Quantitative neutralization assay of fungicidal activity of disinfectants.

A quantitative assay using a neutralization medium was developed to evaluate fungicidal activity of disinfectants. Concentrated Dey-Engley neutralizing broth was used in this study and was demonstrated to inactivate various chemical agents within 5 min when disinfectant concentrations were reduced to specific levels. Addition of this Dey-Engley broth to test tubes containing fungal cells and disinfectants permitted control of the various interactions times. Subsequent concentration of the disinfectant-treated cells to a 1-ml final volume also permitted examination of a larger population for the presence of resistant cells. After timed exposures of 15, 30, and 60 min, only three of seven disinfectant solutions were found to be lethal for quantitative populations of 11 fungi tested. The recommended use dilution formulations of a quaternary ammonium product and an iodophor product were the least effective agents. Various fungi, including Trichophyton mentagrophytes, Epidermophyton floccosum, and Aspergillus niger, survived 30- to 60-min interactions with these disinfectant solutions. The most resistant organism encountered was Aspergillus fumigatus, which survived 60 and even 90 min of exposure to most disinfectants. Only use dilutions of a chlorine dioxide formulation, a glutaraldehyde formulation, and an ethyl alcohol product were effective against this species and all of the other fungi after a 15-min interaction.