MICROBIOLOGY

• C. albicans: the most important Candida species.
  • Colonizer/commensal of GI and GU tracts and skin.
  • This species accounts for nearly all mucosal candidiasis and is the most common cause of invasive disease.
    • Epidemiology does vary by geographical region, the extent of antifungal (esp. azole class) exposure and local hospital epidemiology.
• Fungal organism has multiple morphologies (depending on environmental conditions):
  
  • Yeast: spherical single cells that can bud [Fig 1].
    
    • Usually about 3-6µm in diameter.
    • Likely important in facilitating dissemination through fluids (e.g., saliva, urine, water, bloodstream) to distant sites.
  • Pseudohyphae: filamentous structures composed of elongated yeast cells in chains and have constrictions at septal junctions (rather than true septa) [Fig 2].
  • True hyphae: filamentous structures composed of cells uniform in width [Fig 3].
    • Have true septa and facilitate invasion of tissues.
  • Germ tube: when incubated in serum, C. albicans and C. dubliniensis form an early hyphal structure called the germ tube (although ~5% may be initially called germ-tube negative).
    • For this reason, preliminary identification reports from the microbiology laboratory will often read "C. albicans/C. dubliniensis."
    • Because it is much more common than C. dubliniensis, almost invariably the actual species in such circumstances is C. albicans.
  • Chlamydospores: rounded, thick-walled structures, several times the size of the yeast, typically found at the ends of hyphae [Fig 4].
    • Seen with C. albicans and C. dubliniensis.
• Diagnostic tests:
  • Stains: organisms may be visualized via KOH, Gram stain, calcofluor white, Grocott-Gomori’s methenamine silver (GMS) and periodic-acid-Schiff (PAS).
  • Culture: grows aerobically in various media, including blood culture broth, blood agar, Sabouraud agar and Mueller Hinton agar.
    • Blood cultures, and continuous monitoring systems: these modern systems are as good as the lysis centrifugation ("fungal isolator") method.
      • False-negative blood cultures: common in candidemia and with any blood culture technique.
    • Identification of individual species: facilitated microscopic morphology, biochemical tests, chromogenic agar and PNA FISH techniques.
  • Serum beta D glucan (BDG): non-specific test for fungal infections.
    
    • Levels >60-80 pg/ml suggest invasive disease.
    • Causes of false positives include severe burns, extensive gauze packing, and other fungal infections.
  • T2Candida: Nano diagnostic tool that utilizes T2 magnetic resonance and a dedicated instrument to detect Candida directly in a whole blood sample.
    • Time to positivity and initiation of antifungal therapy is significantly shorter compared to traditional blood cultures.
    • Sensitivity is generally as good as and even better than standard blood culture techniques. Sensitivity is superior to blood cultures in cases of prior antifungal therapy, neutropenia, and C. albicans candidemia.