novel Gram-negative Spirillum-like bacterium (ASP-1) was isolated from lake water by enrichment culture on desferrioxamine B as sole source of carbon and energy. ASP-1 was able to degrade the siderophores desferrioxamine B and E. The property of siderophore degradation was inducible in the presence of desferrioxamine B. The ferric complexes, however, were not measurably degraded but served as an iron source. Degradation of desferrioxamines in culture was followed by measuring the residual ferrioxamines colorimetrically at 430 nm after addition of iron. Degradation in cell-free assays was followed quantitatively by HPLC on a reversed-phase column measuring the time-dependent disappearance of the desferrioxamines B and E. Cell-free assays also revealed that degradation of the cyclic desferrioxamine E was rapid and complete, whereas degradation of the linear desferrioxamine B yielded two intermediate iron-binding metabolites of shorter chain length. Preparative isolation by HPLC and mass spectrometric analysis of the metabolites revealed masses at 361 and 419 a.m.u., respectively, suggesting a splitting at the two amide bonds. ASP-1 is a nitrogen fixing Spirillum bacterium which could also use ammonium and glucose or several organic acids as a carbon source but grew poorly with amino acids. Physiological comparisons with Aquaspirillum and Azospirillum failed to assign ASP-1 to any of the presently known Spirillum species. Based on 16S rDNA sequence analysis the strain could be placed within the radiation of the Azospirillum/Rhodocista group. The closest relative was Azospirillum irakense, showing 98.8% similarity.