A murine model was used to evaluate the virulence of conidia cultured for 4, 7, 10 or 12 days in Sabouraud's dextrose broth (SDB). A correlation was observed between length of culture and virulence. Mice infected intravenously with conidia cultured for 4 or 7 days showed 40-100% cumulative mortality. In contrast, mice infected with conidia from cultures grown for 10 or 12 days in SDB showed no mortality (100% survival). A much greater accumulation of fungal colony forming units (cfu) was observed in the lungs, livers and spleens of mice inoculated with conidia of cultured for 7 days than in mice infected with conidia cultured for 12 days. The livers of mice from the former group showed a widespread granulomatous reaction whereas mice inoculated with cultured for 12 days showed a more limited response with fewer granulomas. No difference in viability or replicative capacity was discerned for these cultured cells. However, the more virulent forms of the fungus showed differences in cell-wall sugar composition with rhamnose:mannose molar ratios of 1.7:1.0 for cells cultured for 4 days and 1.0:1.7 for conidia cultured for 12 days. These results suggest that the virulence of conidia may be determined by their cell-wall composition.


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