Although members of the Ascomycota phylum, particularly Sacchromyces cerevisiae, are the most studied fungi, there are 80,000 known species of the Fungi kingdom. There is a great deal of diversity in the kingdom, ranging from small harmless unicellular yeast such as S. cerevisiae to the great plant pathogen Armillaria ostoyae, one of the largest organisms in the world. This latter species is a member of the Basidiomycota phylum, a phylum less well understood than Ascomycota.
While no basidiomycete species has been studied in as much detail as S. cerevisiae
, it is a fascinating and diverse group of organisms. Basidiomycetes produce many interesting secondary metabolites used in medicine, industry, and research. Members of the phylum account for about 10% (40 species) of known human fungal pathogens (Morrow and Fraser, 2009
). With the onset of the AIDS epidemic, one basidiomycete in particular, Cryptococcus neoformans
, has risen from a little-known pathogen to one of the top fungal killers of immunocompromised patients.
is primarily found as a haploid yeast, and is widely present in the environment worldwide, including in avian excreta, soil, and tree bark. Studies have shown that humans come into frequent contact with C. neoformans
: individuals with no history of cryptococcosis possess antibodies against the yeast (Chen et al., 1999
), and most children appear to have been exposed by the age of five (Goldman et al., 2001
). This suggests that the majority of individuals encounter C. neoformans
in the environment, most likely through inhalation into the lungs. Immunocompetent individuals are usually able to control and contain the infection, often leading to an asymptomatic latent state of infection. If the patient’s immune system becomes compromised at a later date, the latent infection can reactivate. In the case of the immunocompromised individual, pulmonary infection can lead to pneumonia followed by dissemination via the bloodstream to other organs. C. neoformans
is one of only a few fungal species known to cross the blood-brain barrier and infect the brain (Kim, 2006
), leading to meningitis that is fatal if left untreated. When the AIDS epidemic began in the 1980s, there was a concomitant surge in cryptococcosis cases worldwide. In recent years, the increased usage of antiretroviral therapy and antifungals has reduced the overall incidences of fatal cryptococcal meningitis. Yet in areas where access to treatment is limited, C. neoformans
remains an important concern in the care of the immunocompromised, including AIDS, cancer, and organ transplant patients. In addition, recent outbreaks of cryptococcosis in immunocompetent individuals in the Pacific Northwest raise concerns about the risk of cryptococcal infection even in otherwise healthy individuals (Bartlett et al., 2008
; Hoang et al., 2004
As a haploid yeast cell, C. neoformans
is amenable to many of the extensive protocols that have been developed for S. cerevisiae
, requiring in most cases only a few adjustments. However, having diverged from the ascomycete lineage some 400 million years ago (mya) (Taylor and Berbee, 2006
), there are significant differences in its cellular machinery and life cycle (see below). Comparative genomics promises to yield rich information about the evolution of shared and diverged genes, proteins, and pathways, as well as offering insight into the differences between species that allow one yeast to exist as a benign saprophyte and another to cause lethal infection in a mammalian host.