Phenotypic differences of cryptococcus molecular types and their implications for virulence in a drosophila model of infection

Compared to Cryptococcus neoformans, little is known about the virulence of the molecular types in Cryptococcus gattii. We compared in vitro virulence factor production and survival data using a Drosophila model of infection to further characterize the phenotypic features of different cryptococcal molecular types. Forty-nine different isolates were inoculated into wild-type flies and followed for survival. In vitro, isolates were assessed for growth at 30 and 37°C, melanin production, capsule size, resistance to H₂O₂, and antifungal susceptibility. A mediator model was used to assess molecular type and virulence characteristics as predictors of survival in the fly model. VGIII was the most virulent molecular type in flies (P<0.001). At 30°C, VGIII isolates grew most rapidly; at 37°C, VNI isolates grew best. C. gattii capsules were larger than those of C. neoformans (P < 0.001). Mediator model analysis found a strong correlation of Drosophila survival with molecular type and with growth at 30°C. We found molecular-type-specific differences in C. gattii in growth at different temperatures, melanin production, capsule size, ability to resist hydrogen peroxide, and antifungal susceptibility, while growth at 30°C and the VGIII molecular type were strongly associated with virulence in a Drosophila model of infection.