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Background  The unique, black, hypersaline mud mined from the Dead Sea shores is extensively used in mud packs, masks, and topical body and facial treatments in spas surrounding the lake, and in cosmetic preparations marketed worldwide, but little is known about its antimicrobiological properties.

Methods  We performed detailed microbial and chemical analysis of Dead Sea mineral mud compounded in dermatological and cosmetic preparations.

Results  Using conventional bacteriological media (with or without salt augmentation), we found surprisingly low numbers of colony-forming microorganisms in the mud. The highest counts (up to 20,000 colonies per gram, mostly consisting of endospore-forming bacteria) were obtained on sheep blood agar. Test microorganisms (i.e. Escherichia coli, Staphylococcus aureus, Propionibacterium acnes, Candida albicans) rapidly lost their viability when added to the mud. Zones of growth inhibition were observed around discs of Dead Sea mud placed on agar plates inoculated with Candida or with Propionibacterium, but not with Staphylococcus or Escherichia. The effect was also found when the mud was sterilized by gamma irradiation. Using 35S-labeled sulfate as a tracer, bacterial dissimilatory sulfate reduction could be demonstrated at a low rate (0.13 ± 0.03 nmol/cm3.d).

Conclusion  The antibacterial properties of Dead Sea mud are probably owing to chemical and/or physical phenomena. Possible modes of antimicrobial action of the mud in relation to its therapeutic properties are discussed.