Functional genomic and biochemical analysis reveals pleiotropic effect of Congo Red on Aspergillus fumigatus

Inhibition of fungal growth by Congo Red (CR) has been putatively associated to a specific binding to β-1,3-glucans which blocks cell wall polysaccharide synthesis. In this study, we searched for transcription factors (TF), which regulate the response to CR and interrogate their regulon. During the investigation of the susceptibility to CR of the COFUN TF mutant library, several CR resistant and hypersensitive mutants were discovered and further studied. Abnormal distorted swollen conidia called Quasimodo cells were seen in presence of CR. Quasimodo cells in the resistant mutants were larger than the one of the sensitive and parental strain; consequently the conidia of the resistant mutants absorbed more CR than the germinating conidia of the sensitive or parental strains. Accordingly, this higher absorption rate by Quasimodo cells resulted in the removal of CR from the culture medium, allowing a subset of conidia to germinate and grow. In contrast, all resting conidia of the sensitive mutants and parental strain were killed. This result indicated that heterogeneity of the conidial population is essential to promote survival of A. fumigatus in presence of CR. Moreover, amorphous surface cell wall polysaccharides such as the galactosaminogalactan, control the influx of CR inside the cells and accordingly resistance to the drug. Finally, long-term incubation with CR lead to the discovery of a new CR-induced growth effect, called Drug Inducted Growth Stimulation (DIGS), since the growth of one of them could be stimulated after recovering from the CR stress.