Answer to December 2021 Photo Quiz.

Abstract

In the United States, Histoplasma capsulatum causes histoplasmosis east of the Mississippi River, historically in the Mississippi and Ohio River valleys. The mold grown on a fungal culture at the presenting hospital was sent to our reference laboratory for identification. Macroscopic and microscopic morphology strongly suggested Histoplasma capsulatum. The culture morphology was that of a white, cottony mold that grew slowly on an inhibitory mold agar plate, and the microscopic morphology demonstrated hyaline hyphae with microconidia and tuberculate, thick-walled macroconidia (1). Upon observation of the characteristic macroconidia for Histoplasma on the microscopic examination, an H. capsulatum-specific PCR was performed from the culture growth for confirmation of identification but the PCR result was negative. The PCR assay targets a 192-bp region of the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene of H. capsulatum (2). Given that the PCR result did not correlate with the microscopic morphology, Sanger nucleic acid sequencing was also performed utilizing the MicroSeq D2 large-subunit ribosomal DNA (rDNA) sequencing kit. Using NCBI GenBank database BLAST analysis, the sequence from the patient isolate was a perfect match for Histoplasma duboisii, while a Histoplasma capsulatum entry in the Mayo Clinic Fungal Database had a 1-bp mismatch (99.62% match). Since H. duboisii is an unusual isolate in our experience, the culture was sent to a second reference laboratory for confirmation of the identification. Sequencing of the α-tubulin 1 and the ADP-ribosylation factor genes and subsequent phylogenetic analyses gave high support (0.99 Bayesian posterior probabilities/84 to 95% similarity with bootstrap evaluation) to match to H. duboisii, further confirming the identification. The isolate was converted to the yeast form after serial passages at 35°C on brain heart infusion (BHI) agar. Figure 1B of the photo quiz shows the characteristic yeast form of H. duboisii , which is larger than the yeast form of H. capsulatum (2 to 5 μm).