Central nervous system infections Archives • Microbe Online

Cryptococcus neoformans is a yeast with a prominent polysaccharide capsule. It is an opportunistic fungal pathogen notoriously known as the most common cause of fungal meningitis (infection and inflammation of the meninges) in immunocompromised patients (such as people with AIDS). It reproduces by budding and the single buds are characteristically narrow at the base. It does not produce pseudohyphae (Candida albicans does). It is a ubiquitous saprophyte often found associated with bird droppings, especially of pigeons. Pathogenesis Infection is initiated by inhalation of the yeast cells. The primary pulmonary infection may be asymptomatic or may mimic influenza-like respiratory infection often resolving spontaneously. In immune-compromised patients with impaired T cell immunity, the yeasts may multiply and 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. The inflammatory reaction is usually minimal or granulomatous. Virulence Factors of Cryptococcus neformans Capsule:Cryptococcus neoformans is the only pathogenic yeast known to have a polysaccharide capsule. The function of capsule is as follows: Anti-phagocytic: Polysaccharide capsule contains compounds that are not recognized by phagocytes. Protection under drying conditions: The capsule collapses and protects the yeast. Ideal size range for alveolar deposition: the cell size reduction, resulting from capsular collapse, places the yeast in the ideal size range for alveolar deposition. Phenoloxidase: This enzyme is responsible for melanin production. Melanin is might act as a virulence factor by making the organism resistant to leukocytes attack. It has been found that increased melanin production can decrease lymphocyte proliferation and tumor necrosis factor production. Phenoloxidase also protects the organism from oxidants released by phagocytic cells. Diseases Cryptococcosis is most often seen in immune-compromised patients with impaired T cell immunity. Important risk factors include AIDS, corticosteroid therapy, lymphoma, and T cell dysfunction. Cryptococcal infection occurs throughout the world sporadically. Infection occurs from the environment usually by inhalation, especially of dust containing excreta of pigeons but the infection is not transmitted from person to person. People with immune-compromised status are likely to be affected more and have serious outcomes; Cryptococcal meningitis: Only in a small proportion of cases, hematogenous spread results in subacute or chronic meningitis or meningoencephalitis. All untreated cases of cryptococcal meningitis are ultimately fatal. About 5-8% of patients with AIDS develop cryptococcal meningitis. Signs and symptoms of cryptococcal meningitis Headache Fever Change in mental status (ranging from confusion to lethargy to coma) Blurry vision (and other cranial nerve deficits) Neck stiffness Sensitivity to light Nausea and vomiting Seizures Papilledema Lung infections: Pulmonary disease is increasing nowadays, particularly in immunocompromised hosts. Reactivation of old healed lesions may occur. Skin and other infections: Sometimes skin, lymph nodes, bones are involved. Laboratory diagnosis Specimens: Specimens depends on clinical presentation and suspected disease conditions. Common specimens include spinal fluid (CSF), tissue, exudates, sputum, blood, and urine. Microscopy and staining Cryptococcus neoformans appear as a spherical, single or multiple budding, thick-walled yeast that is 2-15 μm (wide variation in size) in diameter. It is usually surrounded by a wide refractile capsule. Image-1: India ink preparation of CSF sample India ink preparation is used as a rapid and inexpensive diagnostic tool for detecting cryptococcal infection in many institutions and resource-poor settings. Demonstration of heavily capsulated yeast cells (see the image) in CSF, exudates, and urine establishes the diagnosis. India ink preparation when positive in CSF is diagnostic of cryptococcal meningitis but its sensitivity is low. Many diagnostic laboratories have replaced this test with a more sensitive cryptococcal latex agglutination test. Culture Colonies develop within a few days on most media (e.g., Sabouraud’s dextrose agar) at room temperature or 37 °C. Cryptococcus neoformans are sensitive to cycloheximide so media containing cycloheximide should be avoided. Other culture media are Blood Agar, BHI Agar, Bird seed agar, etc. Colonies in SDA are creamy, white, and mucoid (because of capsule). Image-2: Mucoid colonies of C. neoformans in Bird Seed Agar (source: The University of Adelaide) Identification: Cryptococcus neoformans is identified by urease production and carbohydrate assimilation test, and confirmed by direct immunoflurorescence using a fluorescein-labeled anti-neoformans antibody. Detection of Antigen and/or Antibody Detection of Antigen: Tests for capsular antigen can be performed on CSF and serum. Latex agglutination test is most useful in the detection of cryptococcal polysaccharide antigen. Slide latex agglutination test has a sensitivity of 90% in the cases of cryptococcal meningitis. Detection of Antibody: Serum antibodies can be detected by agglutination and immunofluorescence. Molecular diagnosis Not common for diagnostic purposes. Key Facts Only pathogenic yeast known to have a polysaccharide capsule. The most common cause of fungal meningitis. Cryptococcal meningitis is a common cause of death among HIV/AIDS patients India ink preparation and cryptococcal antigen agglutination are commonly used diagnostic methods. Association with birds excreta (pigeons droppings) and rapid urease positive References Rathore, S. S., Sathiyamoorthy, J., Lalitha, C., & Ramakrishnan, J. (2022). A holistic review on Cryptococcus neoformans. Microbial pathogenesis, 166, 105521. https://doi.org/10.1016/j.micpath.2022.105521 Lin X. (2009). Cryptococcus neoformans: morphogenesis, infection, and evolution. Infection, genetics and evolution : journal of molecular epidemiology and evolutionary genetics in infectious diseases, 9(4), 401–416. https://doi.org/10.1016/j.meegid.2009.01.013