An anti-Aspergillus protein from Escherichia coli DH5α: putative inhibitor of siderophore biosynthesis in Aspergillus fumigatus.

An antifungal protein designated as anti-Aspergillus protein (AAP), produced by Escherichia coli DH5α, was purified and characterised. It exhibited a molecular weight of 60 kDa on Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis and depicted 99% purity on ultra performance liquid chromatography. The purified protein manifested antimycotic potential against pathogenic isolates of Aspergillus spp., depicting a minimum inhibitory concentration in the range 15.62-31.25 μg ml(-1) and 5.0-10.0 μg per disc, using microbroth dilution, spore germination inhibition and disc diffusion assays respectively. In vitro toxicity tests demonstrated that it showed no toxicity against human erythrocytes at doses up to 1000 μg ml(-1) . Matrix-assisted laser desorption ionisation-Time-of-flight analysis of trypsin-digested peptides of purified protein and subsequent Mascot search revealed that several peptides of AAP have identity with bacterial siderophore biosynthetic protein, i.e. non-ribosomal peptide synthetase enzyme, involved in critical step of fungal siderophore biosynthesis. Siderophore-based inhibition was further corroborated by Chrome azurol S assay. Hence, the antagonistic effect might be the result of impediment in siderophore-mediated iron uptake and transport process which may cause critical consequences on Aspergillus growth and virulence.