Safety considerations in handling exoantigen extracts from pathogenic fungi
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Abstract:
Currently, exoantigen test procedures for identifying mycelial form cultures of pathogenic molds require that the fungi being extracted be treated with thimerosal to render them safe for handling. Recent studies have demonstrated that thimerosal may not be fungicidal. In view of these reports, we investigated the effects of thimerosal and formaldehyde on a variety of exoantigen preparations. Mature mycelial form fungal cultures, including cultures of Blastomyces dermatitidis, Coccidioides immitis, and Histoplasma capsulatum and morphologically similar fungi, were grown on Sabouraud dextrose agar slants and treated with 0.02, 0.04, and 0.08% thimerosal for 24 and 48 h and with 0.2 and 0.5% formaldehyde for 24 and 48 h. We found that 0.5% formaldehyde killed all of the fungi studied, whereas 0.2% formaldehyde permitted the growth of only one fungus; 0.02, 0.04, and 0.08% thimerosal were fungistatic. Furthermore, 0.2 and 0.5% formaldehyde and 0.08% thimerosal affected certain antigens adversely. For those investigators who prefer to use 0.02% thimerosal and to work with sterile extracts, we recommend that the procedure be modified, and we advocate sterilization of extracts by passage through membrane filters.
Insights
Thimerosal is not reliably fungicidal for pathogenic molds; formaldehyde is more effective but can damage antigens. Membrane filtration is recommended for sterilizing fungal extracts for exoantigen testing.
Area of Science:
- Medical Mycology
- Clinical Microbiology
- Fungal Pathogenesis
Background:
- Exoantigen testing identifies pathogenic molds but requires safe handling of fungal cultures.
- Thimerosal is commonly used for decontamination, but its fungicidal efficacy is questioned.
- Formaldehyde is an alternative disinfectant with potential antigen-altering effects.
Purpose of the Study:
- To evaluate the fungicidal and fungistatic effects of thimerosal and formaldehyde on pathogenic mold cultures.
- To assess the impact of these chemicals on exoantigen integrity.
- To recommend safer and more effective methods for preparing fungal extracts.
Main Methods:
- Culturing of pathogenic molds (Blastomyces dermatitidis, Coccidioides immitis, Histoplasma capsulatum) on Sabouraud dextrose agar.
- Treatment of cultures with varying concentrations of thimerosal (0.02-0.08%) and formaldehyde (0.2-0.5%) for 24-48 hours.
- Microscopic examination for fungal growth and assessment of antigen integrity.
Main Results:
- 0.5% formaldehyde demonstrated complete fungicidal activity against all tested molds.
- 0.2% formaldehyde was fungistatic, allowing one mold species to grow.
- Thimerosal (0.02-0.08%) was fungistatic, and higher concentrations of both chemicals adversely affected certain antigens.
Conclusions:
- Formaldehyde is more effective than thimerosal for killing pathogenic molds but can degrade antigens.
- Current thimerosal decontamination protocols may not ensure fungal inactivation.
- Sterilization of fungal extracts via membrane filtration is a recommended alternative for safe exoantigen testing.