MICROBIOLOGY

• General description:
  • A darkly pigmented (dematiaceous) filamentous fungus that lives in soil and grasses, especially in warm and humid conditions.
  • E. rostratum is by far the most common infecting species.
    • E. longirostratum and E. mcginnisii have also been reported, but these may be the same organism as E. rostratum.
• Appearance:
  • Under direct microscopy with KOH, Exserohilum spp. appear as brown septate hyphae.
• Culture:
  • In selective culture media, Exserohilum spp. tend to grow fast.
  • Conidia may take up to 3 weeks to grow.
  • Colonies of Exserohilum spp. have a dark (brown-black or “olivaceous”) color.
  • Appearance of conidia:
    • E. rostratum: long (25-90 by 9-22 μm), straight or slightly curved with 4 to 9 internal septations [Fig 1].
      
      • Dark bands at both ends and one prominent septum at the base.
      • The color is brown to olivaceous.
      • A protruding hilum that looks like a bird’s beak is at the site where the conidia attach at a sharp angle to the conidiophore (conidia-bearing filament).
    • E. longirostratum: similar in appearance to E. rostratum, but with larger conidia (up to 228 by 12-19 μm).
    • E. mcginnisii: 44 to 76 by 11 to 18 μm, with 4 to 8 internal septations, warty projections on their outer walls and without dark bands at the ends
• Non-culture-based diagnostics:
  • Polymerase chain reaction (PCR) has facilitated the prompt identification of Exserohilum but is highly susceptible to contamination.
    • Results should be used only in conjunction with clinical and epidemiological data.
  • CSF (1-3)-β-D-glucan (BDG) level measurements in outbreak-associated E. rostratum CNS infections:
    • A 138 pg/mL cutoff provided 100% sensitivity and 98% specificity for diagnosing fungal meningitis.
    • The decline in CSF BDG correlated with clinical improvement.
• Biopsies from tissue-invasive disease with Exserohilum spp. will show broad septate hyphae with acute angle branching [Fig 2]. These organisms can be angioinvasive, and hyphal elements can be seen in and around blood vessels.