Cryptococcus

Cryptococcus (Greek for 'hidden sphere') is a genus of fungi, which grow in culture as yeasts. The sexual forms or teleomorphs of Cryptococcus species are filamentous fungi in the genus Filobasidiella. The name Cryptococcus is used when referring to the yeast states of the fungi. The cells of these species are covered in a thin layer of glycoprotein capsular material that has a gelatin-like consistency, and that among other functions, serves to help extract nutrients from the soil. The C. neoformans capsule consists of several polysaccharides, of which the major one is the immunomodulatory polysaccharide called glucuronoxylomannan (GXM). GXM is made up of the monosaccharides glucuronic acid, xylose and mannose and can also contain O-acetyl groups. The capsule is functioning as the major virulence factor in cryptococcal infection and disease. Modern molecular technologies increased the validity and readiness in classifying microorganisms in taxonomical level. Cryptococcus spp. has a huge diversity in subspecies level each with different molecular types based on their genetic differences, mainly due to different geographical distributions, molecular characteristics and ecological niches. Serotypes A, D and a hybrid of AD are found among Cryptococcus neoformans var grubii and Cryptococcus neoformans var neoformans. Serotypes B and C are mainly found in Cryptococcus gattii. There are also hybrids between Cryptococcus neoformans and Cryptococcus gattii. There are 9 major molecular types of pathogenic Cryptococcus spp. - (VNI, VNII, VNB, VNIII, and VNIV) grouped under C. neoformans isolates; (VGI, VGII, VGIII, and VGIV) grouped under C. gattii isolates. About 37 species of Cryptococcus are recognized, but the taxonomy of the group is currently being re-evaluated with up-to-date methods. The majority of species live in the soil and are not harmful to humans. Very common species include C. laurentii and C. albidus. Of all species, C. neoformans is the major human and animal pathogen. However, C. laurentii and C. albidus have been known to occasionally cause moderate-to-severe disease, to be specific meningitis, in human patients with compromised immunity (owing to HIV infection, cancer chemotherapy, metabolic immunosuppression, etc.). C. neoformans is the most prominent medically important species. It is best known for causing a severe form of meningitis and meningoencephalitis in people with HIV/AIDS. It may also infect organ-transplant recipients and people receiving certain cancer treatments. C. neoformans is found in the droppings of wild birds, often pigeons; when dust of the droppings is stirred up, it can infect humans or pets that inhale the dust. Infected humans and animals do not transmit their infection to others; they are not infectious. When plated on Niger or birdseed agar, C. neoformans produces melanin, which causes the colonies to have a brown color, and this melanin production is believed to be an important virulence factor. C. neoformans produces extracellular vesicles that contain protein components associated with virulence. These vesicles interact with the cell wall and their formation is changed upon mutations in genes responsible for capsule formation. The taxonomy of this species has been reviewed: it has been proposed that it be divided into two species - Cryptococcus neoformans and Cryptococcus deneoformans.