Closing the Brief Case: Mold Infection of an Indwelling Cranial Device—a Perplexing Combination of “Classic” Laboratory Findings

ANSWERS TO SELF-ASSESSMENT QUESTIONS

1. Which of the following infections is commonly associated with the Aspergillus niger complex in otherwise healthy individuals?

   • a.
     Soft tissue hyphomycoses
   • b.
     Outer ear infections
   • c.
     Meningitis
   • d.
     Invasive rhinocerebral infections

Answer: b. A. niger is among the most commonly encountered causes of otitis externa in clinical practice and does not require immunosuppression. Potential risk factors include chronic exposure to dusty or warm/humid conditions, chronic wetness of the ear, and poor otologic hygiene. Otherwise, like other Aspergillus species, A. niger can occasionally elicit invasive hyphal infections of tissue, although not in immunocompetent patients, A. niger and other Aspergillus species are not typically associated with meningitis, and A. niger can cause luminal sinus colonization with resultant symptomology, but tissue invasion is rare in healthy hosts.

1. Which of the following characteristics, when visualized by a tape preparation of a cultured isolate, would be expected for Aspergillus niger?

   • a.
     Hyphae that are pauciseptate and broad
   • b.
     Short conidiophores with ovoid terminal vesicles
   • c.
     Phialides that are uniseriate with adherent conidia in clusters
   • d.
     Melanized conidia

Answer: d. The conidia of A. niger are melanized and appear dark when visualized by tape preparation. Hyphae that are pauciseptate and broad are characteristic of mucoraceous molds, not A. niger. A. niger conidiophores are notoriously long among the species of this genus, with round terminal vesicles. The phialides of Aspergillus niger are typically biseriate, although uniseriate genomospecies have also been described. The conidia, however, extend in chains (not clusters) from the phialides over the entire surface of the vesicles.

1. What is the underlying morphologic reason that clinical laboratories should not grind tissue specimens when homogenizing them for fungal culture?

   • a.
     The hyphae of mucoraceous molds are pauciseptate
   • b.
     The conidia of Aspergillus niger are melanized
   • c.
     The tissue morphology of Histoplasma and Blastomyces spp. is yeast phase
   • d.
     In situ conidiation can occur when molds infect air-exposed tissues

Answer: a. Mucoraceous molds possess characteristic pauciseptate hyphae, consisting of long cells with many nuclei (“coenocytic”); these hyphae are the morphologic forms present in situ during invasive mucormycosis (aka zygomycosis). If the contiguous cell is damaged at any point (for instance, by mechanical grinding), the cytoplasm leaks, and the entire cell will die. In contrast, the hyphae of nonmucoraceous molds possess regular septations that serve to “close off” adjacent cells from damage. While the other statements are true, the structures/phenomena described are unrelated to the rationale for specimen preparation.

TAKE-HOME POINTS

• •The Aspergillus niger complex is a ubiquitous environmental mold that can elicit opportunistic disease in anatomically or immunologically comprised hosts. Its “classic” features include melanized conidia and extremely long conidiophores (when air exposed), as well as calcium oxalate formation.
• •While the morphologic characterization of molds from infected tissue (histologic or cytologic, including Gram stain) is often limited to hyphal features, the air exposure of an infected site can stimulate the in situ formation of more diverse reproductive structures.
• •Advancing medical technologies, such as indwelling devices, create opportunities for infections by an increasingly diverse range of organisms, including species that are not traditionally associated with a given anatomic site.

See https://doi.org/10.1128/JCM.01116-19 in this issue for case presentation and discussion.