Fungal siderophores and their analogues alter microbial growth and biochemistry: expanding the repertoire of antimicrobial strategies.

Abstract

In recent years, the concerns of rising anti-microbial resistance (AMR) in bacteria and fungi have grown due to the rate at which pathogens gain resistance to antimicrobials compared to the rate of antibiotic discovery. As a result, investigations into the role of siderophores, natural iron chelators produced by microbes under iron limiting conditions, are being carried out to see if these small metabolites can be used to bypass AMR by either limiting the amount of available iron or using them as carriers for ‘Trojan-Horse’ antibiotics. It was observed that Triacetylfusarinine C (TAFC), a siderophore native to Aspergillus fumigatus, significantly inhibited Klebsiella pneumoniae and Acinetobacter baumannii growth in a dose-dependent manner (p < 0.05) and increased the production of a hitherto unknown catecholate, potentially enterobactin-related, siderophore in K. pneumoniae with a singly charged [M+1H]1+ of 924.34. Quantitative proteomic analysis revealed that K. pneumoniae treated with TAFC or gliotoxin (GT), another metabolite produced by A. fumigatus, exhibited disruption of metal homeostasis pathways, protein synthesis and electron transfer. Furthermore, Diacetylfusarinine C (DAFC), a TAFC analogue, was successfully conjugated to chitosan, a biopolymer, and separately to the chemotherapy drug methotrexate (MTX). The DAFC polymer exhibited increased iron-binding activity relative to free TAFC and the gallium chelate of DAFC-MTX (GaDAFC-MTX) conjugate significantly inhibited the growth of A. fumigatus on solid agar by 50 % (p < 0.005) and in liquid culture by 60% (p < 0.005). Interestingly, the GaDAFC-MTX conjugate significantly inhibited TAFC production by 95% (p < 0.005) and overall siderophore production by 50% (p < 0.05) whereas free MTX only inhibited the production of TAFC (p < 0.005). It is concluded that siderophores not native to K. pneumoniae can be used to limit the available environmental iron and that TAFC and derivatives are suitable candidates for developing Trojan-Horse antifungals