Stress tolerance and pathogenic potential of a mannitol mutant of Cryptococcus neoformans

Cryptococcus neoformans produces large amounts of the acydic hexitol mannitol in culture and infected animals, but the functional and pathogenic significance of mannitol production by this fungus is not known. We exposed neoformans H99 (Cn H99) to UV irradiation (1 x LD₅₀) and screened survivors for mannitol production. A mutant, Cn MLP (Mannitol Low Producer), synthesized less mannitol from glucose (2.7 vs 8.2 nmol per 10⁸ cells min-¹ at 37°C) and contained less intracellular mannitol (1 vs 11 μmol per 10⁶ cells at 37°C) than did Cn H99. Cn MLP and Cn H99 were similar with respect to carbon assimilation patterns, rates of glucose consumption, growth rates at 30°C, urease and phenoloxidase activities, morphology, capsule formation, mating type, electrophoretic karyotype, rapid amplification of polymorphic DNA (RAPD) patterns and antifungal susceptibility. However, Cn MLP was more susceptible than was Cn H99 to growth inhibition and killing by heat and high NaCl concentrations. Also, the LD₅₀ values in mice injected intravenously were 3.7 x 10⁶ c.f.u, for Cn MLP compared to 6.9 x 10² c.f.u. for Cn H99. Moreover, 500 c.f.u. Cn H99 intravenously killed 12 of 12 mice by 60 d, whereas all mice given the same inoculum of Cn MLP survived. Classical genetic studies were undertaken to determine if these differences were due to a single mutation, but the basidiospores were nonviable. These results suggest that the abilities of C. neoformans to produce and accumulate mannitol may influence its tolerance to heat and osmotic stresses and its pathogenicity in mice.