Cryptococcus neoformans

University of Oklahoma Study Abroad Microbiology in Arezzo, Italy[1]

File:Cdiphtheriametachromatic.jpg

C. diphtheriae bacteria, in green, showing metachromatic granules on the ends of the cell. From: http://textbookofbacteriology.net/diphtheria.html

Etiology/Bacteriology

Higher order taxa

Eukaryota (Kingdom); Fungi (Domain); Basidiomycota (Phylum); Tremellomycetes (Class); Tremellales (Order); Tremellaceae (Family); Cryptococcus (Genus)

Species

C. neoformans v. neoformans, C. neoformans v. grubii. A third variety, C. neoformans v. gattii, is now considered a distinct species, Cryptococcus gattii. Its teleomorph is Filobasidiella neoformans.

NCBI: Taxonomy Genome: Cryptotoccus neoformans

Description

Cryptococcus neoformans is a spherical, encapsulated, non-myceliated, non-fermenting fungal cell [[#References|500]. Among pathogenic fungi, C. neoformans is unique in that it possesses a mucinous capsule. Small-sized basidiospores (1.8 to 3.0 μm) can turn into yeast cells, the form preferred at 37°C, or can form dikaryotic hyphae which are favoured at 24°C [#References|2500]. C. neoformans causes fatal meningitis primarily in immunosuppressed humans. It is usually found in tissues in the yeast form. Besides a prevalent asexual life cycle, C. neoformans also presents a bipolar mating cycle with two mating types, MATa and MATα, with the latter being the most prevalently isolated from hosts and the environment. Filobasidiella neoformans is the teleomorph (sexual state). During sexual recombination, either filaments with clamp connections or basidiospores are produced. Recombinant basidiospores are produced via same-sex mating and are thought to be the propagules responsible for infection.

Genome structure

Most isolates of C. neoformans are haploid. The size of the genome is approximately 19 Mb with 14 chromosomes. Cryptococcus neoformans has a defined sexual cycle involving mating between cells of the MATalpha and MATa types. Thus, classical genetic approaches can be applied to study this organism.

Pathogenesis

Transmission

An environmental survey found that Cryptococcus neoformans was prevalently isolated from pigeon and other bird excreta and less frequently in other environmental and veterinary (cat, dog, horse, sheep, cow) sources [2000]. The primary mode of transmission is inhalation of spores, affecting both humans and other domestic and wild animals. Transmission from humans and other animals have not been documented.

Epidemiology

A close correlation between the occurrence of C. neoformans and pigeon droppings has been shown [1000]. C. neoformans var. grubii has a worldwide distribution with the most cases being reported in the United States and Australia, while C. neoformans var. neoformans is is mostly restricted to France, Italy, and Denmark [1500]. Among immmunocompetent individuals, groups at risk are the elderly and those using corticosteroids [1000]. The fungus predominantly affects immunosuppressed hosts. Among HIV/AIDS patients, it is estimated that approximately 957,900 cases occur each year, resulting in nearly 625,000 deaths [2000]. Worldwide, 7-10% of patients with AIDS are affected [2000]. AIDS-associated cryptococcosis accounts for 50% of cryptococcal infections reported worldwide, usually occurring in HIV patients when their CD4 lymphocyte count falls below 200/mm3. In sub-Saharan Africa, the mortality rate is estimated to be between 50-70% . In developed countries such as the United States where therapy is available, the mortality rate is as low as 12% [2000].

Treatment

In healthy individuals, treatment is usually not necessary. However, it is recommended to receive check-ups for up to monitor any spreading of the infection. If there are lung lesions or the disease spreads, antibiotics such as fluconazole, itraconazole, and voriconazole may be prescribed.

References

500. http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1000953

1000. http://digital.library.okstate.edu/oas/oas_pdf/v45/p277_281.pdf

1500. http://www.mycology.adelaide.edu.au/Fungal_Descriptions/Yeasts/Cryptococcus/C_neoformans.html

2000. http://www.hindawi.com/journals/scientifica/2013/675213/

2500. http://www.phac-aspc.gc.ca/lab-bio/res/psds-ftss/cryptococcus-eng.php