MICROBIOLOGY

• The nomenclature is currently in flux.
• The clinical entity of mucormycosis refers to infection by organisms previously classified as Zygomycetes that are within the phylum Mucormycota order Mucorales.
  • These include at least 38 species reported to cause infections in humans. These include Rhizopus species (most common), Rhizomucor pusillus, Cunninghamella bertholletiae, Apophysomyces elegans, Saksenaea vasiformis, Lichtheimia (Absidia) species, Mucor circinelloides.
  • Basidiobolus ranarum and Conidiobolus species, which typically cause subcutaneous infections in tropical regions, were also previously classified as Zygomycetes prior to the change in nomenclature. They are now classified separately.
• Micro considerations:
  
  • Agents of mucormycosis frequently fail to grow in culture.
    • Sensitivity can be improved by placing tissue sections directly on the culture plate without first grinding or homogenizing the material prior to inoculation.
    • Mincing of tissue during specimen preparation can disrupt the fungal structure, impairing diagnosis.
  • Appearance in tissue and culture:
    • In tissue: agents of mucormycosis grow in tissue as hyphae with variable width (from 6-25µm, Fig 1), zero or sparse septations, irregular ribbon-like appearance, non-pigmented and wide-angle bifurcations including 90-degree angles [Fig 2].
      • Visualization with direct microscopy of tissue samples is facilitated by blankaphor or calcofluor white stains which bind to the fungal chitin and fluoresces in UV light.
      • The organisms may be seen in histopathology specimens using H+E stains, but visualization is improved with GMS or PAS stains.
  • In culture:
    • Growth, when it occurs, is usually within 3-5 days on typical fungal culture media (e.g. Sabouraud or potato dextrose agar).
    • Appearance differs by species.
      • Characteristics such as the arrangement of spores [Fig 3], and presence, absence, or configuration of specific structures allow for the identification of the specific organisms.
  • Molecular tests
    • PCR diagnostic assays are increasingly employed. Standardization and clinical validation remain to be worked out.
    • MALDI-TOF mass spectrometry, if there is fungal growth, offers another approach to specific identification.