A New Method for the Evaluation of Germicidal Substances

Abstract

The determination of the phenol coefficient is not an accurate basis for the evaluation of disinfectants employed under all conditions. The method is practical for rating disinfectants to be employed for the sterilizaton of nonliving material, but it fails to take into account the toxicity of germicides for living tissues when used internally or on mucous surfaces. A more valuable expression would be one based on a combination of the killing power of the disinfectant for bacteria with its toxic action toward living embryonic tissue. This method gives a figure, the toxicity index, which is defined as the ratio of the highest dilution of disinfectant required to prevent the growth of embryonic chick heart tissue during 48 hrs. to the dilution required to kill a given test organism in 10 min. The toxicity indices are not always proportional to the phenol coefficients. Toxicity indices for germicides to chick tissue and Staphylo-coccus aureus are: iodine, 0.09; iodine trichloride 0.40; HgCI2, 2.8; Hexylresorcinol, 3.0; Metaphen, 12.7; phenol, 12.9; potassium mercuric iodide, 13.3; Merthiolate 35.3; Mercurochrome, 262.0. Toxicity indices for germicides to chick tissue and Eberthella typhosa are: I, 0.08; potassium mercuric iodide, 0.11; HgCl2, 0.25; iodine trichloride, 0.28; Metaphen, 0.84; Hexylresorcinol, 2.8; phenol, 8.4; Merthiolate, 35; Mercurochrome, 35.