Incorporation of Cystine by Yeast Cells of Histoplasma capsulatum

Dimorphism in Histoplasma capsulatum: a model for the study of cell differentiation in pathogenic fungi

Several fungi can assume either a filamentous or a unicellular morphology in response to changes in environmental conditions. This process, known as dimorphism, is a characteristic of several pathogenic fungi, e.g., Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides brasiliensis, and appears to be directly related to adaptation from a saprobic to a parasitic existence. H. capsulatum is the most extensively studied of the dimorphic fungi, with a parasitic phase consisting of yeast cells and a saprobic mycelial phase. In culture, the transition of H. capsulatum from one phase to the other can be triggered reversibly by shifting the temperature of incubation between 25 degrees C (mycelia) and 37 degrees C (yeast phase). Mycelia are found in soil and never in infected tissue, in contrast to the yeast phase, which is the only form present in patients. The temperature-induced phase transition and the events in establishment of the disease state are very likely to be intimately related. Furthermore, the temperature-induced phase transition implies that each growth phase is an adaptation to two critically different environments. A fundamental question concerning dimorphism is the nature of the signal(s) that responds to temperature shifts. So far, both the responding cell component(s) and the mechanism(s) remain unclear. This review describes the work done in the last several years at the biochemical and molecular levels on the mechanisms involved in the mycelium to yeast phase transition and speculates on possible models of regulation of morphogenesis in dimorphic pathogenic fungi.

Factors influencing the production of H and M antigens by Histoplasma capsulatum: effect of physical factors and composition of medium

Stagnant culture methods have permitted only limited physiological studies of the production of H and M antigens by Histoplasma capsulatum because, with such methods, antigen production is uncontrolled. In this investigation, a shake culture method was used to convert yeast-phase inoculum to mycelialphase growth at 25 C. Results strongly suggest that the release of H and M antigens relates to autolysis of the cells. Among the factors influencing production of H and M antigens under shaking conditions, choice of strain was the most important. Alterations of carbon or nitrogen source or variations in amino acid to carbohydrate ratios had limited influence on antigen production. With a strain that produced both H and M antigens, however, proportions of titers of M to H antigens could be made to vary considerably by changes in the medium, the pH, and the temperature. Results suggest that the source of M antigen during autolysis is enzymatic dissolution of the cell wall. The source of H antigen is more obscure. Production of both antigens may be differentially controlled under conditions of good reproducibility by a correct choice of strain and manipulation of culture medium.