Fungal Pathogen Species
The Proteome module currently covers eighteen fungal pathogens of humans, including Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, Blasto-myces dermatitidis, Candida albicans, Candida dubliniensis, Candida glabrata,Candida guilliermondii, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida pseudotropicalis, Candida tropicalis, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum, Pneumocystis carinii and Pneumocystis jiroveci. These species are described in detail below.
Aspergillus species
Aspergillus species are members of the family Trichocomaceae of the Ascomycetes. They are haploid filamentous fungi that grow as a mycelium of branched, multinucleate hyphae. Infection with these species often begins with inhalation of conidia (asexual spores) which germinate in the lungs and begin to grow hyphally. Aspergillus spp. can cause the formation of a fungal mass (aspergilloma) or invasive disease, and can provoke allergic reactions. In addition, contamination of food with the carcinogenic aflatoxin produced by A. flavus has serious implications for human health.
The Aspergillus species included are:
- A. fumigatus
- A. flavus
- A. niger
Blastomyces dermatitidis (Ajellomyces
dermatitidis; Blastomycoides dermatitidis)
Histoplasma capsulatum (Ajellomyces capsulatus; Emmonsiella capsulata)
These relatively closely related pathogens are classified within the family Onygenaceae of the Ascomycetes. The mycelial form of each grows in soil and produces conidia (asexual spores) which are inhaled by mammals. in vivo in mammalian lungs, B. dermatitidis and H. capsulatum begin to grow as yeast-form cells; H. capsulatum grows intracellularly in the phagolysosome after being ingested by white blood cells. Both of these fungi can cause relatively benign respiratory infections or disseminated, chronic, sometimes fatal infections, in both immunocompromised and in otherwise healthy patients.
Candida species
The genus Candida belongs to the class Saccharomycetes (Hemiascomycetes) along with the model organism S. cerevisiae. It includes many yeast-like species, most of which are not known to mate. However, as molecular taxonomic studies progress, some Candida species are being recognized as true members of other genera such as Kluyveromyces. Of the Candida species covered, C. glabrata is most closely related to S. cerevisiae. C. albicans, which can cause both superficial and systemic infections in humans, is more commonly found as a human pathogen than any other fungal species. The other Candida species are also known to be opportunistic pathogens, and some seem to be particularly suited for the infection of certain niches such as mucous membranes.
The Candida species included are:
- C. albicans
- C. dubliniensis
- C. glabrata (Torulopsis glabrata
- C. guilliermondii (Pichia guilliermondii; Yamadazyma guilliermondii)
- C. krusei (Issatchenkia orientalis)
- C. lusitaniae (Clavispora lusitaniae)
- C. parapsilosis
- C. pseudotropicalis (C. kefyr; Kluyveromyces cicerisporus; K. fragilis; K. marxianus)
- C. tropicalis
Coccidioides immitis
C. immitis is a member of the order Onygenales of the Ascomycetes. It grows in mycelial form in the soil, producing infectious arthroconidia (spores derived from hyphal fragmentation) that are easily inhaled. Once in the lungs C. immitis differentiates into a structure termed a spherule that becomes filled with hundreds of endospores. Upon rupture of the spherule, each endospore can in turn become a new endospore-filled spherule. Because of this efficient mode of replication and several other characteristics, C. immitis is highly infectious and more virulent than many other fungal pathogens, infecting immunocompetent as well as immunocompromised hosts.
Cryptococcus neoformans (Filobasidiella neoformans)
C. neoformans is the only major pathogen of humans that is a member of the phylum Basidiomycetes. It is usually found as haploid yeast-form cells, and although it can grow in a filamentous form, hyphae are rarely seen in infected patients, suggesting that morphological transitions do not play a role in virulence. C. neoformans is almost exclusively an opportunistic pathogen, entering the body through the lungs and, in immunocompromised patients, disseminating to the central nervous system. A significant proportion of AIDS patients contract it, and those who do need lifelong antifungal therapy.
Pneumocystis species
Pneumocystis is classified in its own order within the Ascomycetes, the Pneumocystidomycetes, and is more closely related to the model Ascomycete Schizosaccharomyces pombe than to Saccharomyces cerevisiae. However, several aspects of its morphology and physiology are quite different from those of any other fungus yet described, which led to early controversy about whether it was a fungus. Pneumocystis is an opportunistic pathogen that infects most mammals. Infection is known to begin with inhalation of the organism, but there are still many open questions surrounding the details of transmission. Since Pneumocystis cannot yet be cultured outside a mammalian host, the details of its life cycle are still unclear. In the lungs of infected animals, two cell types are found: trophic forms, presumed to be the haploid, vegetatively propagating phase, and spore cases or cysts, which are thought to contain the haploid products of mating and meiosis. Each different host carries a different strain or special form (forma specialis or f. sp.); the special forms are thought to represent separate species. P. carinii f. sp. hominis, the special form that exists in humans, has been recognized as a distinct species and renamed Pneumocystis jiroveci (Stringer et al., 2002). P. jiroveci is a frequent cause of pneumonia in AIDS patients.
The Pneumocystis species included are:
- P. carinii
- P. jiroveci