The role of siderophores in iron uptake and virulence of the opportunistic fungal pathogen, Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic fungal pathogen that can cause life-threatening invasive aspergillosis and is a leading cause of mortality in bone marrow and lung transplant recipients. Currently, the mortality rate of invasive aspergillosis is approximately 50%. The virulence factors of Aspergillus species remain largely unknown. The purpose of this research was to determine the mechanisms) by which A. fumigatus survives within the iron-limiting environment of the host.; A. fumigatus conidia germinated and grew extremely well in the presence of human serum, which is normally fungistatic due to the presence of the iron-binding protein, transferrin. This result implied that A. fumigatus possesses highly effective mechanisms for iron uptake. A. fumigatus was able to remove transferrin-bound iron across a dialysis membrane, indicating that molecules <10 kDa were responsible for iron acquisition. Siderophores, which are small iron-binding molecules, were detected during the first 8 hours of culture. Several hydroxamate siderophores were purified from A. fumigatus cultures, and the most abundant were identified as N'N″N'″-triacetylfusarinine C (TAF) and ferricrocin.; The individual microscopic rate constants for removal of iron from the two iron-binding sites on human transferrin were calculated for both TAF and ferricrocin. Both A. fumigatus siderophores could remove iron from all transferrin species, but preferentially removed C-terminal iron. The rates of iron removal from diferric transferrin were 2.4 +/- 0.2 M-1min-1 for TAF and 1.6 +/- 0.3 M-1min-1 for ferricrocin. These rates were similar to those of bacterial siderophores which are known virulence factors.; A siderophore secretion mutant was constructed in A. fumigatus by deletion of sidA, which encodes L-ornithine N5-oxygenase, the first committed step in siderophore biosynthesis. This mutant was unable to grow in low iron medium, or in medium containing serum. It was also unable to remove iron from human diferric transferrin. The sidA mutant showed significantly reduced virulence in a mouse model of invasive aspergillosis.; In conclusion, hydroxamate siderophores produced by A. fumigatus play an important role in iron uptake in vivo. Therefore, siderophores represent one of the few virulence factors identified in A. fumigatus . The siderophore-iron uptake pathway represents a potential target for anti-Aspergillus drug development.