4. Researches and Findings
C. neoformans var.
neoformans isolates cultivated in Germany show a remarkable phenotypic diversity including capsule size, production of melanin, urease and phospholipase activity. This may impact the differences in uptake into phagocytic amoebae, cytotoxicity for amoebae and virulence in the
G. mellonella infection model. Isolates from disseminated infections showed larger capsules, cell bodies and higher melanin production as compared to isolates from localized skin infections. Although isolates from disseminated infections showed variable degrees of urease and phospholipase production, they demonstrated higher uptake into amoeba and cytotoxicity and shorter survival of
G. mellonella.
Epidemiologic studies performed in the context of disseminated infections by
C. neoformans var.
grubii in HIV-infected patients demonstrate that fungal genotype may affect clinical outcomes
[4]. It was suggested that cell surface changes, including capsule size, melanization and changes in cell size, i.e., large titan cells and small micro cells are phenotypic changes that correlate with virulence potential when environmental fungi enter hosts
[4]. It was demonstrated that isolates from human cryptococcosis patients, mainly
C. neoformans var.
grubii and
C. gattii, show substantial variation in virulence in infection models including the
G. mellonella infection model
[21,22][15][16].
So far, few studies have been performed to correlate the virulence potential of
C. neoformans var.
neoformans and the clinical presentation of these infections.
C. neoformans var.
neoformans are the second most prevalent agents of cryptococcosis in Europe
[5,6][5][6]. In addition, they can be cultivated from environmental sources including trees, dust, pigeon droppings and animals including cats, cows and pigeons
[9]. Based on environmental sampling from non-random sites, it was suggested that this fungus may be adapted to colder, wetter climates currently found in central Europe
[10]. However, a recent report on the cultivation from Saudi Arabian desert soil suggests that this fungus is more widespread and potentially more adaptable to various environmental conditions than previously thought
[23][17]. Interestingly, the population structure of
C. neoformans var.
neoformans isolates from Europe shows high genotypic diversity potentially facilitated by sexual reproduction
[9]. As this process may generate phenotypic diverse offspring, laboratory tests might be needed address to virulence potential and therefore the public health potential of
C. neoformans var.
neoformans isolates.
Different models were used to study fungal virulence. Amoebas are a relatively easy and cheap model system. They have gained interest as some aspects of the interaction between amoeba and fungi resemble the interaction between fungi and phagocytic cells. The capsule, melanin production and phospholipase activity were shown to be important for the survival of
Cryptococcus in coincubation with amoeba, while urease activity and mating-type were not. This model does not reflect other disease determinants such as immune-mediated host damage, an important part of animal cryptococcosis
[24][18]. However, the significant differences in uptake by amoeba and cytotoxicity suggest that this model may be used to screen for the potential of strains to cause disseminated infections.
G. mellonella has emerged as a model system to explore fungal virulence and pathogenesis.
G. mellonella and the mammalian immune system share some similarities including pathogen recognition, inflammatory responses, phagocytosis of pathogens and phagocyte responses to fungal uptake
[25][19].
Primary cutaneous cryptococcosis typically presents as a solitary skin infection site after transcutaneous injury with a contaminated source. These infections are thought to be mostly caused by
C. neoformans var.
neoformans (genotype VNIV) isolates and generally have a favorable outcome without systemic signs of infection and mostly negative serum antigen. They are often diagnosed in patients without underlying immunosuppression
[8]. The absence of dissemination in these cases might be a consequence of the host’s immune system and fungal attributes. Genotype VNIV strains were shown to be less virulent in animal models than VNI strains. In addition, cryptococci with the mating-type are less virulent in infection models and this was confirmed for
C. neoformans var.
neoformans [7,26][7][20].
Nevertheless, even Da strains may cause disseminated infections despite low virulence potential in infection models as demonstrated here by the isolate 19-0346 from a disseminated infection and low virulence in
G. mellonella. This has previously been documented for another Da isolate from a disseminated infection in an AIDS patient and low virulence in a mouse model
[7].