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Amphotericin B-induced changes in K+ content, viability, and ultrastructure of yeast-phase Histoplasma capsulatum
Abstract:
Yeast-phase cells of Histoplasma capsulatum were challenged with amphotericin B, and membrane perturbation was monitored by K+ efflux. Suspensions of washed cells readily absorbed about 1.12 microgram of amphotericin B per mg (dry weight) and further nonspecific sites were also apparent. The dose-response curve for initial rate of K+ efflux was sigmoidal within the range 0.1 to 1.0 microgram of amphotericin B per ml. A fungistatic concentration of amphotericin B (0.3 microgram/ml) evoked an efflux of 85 to 90% K+ from the cells within 15 min, but cell viability decreased only 13% (yeast phase) or 33% (transformed to mycelial units). Ultrastructural changes in treated cells were detected within 5 min, and the hallmark was expansion of vacuoles during the 1-h monitoring period. In contradistinction to a previous report, the appearance of the protoplasmic membrane was not altered by fungistatic concentration. When treated cells were returned to a fresh growth medium, there was a pronounced lag (20 h). During this apparent recovery phase, the large vacuoles fragmented and returned to normal size. It is proposed that vacuoles of H. capsulatum act as a spatial buffer of considerable survival value to stressed cells.
Insights
Histoplasma capsulatum cells treated with amphotericin B showed significant potassium (K+) efflux and vacuole expansion. Vacuoles appear to buffer cells against antifungal stress, aiding survival.
Area of Science:
- Mycology
- Cell Biology
- Antifungal Drug Action
Background:
- Histoplasma capsulatum is a fungal pathogen causing histoplasmosis.
- Amphotericin B is a key antifungal agent targeting fungal membranes.
- Understanding drug-induced cellular responses is crucial for effective treatment.
Purpose of the Study:
- To investigate the effects of amphotericin B on Histoplasma capsulatum yeast-phase cells.
- To monitor membrane perturbation and cellular changes in response to antifungal challenge.
- To explore the role of cellular vacuoles in fungal stress response.
Main Methods:
- Challenging yeast-phase Histoplasma capsulatum with amphotericin B.
- Monitoring potassium (K+) efflux as an indicator of membrane perturbation.
- Assessing cell viability and ultrastructural changes, including vacuole morphology.
- Observing cellular recovery after drug removal.
Main Results:
- Amphotericin B induced rapid and significant K+ efflux from fungal cells.
- A fungistatic concentration caused substantial K+ loss with limited immediate impact on cell viability.
- Ultrastructural analysis revealed prominent vacuole expansion within minutes of drug exposure.
- Treated cells exhibited a prolonged lag phase upon transfer to fresh medium, during which vacuoles normalized.
Conclusions:
- Vacuoles in Histoplasma capsulatum function as a spatial buffer, enhancing cell survival under antifungal stress.
- The observed vacuole dynamics are a critical survival mechanism for the fungus.
- Amphotericin B's primary effect at fungistatic levels involves membrane disruption leading to ion leakage and vacuole response.