Cryptococcus neoformans: Introduction, Morphology, Pathogenecity, Lab Diagnosis,Treatment and Keynotes

Cryptococcus neoformans: Introduction, Morphology, Pathogenecity, Lab Diagnosis,Treatment and Keynotes

 Introduction of Cryptococcus neoformans

Cryptococcus neoformans is a fungus found in the environment globally.  Even though most people who are exposed to the fungus never get infected from it. Weakened immune systems, particularly those who have advanced HIV/AIDS are most prone to Cryptococcal infection. It causes a disease called Cryptococcosis. Cryptococcosis is acute, subacute, or chronic pulmonary meningeal mycosis.

Classification of Cryptococcus

  • Kingdom: Fung
  • Phylum: Basidiomycota
  • Class: Tremellomycetes
  • Order: Tremellales
  • Family: Tremellaceae
  • Genus: Cryptococcus
  • Species: C. neoformans

Other species are-

  • C. albidus ( nitrate positive)
  • C. laurentii ( melibiose fermentation)
  • C. gattii ( Trehalose positive)
  • C. neoformans: All are negative
  • C. neoformans var. grubii ( serptype A)- world-wide distribution
  • C. neoformans var. neoformans  (serotype D)-Restricted distribution and prevalent in France, Italy, and Denmark.

Morphology of Cryptococcus neoformans

It is a true yeast characterized by wide carbohydrates capsule and budding, found both in culture and tissue fluid. It is normally found in pigeon’s dropping.

Size and shape: It is 5-20 µm in diameter and round or ovoid budding cell.

Antigenic structure: based on carbohydrates antigen, there are 4 serotypes eg. A, B, C, D. Infection due to serotypes  A and D are common.

Virulence of Marker

  • Polysaccharide capsule
  • Phenoloxides
  • Growth rate at 37 °C


Mode of transmission: Infection is acquired by inhalation of aerosol forms of Cryptococcus. Pulmonary infection is associated with signs and symptoms. It may be disseminated to various organs. The organism has a predilection to Central Nervous System (CNS). The organism is opportunistic and causes infection in an immunosuppressed patient. AIDS, Diabetes, organ transplantation, cancer, chemotherapy taker are conditions that make a favorable environment for organisms.

Risk factors

  •  Patients with advanced HIV/AIDS having  CD4 T cell counts <200/µl.
  • Hodgkin lymphoma
  • Other lymphomas
  • Sarcoidosis
  • Long-term corticosteroid therapy
  • Solid-organ transplantation


Clinical Features

1. Pulmonary Cryptococcosis: Respiratory tract is the most common entrance of C. neoformans. It is mainly in immunocompetent hosts like AIDS,  but also rarely in diabetes, organ transplantation, cancer, chemotherapy users.

2. Disseminated infections: It may lead to visceral, cutaneous, meningoencephalitis disease, or ocular cryptococcosis.

A.  A central nervous system (CNS) cryptococcosis is the most common fungal infection and the second most common opportunistic infection of the CNS.

B. Visceral cryptococcus/osseous cryptococcosis

C.  Cutaneous cryptococcus: It results from hematogenous dissemination of infection.

D.  Ocular Cryptococcal: Patients develop keratitis, papilledema, scotoma, chorioretinitis, and ocular palsy.

Laboratory Diagnosis of Cryptococcus neoformans

Specimen: It depends on the site of infection and the common sample are CSF, sputum, blood, pus, brain tissue, and skin scrapings.

Microscopic examination: India ink preparation

Other stains: Mucicarmine stain which stains carminophilic cell wall of C. neoformans. Fontana-Masson silver (FMS) stain oxidizes melanin and melanin-like pigments as it reduces silver.

Culture: Sabrose dextrose agar (SDA) without cycloheximide-Colony morphology on SDA are creamy colored and shiny, smooth, mucoid colony.

Selective culture medium: Birdseed agar, niger seed agar, and caffeic acid agar

These media show melanin production i.e. brown colored colonies.

Brain heart infusion (BHI) broth

Inositol agar with chloramphenicol: It inhibits Candida growth and mainly uses for inoculation of urine.

Colony characteristics:

Bird Seed Agar:  Brownish colony physiology

C. neoformans produces brown colonies within 1 week at 30°C, a property not shown by other yeast, including other Cryptococcus spp.

Phenol  oxidase ( C. neoformans )

Caffeic acid —Brown pigment

For the rapid formation of melanin by C. neoformans, it is important that glucose constitute no more than  0.1% of the medium because the phenoloxidase of the fungus is repressed by glucose.

Biochemical test: Sugar fermentation and assimilation – Do not ferment Carbohydrates

Urea hydrolyzation test:   positive


Antigen detection in the specimen

Method: Latex agglutination test, EIA.

Pathogenicity test: Inoculation of colonies in mice via intracerebral or intraperitoneally which is fatal for mice.

Molecular Test

Detection of C.  neoformans DNA in tissue samples by Nested and Real-Time PCR Assays

Treatment of Cryptococcus neoformans

Following antifungal drugs are used to treat Cryptococcosis.

  • Amphotericin B
  • 5- fluorocytosine
  • Imidazoles (miconazole, keratonazole)
  •  Triazoles (itranazole, fluconazol)

Prevention and Control

  1. People having Immunocompromized or Immunosuppressed status should avoid contact with birds and avoid digging and dusty activities in areas heavily contaminated with bird droppings.
  2. Avoid the area where availability of dried pigeon feces.
  3. Wear masks to prevent the inhalation of C. neoformans.


  • Cryptococcosis is also called Torulosis.
  • Cryptococcus has the ability to cross the blood-brain barriers.
  • Other agents of cryptococcosis are teleomorphs ( the sexual reproductive stage, typically a fruiting body) for fungus.
  • For localized pulmonary disease, fluconazole is commonly used while in case of meningitis or other severe infection, amphotericin B with or without flucytosine is used, followed by fluconazole.
  • In healthy people, infection is typically asymptomatic and self-limited.
  • Cryptococcosis is occurred by inhaling dust from soil contaminated with bird droppings, particularly those of pigeons.
  • All fungi are gram-positive.
  • For Cryptococcus spp. identification other physiological tests like sugar  (glucose, galactose, sucrose, maltose, lactose, trehalose) fermentation and sugar assimilation tests are used.
  • For the rapid formation of melanin by C. neoformans , it is important that glucose constitute no more than 0.1% of the medium since the phenoloxidase of this organism is repressed by glucose.

Further Readings

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