Cryptococcus

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.[7] 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.[7] Infected humans and animals do not transmit their infection to others; they are not infectious.[7] 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.[8] C. neoformans produces extracellular vesicles that contain protein components associated with virulence.[9] These vesicles interact with the cell wall and their formation is changed upon mutations in genes responsible for capsule formation.[10][11]

The taxonomy of this species has been reviewed: it has been proposed that it be divided into two species - Cryptococcus neoformans and Cryptococcus deneoformans.[12]

Field stain showing Cryptococcus species in lung tissue

C. gattii (formerly C. neoformans var gattii) is endemic to tropical parts of the continent of Africa and Australia. It is capable of causing disease (cryptococcosis) in nonimmunocompromised people. It has been isolated from eucalyptus trees in Australia. Since 1999, an outbreak of C. gattii infections has occurred in eastern Vancouver Island,[13] an area not generally thought to be endemic for this organism. Cases have since been described in the Pacific Northwest, in both Canada and the United States.[14]

The taxonomy of this species has been reviewed; it has been proposed that it be divided into five species - C. gattii, C. bacillisporus, 'C. deuterogattii, C. tetragattii, and C. decagattii.[12]

C. albidus has been isolated from the air, dry moss in Portugal, grasshoppers in Portugal, and tubercular lungs.[15] The colonies on a macroscopic level are cream-color to pale pink, with the majority of colonies being smooth with a mucoid appearance. Some of the colonies have been found to be rough and wrinkled, but this is a rare occurrence.[16] This species is very similar to C. neoformans, but can be differentiated because it is phenol oxidase-negative, and, when grown on Niger or birdseed agar, C. neoformans produces melanin, causing the cells to take on a brown color, while the C. albidus cells stay cream-colored.[8][17] On a microscopic level, C. albidus has an ovoid shape, and when viewed with India ink, a capsule is apparent. This species also reproduces through budding. The formation of pseudohyphae has not been seen. C. albidus is able to use glucose, citric acid, maltose, sucrose, trehalose, salicin, cellobiose, and inositol, as well as many other compounds, as sole carbon sources. This species is also able to use potassium nitrate as a nitrogen source. C. albidus produces urease, as is common for Cryptococcus species.[18] C. albidus is very easily mistaken for other Cryptococcus species, as well as species from other genera of yeast, so should be allowed to grow for a minimum of 7 days before attempting to identify this species.

While this species is most frequently found in water and plants and is also found on animal and human skin, it is not a frequent human pathogen. Cases of C. albidus infection have increased in humans during the past few years, and it has caused ocular and systemic disease in those with immunoincompetent systems, for example, patients with AIDS, leukemia, or lymphoma.[16][17] While systemic infections have been found with increasing regularity in humans, it is still relatively rare in animals. The administration of amphotericin B in animals has been successful, but in humans, the treatment usually has poor results.[8]

C. albidus var. albidus is a variety of C. albidus that has been considered unique. It differs from C. neoformans because of its ability to assimilate lactose, but not galactose. This species is also considered unique because its strains have a maximum temperature range from between 25 and 37°C. This is important because it violates van Uden’s rule, which states that the yeast strains of a particular species cannot have their maximum growth temperature vary by more than 5°C.[19] However, some debate exists as to whether or not this maximum temperature range for the strains of C. albidus is accurate, because other research has shown that C. albidus var. albidus strains cannot grow at 37°C.[17] Another variety, 'C. albidus var. diffluens is different from C. neoformans in that it can assimilate melibiose, but not galactose.[8]

C. uniguttulatus (Filobasidium uniguttulatus is a teleomorph) was the first non-neoformans Cryptococcus to infect a human. It was isolated from ventricular fluid from a patient having had a neurosurgical procedure. This species was found to be very sensitive to amphotericin B at the minimum inhibitory dose. This species was first isolated from a human nail.