Yeast-suspension as soiling matter in disinfectant testing

Using the Kelsey-Sykes capacity-test, it was found that a sterile yeast suspension used to simulate 'dirty' conditions, gave an increased effect of Chloramine T against the fungi Candida albicans, Aspergillus fumigatus, Geotrichum candidum and Penicillium sp. compared with the effect under 'clean' conditions. This effect was not found with the fungus Rhodotorula rubra nor on the various bacteria tested. The enhanced effect was found with respect to both Chloramine T and Chloramine B, but not with the sodium hypochlorite solution when tested on C. albicans. This effect was due to a diffusible factor from the yeast cells. The factor was evident in the solution after heating of the yeast-cell suspension and in unsterilized yeast-cell suspension left at room temperature for 2 h or more. The effect of Chloramine T on the fungi C. albicans and A. fumigatus was reduced as expected when the yeast suspension was replaced by 20% normal horse serum. The results indicate that using sterile yeast suspensions in this type of test, may erroneously give high fungicidal effects of Chloramine, and thus lead to an incorrect use-dilution concentration, especially if the determination is made on the basis of the effect observed only under dirty conditions.