Cryptococcus (Filobasidiella) is a genus of fungus in the Phylum Basidiomycota of the Subphylum, Basidimycotina, of the Order Sporidiales and Family Sporidiobolaceae. It has more than 100 species ubiquitously distributed in the environment, with two species commonly known to cause cryptococcosis in humans, Cryptococcus neoformans and Cryptococcus gattii [1]. C. neoformans is a fungus that is distributed globally, being found in bird feces, trees and soil [2]. For the most part, it affects immunocompromised patients, causing a symptomatic and disseminated infection [3]; however, there are reports of C. neoformans var grubii infecting immunocompetent patients [4]. C. gattii is seen as a tropical and subtropical fungus, being found in reforestation trees, such as pine and eucalyptus, and in the soil close to these trees [5], and differently from C. neoformans, C. gattii infection occurs in both immunocompetent and immunocompromised patients [6]. Another species of the same genus, C. laurentii, is distributed in a dispersed way in the environment, in soil and plants, even maintaining a role for symbiosis with mycorrhizas, helping in plant metabolism [7]. It is known not to cause infection in humans and immunocompetent animals ^[8][9][8,9]; however, there are reports of infected immunocompromised patients in tropical regions ^[10][11][12][10,11,12].

2. First Historical Aspects of Cryptococcus spp.

The discovery of the C. gattii fungus was carried out after several attempts to classify other species of the same genus throughout the years. It started in 1894 in Germany, when the pathologist Otto Busse and the surgeon Abraham Buschke isolated a yeast similar to Saccharomyces from a bone infection in a patient, calling it “Saccharomycosis hominis”. Later in the same year in Italy, Francesco Sanfelice isolated a similar yeast from peach juice ferment and named it Saccharomyces neoformans because of its peculiar colony formation ^[3][13][3,13]. In 1901, in France, Jean-Paul Vuillemin would give new nomenclatures to these yeasts, as they did not form ascospores, which are clusters of spores within a single mother cell called an ascus [14], and did not ferment carbohydrates, both hallmarks of Saccharomyces. Thus, the yeast discovered by Busse was named Cryptococcus hominis and that of Sanfelice was renamed Cryptococcus neoformans. It was only in 1950 that Benham in the United States would recommend the definitive nomenclature for the species (C. neoformans), which would be adopted in later works [15]. In the same year, Evans and authors, for the first time, described an antigenic feature of the fungus in three specific serotypes: A, B and C [16]; only in 1968, Wilson described a fourth serotype, D [3]. Decades later, based on agglutination reactions of the capsular constituents of the fungus, the complex of species of Cryptococcus neoformans could be divided into serotypes A, D and hybrid AD, with serotype A corresponding to the grubii variant, whereas C. gattii has distinguishing serotypes B and C ^[17][18][17,18]. In addition to this classification, the division can also be made from DNA polymorphisms found in these species. In this way, the species C. neoformans of serotype A, known as C. neoformans variant grubii, can be divided into molecular types VNI, VNII and VNB, the latter referring to a specific variant [19], and serotype D, known as C. neoformans variant neoformans with molecular type VNIV and the hybrid of the two serotypes AD with molecular type VNIII [17]. C. gattii has a B serotype with the most common variants VGI and VGII and C serotype with VGIII and VGIV [20]. Although the characterization studies were carried out at the beginning of the century and since then, this pathogen has been known to cause disease in humans [13], C. neoformans became more important at the end of the 20th century with the AIDS crisis, where it was responsible for a large part of the morbidity in these patients. Recent data show that about 181,000 deaths from cryptococcal meningitis are recorded annually [21]. C. gattii infections are more frequent in tropical and subtropical regions and even though their incidence has increased globally over the last few decades, they do not comprise the highest incidence globally, with about 80% of cases being caused by C. neoformans. However, C. gattii is responsible for more severe cases of the disease [3], with studies suggesting these cases may be underdiagnosed, either because the tests used rely on cryptococcal serum antigen, which is very hard to detect in cases of localized pulmonary cryptococcosis, or are unable to discriminate between different species of Cryptococcus ^[22][23][22,23].

3. Cryptococcus gattii

The first clinical sample finding of C. gattii came from the cerebrospinal fluid of a Congolese individual, initially reported as C. neoformans in 1894. Only in later years, the yeast with an atypical elliptical shape would be confirmed as C. gattii [2]. Initially, C. gattii was not classified as a species on it own, but considered a Cryptococcus neoformans variant [24]. Differentiation between the yeasts was made using CBD medium, still used to differentiate the species and consisting of an agar solution containing canavanine-glycine-bromothymol blue. C. neoformans only uses glycine as a carbon source and does not show favorable growth in the presence of canavanine [25]; however, C. gattii grows well on this medium and uses glycine not only as a carbon source, but also as a source of nitrogen, thus, alkalinizing the medium, granting it a bluish tinge due to the pH indicator bromothymol blue [26]. Unlike C. neoformans, which is ubiquitously distributed in soil and plant species, C. gattii is found specifically in plant material, mainly in urban landscape trees and in tree species associated with tropical and subtropical climates, such as in forests of Eucalyptus spp. in Australia [27]. The most common variants of C. gattii are the globally distributed VGI and VGII, both responsible for causing most cases of C. gattii cryptococcosis in immunocompetent individuals ^[28][29][28,29]. The variants of C. gattii are more frequent in warmer morphoclimatic domains, such as the predominant VGII in the Brazilian territory, with findings in the Amazon biome ^[30][31][32][30,31,32], clinical isolates from the Brazilian Northeast [33], Central-West [34] and the Brazilian Southeast ^[35][36][37][35,36,37], as well as in northern Australia and the islands of Papua New Guinea [8]. However, there is a trend of VGII findings causing outbreaks of C. gattii in temperate regions around the world, as has been observed with large numbers of domestic animals infected with C. gattii reported in western Australia [38], in outbreaks of C. gattii in North America from Vancouver Island in Canada, which resulted in cases of infections both in wildlife and domestic animals ^[39][40][39,40], along with an outbreak of cases in immunocompetent and immunocompromised humans in 1999 [41]. This outbreak also contributed to the expansion of new cases in the Northwest Pacific Coast of the United States of America, in Washington and Oregon, and more recently in the Southeast of the USA ^[42][43][42,43]. These cases obtained their clinical findings with evidence of a genetic profile from the tropical forests of South America ^[44][45][44,45]. However, more recent analyses indicate a high genetic diversity of the population in the Brazilian variant C. gattii VGII, changing the origin of this variant from the Amazon rainforest to the Brazilian semi-arid northeast [46]. Thus, the data suggest a change in the dynamics of the global dissemination of C. gattii, associating it to the current scenario of climate change and global warming that favors the increase in new cases of C. gattii infections by facilitating its spread [47].

4. Structure and Virulence Factors

Although estimates indicate the genetic separation of ancestors of the genus Cryptococcus at about 30 to 40 million years ago [48], evidence suggests that about 80 to 100 million years ago, ancestors belonging to the species complex of the genus Cryptococcus were physically separated by the drift of the supercontinent Pangea into two continental masses in the region that today corresponds to South America and Africa. This event would have made possible the accumulation of genetic determinants in the genus Cryptococcus in different environments that, in the future, would lead to differentiation in the contemporary species C. neoformans and C. gattii ^[49][50][51][49,50,51]. The role that birds currently play in the aerial dispersal of Cryptococcus spp. between different regions is closely linked with the emergence of bird species in the early Cretaceous and the dispersal of the Cryptococcus genus between continents in the same geological period ^[49][52][49,52]. However, despite the evolutionary distance between the current species of the genus Cryptococcus, the structural determinants that are shared between these species are still well preserved, especially between the pathogenic C. neoformans and C. gattii [27]. Among these components, the cell wall of Cryptococcus spp. itself. has molecules that help the fungus defend against host defenses and environmental stress, in addition to intrinsic characteristics of the yeast, such as resistance to temperatures of 37 °C [53]. A protein that is required for this thermal resistance is a serine/threonine phosphatase called calmodulin, responsible for modulating the yeast response to environmental stresses. Given that calmodulin mutations adversely affect Cryptococcus spp. resistance to the physiological temperature of mammals, it is fundamental in enabling its successful growth in the host ^[54][55][54,55]. Other proteins involved in C. gattii thermotolerance are superoxide dismutase (SOD2p) [56] and trehalose-6-phosphate synthase (TPS1p and TPS2p) [57]. Another important virulence factor in the fungus is the production of melanin [3], present in the cell wall of the fungus and associated with: resistance to oxidative stress ^[58][59][60][58,59,60], resistance to antimicrobial peptides and antifungal components ^[60][61][62][60,61,62] and tolerance to environmental radiation, with the example of some Cryptococcus variants found surviving in environments with high levels of ionizing radiation, such as on the walls of the reactor of the Chernobyl nuclear disaster-UA ^[63][64][63,64]. Regarding melanin synthesis, laccase (Lac1), a phenol oxidase present in Cryptococcus spp., is responsible for this role, depositing this molecule in the yeast cell wall ^[65][66][65,66]. It has already been described that the deposition of melanin in the cell wall of Cryptococcus spp. depends on the composition and flexibility of the cell wall; as the C. gattii variant R265 has a higher chitosan composition than the C. neoformans variant H99, this ends up contributing to a more homogeneous distribution of melanin in the cell wall of C. gattii [67]. Chitosan is a deacetylated form of chitin and its presence in the cell wall of Cryptococcus spp. provides structure and stability to the other molecules in the cell wall [68]. In addition, differences in the amount of chitosan between different species of Cryptococcus have already been seen, with possible implications concerning the the virulence gap between C. gattii variant R265 and C. neoformans, since C. gattii has two- to three-times more chitosan in its cell wall [69]. Other virulence factors have already been described, such as the production of hexitol D-mannitol that helps in the survival of Cryptococcus spp. in the host and confers the resistance to oxidative stress ^[70][71][70,71], as well as the production of phospholipases, contributing to yeast homeostasis and virulence ^[72][73][72,73], provoking the rupture of cell membranes in the host [74] and, thus, contributing to the passage of the fungus to critical sites, such as the central nervous system of mammals [75]. In addition to all these components that are considered important virulence factors for both pathogenic species, C. neoformans and C. gattii, and other species of the same genus, they also produce a capsule containing mostly polysaccharide components that are determinants in the pathogenesis of the fungus [76]. However, due to its complexity, this subject will be addressed in more detail in another topic.