Opportunistic pathogenicity in fungi can transcend species boundaries

The ability to opportunistically infect humans has evolved multiple times across fungi and is a major burden to public health. Opportunistic pathogenicity requires the confluence of pre-existing traits in the fungus that facilitate host colonization (e.g., the ability to grow at 37C) and the existence of host immune filters that permit the survival of some colonizers (e.g., inborn errors of immunity). Numerous studies have previously shown that fungal pathogens can exhibit extensive strain-to-strain variation in the ability to cause disease. Moreover, it is also well established that non-pathogenic fungi can occasionally cause severe infections. Together, these observations provoke the question: what differentiates opportunistic fungal pathogens from non-pathogens? To empirically address this, we directly compared phenotypic, metabolomic, and genomic variation between Aspergillus fumigatus, an organism responsible for more than 300,000 infections per year, and Aspergillus fischeri, a close relative of A. fumigatus that is not considered clinically relevant. By examining 26 phenotypic traits across 16 representative strains of A. fumigatus and 16 of A. fischeri, we find that infection-relevant traits measured under in vitro monoculture conditions show species-specific distributions, whereas traits measured under in vitro coculture with murine macrophages overlap in their distributions. Strikingly, strains of the two species also overlap in their virulence profiles in an immunocompromised murine model of pulmonary aspergillosis; three strains of A. fischeri exhibit lethality rates of >50% while two A. fumigatus strains were among the least virulent of all 32 strains tested. Consistent with the observed overlap, we could not statistically associate variation in virulence to variation in the presence of specific genomic elements, phenotypic traits, or secondary metabolites. Our results raise the hypothesis that opportunistic pathogenicity can extend beyond the boundaries of individual species. We propose a conceptual model where the opportunistic pathogenic potential of any fungal strain is the product of complex interactions among numerous genomic, ecological, and host immunity factors.