Phenotypic characterization of Histoplasma species

<p>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, <em>Histoplasma, </em>is a dimorphic fungus commonly found in the soil where it grows as mycelia. Humans can become infected by <em>Histoplasma</em> 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 <em>Histoplasma</em> 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 <em>Histoplasma</em> to identify phenotypic traits that differentiate between these species: <em>H. capsulatum</em> <em>sensu stricto</em>, <em>H. ohiense</em>, <em>H. mississippiense</em>, <em>H. suramericanum</em>, 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 <em>Histoplasma</em>, 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.</p>