Bituminous shale, developed from marine sediments during the Jura era, is the raw substance for the production of shale oil. The shale oil undergoes heating, distillation, fractionated refining and sulfonation (according to a patented method) resulting in water-soluble sulfonated shale oils (Ichthyol) for medical purposes. The antifungal effect of sulfonated shale oils have been described earlier. In this study an in vitro method is applied which is based upon the CO2 detection as a measure for the sensitivity of fungi to sulfonated shale oils. In addition to the minimal inhibitory concentration values (MIC values), sub-inhibitory concentrations are also determined. The actual efficacy of these antifungal agents is demonstrated via dose-effect curves. Our results show that the fractions of sulfonated shale oils refined at 150 to 210 degrees C (Ichthyol, dark) are fungicidal in concentrations between 0.2 and 16.8% for yeasts, dermatophytes and other hyphomycetes. The fractions of sulfonated shale oils refined at 85 to 150 degrees C (Ichthyol, light), on the other hand, showed a clearly higher antifungal activity (concentrations between 0.1 and 5.9%) for all fungi tested. An extended exposition (24 to 168 h) of Candida albicans to these fractions resulted in a further increase of fungicidal activity. Due to the complex nature of sulfonated shale oils the chemically defined antifungal substance(s) have not yet been identified.
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