Phenotypic characterization of Histoplasma species

Histoplasmosis is an endemic mycosis that often presents as a respiratory
infection in immunocompromised patients. Hundreds of thousands of new
infections are reported annually around the world. The etiological agent
of the disease, Histoplasma, is a dimorphic fungus commonly found in the
soil where it grows as mycelia. Humans can become infected by Histoplasma
through inhalation of its spores (conidia) or mycelial particles. The
fungi transitions into the yeast phase in the lungs at 37°C. Once in the
lungs, yeast cells reside and proliferate inside alveolar macrophages.
Genomic work has revealed that Histoplasma is composed of at least five
cryptic phylogenetic species that differ genetically.  Three of
those lineages have received new names. Here we evaluated multiple
phenotypic characteristics (colony morphology, secreted proteolytic
activity, yeast size and growth rate) of strains from five of the
phylogenetic species of Histoplasma to identify phenotypic traits that
differentiate between these species: H. capsulatum sensu stricto, H.
ohiense, H. mississippiense, H. suramericanum, and an African lineage. We
report diagnostic traits for three species. The other two species can be
identified by a combination of traits. Our results suggest that 1) there
are significant phenotypic differences among the cryptic species of
Histoplasma, and 2) that those differences can be used to positively
distinguish those species in a clinical setting and for further study of
the evolution of this fungal pathogen.