Opportunities and challenges of microbial siderophores in the medical field

Siderophore-producing bacteria from Spitsbergen soils-novel agents assisted in bioremediation of the metal-polluted soils

Siderophores are molecules that exhibit a high specificity for iron (Fe), and their synthesis is induced by a deficiency of bioavailable Fe. Complexes of Fe-siderophore are formed extracellularly and diffuse through porins across membranes into bacterial cells. Siderophores can bind heavy metals facilitating their influx into cells via the same mechanism. The aim of the studies was to determine the ability of siderophore-producing bacteria isolated from soils in the north-west part of Wedel Jarlsberg Land (Spitsbergen) to chelate non-Fe metals (Al, Cd, Co, Cu, Hg, Mn, Sn, and Zn). Specially modified blue agar plates were used, where Fe was substituted by Al, Cd, Co, Cu, Hg, Mn, Sn, or Zn in metal-chrome azurol S (CAS) complex, which retained the blue color. It has been proven that 31 out of 33 strains were capable of producing siderophores that bind to Fe, as well as other metals. Siderophores from Pantoea sp. 24 bound only Fe and Zn, and O. anthropi 55 did not produce any siderophores in pure culture. The average efficiency of Cd, Co, Cu, Mn, Sn, and Zn chelation was either comparable or higher than that of Fe, while Al and Hg showed significantly lower efficiency. Siderophores produced by S. maltophilia 54, P. luteola 27, P. luteola 46, and P. putida 49 exhibited the highest non-Fe metal chelation activity. It can be concluded that the siderophores of these bacteria may constitute an integral part of the metal bioleaching preparation, and this fact will be the subject of further research.

Biosynthesis of novel desferrioxamine derivatives requires unprecedented crosstalk between separate NRPS-independent siderophore pathways

Iron is essential to many biological processes but its poor solubility in aerobic environments restricts its bioavailability. To overcome this limitation, bacteria have evolved a variety of strategies, including the production and secretion of iron-chelating siderophores. Here, we describe the discovery of four series of siderophores from Streptomyces ambofaciens ATCC23877, three of which are unprecedented. MS/MS-based molecular networking revealed that one of these series corresponds to acylated desferrioxamines (acyl-DFOs) recently identified from S. coelicolor. The remaining sets include tetra- and penta-hydroxamate acyl-DFO derivatives, all of which incorporate a previously undescribed building block. Stable isotope labeling and gene deletion experiments provide evidence that biosynthesis of the acyl-DFO congeners requires unprecedented crosstalk between two separate non-ribosomal peptide synthetase (NRPS)-independent siderophore (NIS) pathways in the producing organism. Although the biological role(s) of these new derivatives remain to be elucidated, they may confer advantages in terms of metal chelation in the competitive soil environment due to the additional bidentate hydroxamic functional groups. The metabolites may also find application in various fields including biotechnology, bioremediation, and immuno-PET imaging.IMPORTANCEIron-chelating siderophores play important roles for their bacterial producers in the environment, but they have also found application in human medicine both in iron chelation therapy to prevent iron overload and in diagnostic imaging, as well as in biotechnology, including as agents for biocontrol of pathogens and bioremediation. In this study, we report the discovery of three novel series of related siderophores, whose biosynthesis depends on the interplay between two NRPS-independent (NIS) pathways in the producing organism S. ambofaciens-the first example to our knowledge of such functional cross-talk. We further reveal that two of these series correspond to acyl-desferrioxamines which incorporate four or five hydroxamate units. Although the biological importance of these novel derivatives is unknown, the increased chelating capacity of these metabolites may find utility in diagnostic imaging (for instance, 89Zr-based immuno-PET imaging) and other applications of metal chelators.