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.
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