Gliotoxin, a known virulence factor in the major human pathogen Aspergillus fumigatus, is also biosynthesized by the non-pathogenic relative A. fischeri

Abstract

Aspergillus fumigatus is a major opportunistic human pathogen. Multiple traits contribute to A. fumigatus pathogenicity, including its ability to produce specific secondary metabolites, such as gliotoxin. Gliotoxin is known to inhibit the host immune response, and genetic mutants that inactivate gliotoxin biosynthesis (or secondary metabolism in general) attenuate A. fumigatus virulence. The genome of A. fischeri, a very close non-pathogenic relative of A. fumigatus, contains a biosynthetic gene cluster that exhibits high sequence similarity to the A. fumigatus gliotoxin cluster. However, A. fischeri is not known to produce gliotoxin. To gain further insight into the similarities and differences between the major pathogen A. fumigatus and the non-pathogen A. fischeri, we examined whether A. fischeri strain NRRL 181 biosynthesizes gliotoxin and whether its production, and of secondary metabolites more generally, influence its virulence profile. We found that A. fischeri biosynthesizes gliotoxin in the same conditions as A. fumigatus. However, whereas loss of laeA, a master regulator of secondary metabolite production, has been previously shown to reduce the virulence of A. fumigatus, we found that laeA loss (and loss of secondary metabolite production, including gliotoxin) in A. fischeri does not influence its virulence. These results suggest that gliotoxin and secondary metabolite production are virulence factors in the genomic and phenotypic background of the major pathogen A. fumigatus but are much less important in the background of the non-pathogen A. fischeri. We submit that understanding the observed spectrum of pathogenicity across closely related pathogenic and non-pathogenic Aspergillus species will require detailed characterization of their biological, chemical, and genomic similarities and differences. IMPORTANCEAspergillus fumigatus is a major opportunistic fungal pathogen of humans but most of its close relatives are non-pathogenic. Why is that so? This important, yet largely unanswered, question can be addressed by examining how A. fumigatus and its non-pathogenic close relatives are similar or different with respect to virulence-associated traits. We investigated whether Aspergillus fischeri, a non-pathogenic close relative of A. fumigatus, can produce gliotoxin, a mycotoxin known to contribute to A. fumigatus virulence. We discovered that the non-pathogenic A. fischeri produces gliotoxin under the same conditions as the major pathogen A. fumigatus. However, we also discovered that, in contrast to what has been previously observed in A. fumigatus, loss of secondary metabolite, including gliotoxin, production in A. fischeri does not alter its virulence. Our results are consistent with the "cards of virulence" model of opportunistic fungal disease, where the ability to cause disease stems from the combination ("hand") of individual virulence factors ("cards"), but not from individual factors per se.