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 might act as a virulence factor by making the organism resistant to leukocytes attack.

Exam Important

Cryptococcus neoformans