The physiological changes that occur during the mycelial-to yeast-phase transitions induced by a temperature shift from 25 to 37°C of cultures of Blastomyces dermatitidis and Paracoccidioides brasiliensis can be divided into three stages. The triggering event is a heat-related insult induced by the temperature shift which results in partial uncoupling of oxidative phosphorylation and declines in cellular ATP levels, respiration rates, and concentrations of electron transport components (stage 1). The cells then enter a stage in which spontaneous respiration ceases (stage 2), and finally, there is a shift into a recovery phase during Which transformation to yeast morphology occurs (stage 3). Cysteine is required during stage 2 for the operation of shunt pathways which permit electron transport to bypass blocked portions of the cytochrome system. The mycelial-to yeast-phase transitions of these two fungi are very similar to that of Histoplasma capsulatum. Therefore, these three dimorphic fungal pathogens have evolved parallel mechanisms to adjust to the temperature shifts which induce these mycelial-to yeast-phase transitions.Blastomyces dermatitidis, Paracoccidioides brasiliensis, and Histoplasma capsulatum are dimorphic pathogenic fungi. Although there are some differences in geographic distribution and clinical presentation of disease caused by these fungi, they all can cause systemic infections in humans. Other similarities include the existence as mycelia in nature and yeast in infected tissue, growth as mycelia in cultures incubated at 25°C and as yeast at 370C, and induction of reversible phase transitions by switching between 25 and 37°C (7).In previous studies (5, 5a, 8) we characterized the mycelial-to yeast-phase transition of H. capsulatum induced by the temperature shift from 25 to 37°C and also by shifts to temperatures as high as 43°C. We found that the physiological changes that occur in this fungus when the temperature is raised can be divided into three distinct stages. Stage 1, which immediately follows the temperature shift, is characterized by partial or complete uncoupling of oxidative phosphorylation, an immediate decline in ATP levels, and a progressive decrease in respiration rates over 24 h. After 24 to 40 h, the cells enter stage 2, a dormant period of 4 to 6 days that is characterized by absent or low rates of respiration, decreased concentrations of mitochondrial electron transport components, and inhibition of RNA and protein synthesis. Stage 3 is characterized by increasing concentrations of cytochrome components, resumption of normal respiration, and completion of the transition to yeast morphology.The severity of the changes during the transition depends on the thermal tolerance of the strain of H. capsulatum and the level of the temperature of incubation (7). When the temperature elevation is high enough, spontaneous respiration ceases in stage 2, and cysteine or other sulfhydrylcontaining compounds are required for the operation of shunt pathways which permit electron transport t...

p-Chloromercuriphenylsulfonic acid (PCMS), a sulfhydryl inhibitor, prevented the mycelial-to-yeast transition of the dimorphic fungal pathogen, Histoplasma capsulatum. The effect of PCMS was specific for the mycelial-to-yeast transformation; it had no effect on growth of either the yeast or mycelial forms or on the yeast-to-mycelial transition. The failure of PCMS-treated mycelia to transform to yeast was permanent and irreversible. PCMS-treated mycelia could not infect mice but could stimulate resistance to infection by a pathogenic strain of Histoplasma capsulatum. These results suggest a new general strategy for vaccine development in diseases caused by dimorphic pathogens.

Histoplasma capsulatum isolates from three St. Louis area AIDS patients with disseminated histoplasmosis were found to be closely related to the temperature-sensitive, previously unique, Downs strain based on growth phenotype and restriction fragment length polymorphisms (RFLP) involving mitochondrial DNA, ribosomal DNA, and the yps-3 gene. H. capsulatum isolates from five non-AIDS patients in the St. Louis area with disseminated histoplasmosis or chronic pulmonary histoplasmosis had the growth phenotype and RFLP pattern characteristic of most strains isolated from other regions of the USA.

We compared the mycelial to yeast transitions of the Downs strain of Histoplasma capsulatum (low level of virulence) with those of G184A and G222B, two more virulent strains having different levels of pathogenicity for mice. When the morphological transitions are initiated by a temperature shift from 250 to 37°C, all three strains undergo similar physiological changes, but these are less severe in G184A and G222B than in the Downs strain. The transitions from mycelial to yeast morphology in both of the more virulent strains are also one-third more rapid than in Downs. We also find that the differences in temperature sensitivity of the three strains can be correlated with the temperature required for complete uncoupling of oxidative phosphorylation. The differences in sensitivity to elevated temperatures extend to the growth of yeast cells of all three strains.Considered together, our results suggest that sensitivity to elevated temperatures may be a key factor accounting for differences in virulence and that uncoupling of oxidative phosphorylation may be the primary event in the morphological transition in all three strains.

Histoplasma capsulatum, like many other fungal pathogens, is dimorphic: it exists as mycelia in the soil and yeast in animal hosts. Because only the yeast phase is parasitic, factors which affect morphogenesis have been of interest for understanding and controlling pathogenicity. In culture, the mycelial to yeast transition of H. capsulatum is induced by a temperature shift from 25 to 37 degrees C (ref. 1). The transition occurs over several days and is accompanied by marked changes in metabolic processes, including respiration and cysteine metabolism. Here, we show that the triggering event for these morphological and biochemical changes is a rapid decline in intracellular ATP levels that follows uncoupling of oxidative phosphorylation when mycelia are shifted from 25 to 37 degrees C. We also show that respiration in the yeast phase is coupled at 37 degrees C and thus that the morphological transition may be viewed as a heat shock followed by cellular adaptation to higher temperature.