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The Use of Sodium Thiosulphate Dilution Blanks in Determining the Germicidal Efficiency of Chlorine Sterilizers^*

Division of Dairy Husbandry and Department of Bacteriology, State College of Washington, Pullman, Washington

ABSTRACT

Simultaneous platings of inoculated chlorine solutions immediately after inoculating and after 2.5, 5.0, 7.5, 10 and 15 minutes, using ordinary dilution blanks and dilution blanks containing sodium thiosulphate were made and compared. In most cases an increased colony count resulted when sodium thiosulphate dilution blanks were used. Colony development following brief periods of exposure of the organisms to the action of chlorine showed, in most cases, an increased count when sodium thiosulphate dilution blanks were used. After longer periods of exposure, ten or more minutes, the presence of sodium thiosulphate had little or no effect on subsequent colony development.

The results of these experiments show that plate counts, in which ordinary dilution blanks have been used are likely to be misleading in evaluating the germicidal efficiency of chlorine sterilizers.

In an effort to study the effect of dilution in the bacteriostatic effect of the residual chlorine, plate counts of inoculated chlorine solutions, using ordinary dilution blanks, were made and the numbers of colonies developing on the plates of the 1–1, 1–10 and 1–100 dilutions compared. In no instance were more colonies found to develop on the plates of the higher dilutions than on plates of the lower dilutions.

^* Published as Scientific Paper No. 248, College of Agriculture and Experimental Station, State College of Washington.