Cryptococcus neoformans

Cryptococcus neoformans


Cryptococcus neoformans

  • Opportunistic fungal pathogen 
  • Most common cause of fungal meningitis:
  • In immuno-compromised patients (such as people with AIDS).
Morphology
  • Cell wall is gram-positive.
  • Yeast with prominent polysaccharide capsule.
  • Unicellular

Important features

  •  Possess urease.
  • It reproduces by budding 
  • Single buds are characteristically narrow at the base.
  • It does not produce pseudohyphae
Serotypes
  • Four capsular serotypes – A, B, C and D
  • Most infections in immunocompromised patients are caused by serotype A.
  • Abundant in pigeon droppings :
  • Contain serotype A and D.
  • Eucalyptus tree contains serotype B.

Culture: 

  • Colonies develop within a few days on most media
  • Sabouraud’s dextrose agar
  •  Other culture media are Blood Agar, BHI Agar, Bird seed agar etc.

Temp:

  • Room temperature or 37 °C. 
  • Colonies in SDA are creamy, white and mucoid (because of capsule)
  • Cryptococcus neoformans is sensitive to cycloheximide

Pathogenesis

  • Infection is initiated by inhalation of the yeast cells.
  • Cryptococcus has predilection for lung and meningis.
  • The primary pulmonary infection may be asymptomatic .
  • In immune-compromised patients with impaired T cell immunity, the yeasts may multiply
  • Disseminate to other parts of the body
  • But preferentially to the central nervous system (neurotropic), causing cryptococcal meningitis.
  • Other common sites of dissemination include the skin, adrenals, bone, eye and prostate gland.

Virulence Factors of Cryptococcus neformans

Capsule:

  • Anti-phagocytic
  • Protection under drying condition
  • Ideal size range for alveolar deposition

Phenoloxidase:

  • Responsible for melanin production
  • Melanin is might act as a virulence factor by making the organism resistant to leukocytes attack.

Exam Question

Cryptococcus neoformans

  • Most common cause of fungal meningitis:
  • In immuno-compromised patients (such as people with AIDS).
Morphology
  • Yeast with prominent polysaccharide capsule.
  • Unicellular
  •  Possess urease.
  • It reproduces by budding 
  • It does not produce pseudohyphae
Serotypes
  • Four capsular serotypes – A, B, C and D
  • Abundant in pigeon droppings :
  • Contain serotype A and D.
  • Eucalyptus tree contains serotype B.

Culture: 

  • Sabouraud’s dextrose agar 
  • Temp:
  • Room temperature or 37 °C. 

Pathogenesis

  • Cryptococcus has predilection for lung and meningis.

Virulence Factors of Cryptococcus neformans

  • Capsule:
  • Anti-phagocytic
  • Phenoloxidase:
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